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Advanced Functional Materials

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ISSN (Print) 1616-301X - ISSN (Online) 1616-3028
Published by John Wiley and Sons

[1587 journals]

Masthead: (Adv. Funct. Mater. 19/2013)
- Pages: n/a - n/a
  PubDate: 2013-05-13T15:12:57.678752-05:
  DOI: 10.1002/adfm.201370093
Single‐Walled Carbon Nanotube/Phase Change Material Composites: Sunlight‐Driven, Reversible, Form‐Stable Phase Transitions for Solar Thermal Energy Storage
- Authors: Yunming Wang; Bingtao Tang, Shufen Zhang
  Pages: n/a - n/a
  Abstract: The development of solar energy conversion materials is critical to the growth of a sustainable energy infrastructure in the coming years. A novel hybrid material based on single‐walled carbon nanotubes (SWNTs) and form‐stable polymer phase change materials (PCMs) is reported. The obtained materials have UV‐vis sunlight harvesting, light‐thermal conversion, thermal energy storage, and form‐stable effects. Judicious application of this efficient photothermal conversion to SWNTs has opened up a rich field of energy materials based on novel SWNT/PCM composits with enhanced performance in energy conversion and storage. Novel single‐walled carbon nanotube/phase change material (SWNT/PCM) composites have UV‐vis sunlight harvesting, light‐thermal conversion, thermal energy storage, and form‐stable effects. Upon UV‐vis light irradiation, the light‐to‐heat conversion and thermal storage efficiency (η) of the obtained SWNT/PCM composites is over 0.84 using the photothermal calculation method.
  PubDate: 2013-05-10T11:23:08.697285-05:
  DOI: 10.1002/adfm.201203728
Graphene Paper Doped with Chemically Compatible Prussian Blue Nanoparticles as Nanohybrid Electrocatalyst
- Authors: Nan Zhu; Shuang Han, Shiyu Gan, Jens Ulstrup, Qijin Chi
  Pages: n/a - n/a
  Abstract: Along with reduced graphene oxide (RGO), water soluble Prussian blue nanoparticles (PBNPs, around 6 nm) are synthesized and broadly characterized. These two types of highly stable, low‐cost and chemically compatible nanomaterials are exploited as building ingredients to prepare electrically enhanced and functionally endorsed nanohybrid electrocatalysts, which are further transformed into free‐standing graphene papers. PBNPs doped graphene papers show highly efficient electrocatalysis towards reduction of hydrogen peroxide and can be used alone as flexible chemical sensors for potential applications in detection of hydrogen peroxide or/and other organic peroxides. The as‐prepared PBNPs–RGO papers are further capable of biocompatible accommodation of enzymes for development of free‐standing enzyme based biosensors. In this regard, glucose oxidase is used as an example for electrocatalytic oxidation and detection of glucose. The present work demonstrates a facile and highly reproducible way to construct free‐standing and flexible graphene paper doped with electroactive catalyst. Thanks to high stability, low‐cost and efficient electrocatalytic characteristics, this kind of nanohybrid material has potential to be produced on a large scale, and offers a broad range of possible applications, particularly in the fabrication of flexible sensing devices and as a platform for electrocatalytic energy conversion. Water soluble and chemically compatible Prussian blue nanoparticles (PBNPs) and reduced graphene oxide (RGO) are synthesized. These two kinds of stable and low‐cost nanoscale materials are used as building blocks to prepare electrically enhanced and functionally endorsed hybrid nanosheets, which are further transformed into free‐standing graphene paper for high‐performance electrocatalysis and application as flexible sensors.
  PubDate: 2013-05-10T03:23:05.169605-05:
  DOI: 10.1002/adfm.201300605
Sensory Arrays of Covalently Functionalized Single‐Walled Carbon Nanotubes for Explosive Detection
- Authors: Jan M. Schnorr; Daan van der Zwaag, Joseph J. Walish, Yossi Weizmann, Timothy M. Swager
  Pages: n/a - n/a
  Abstract: Chemiresistive sensor arrays for cyclohexanone and nitromethane are fabricated using single‐walled carbon nanotubes (SWCNTs) that are covalently functionalized with urea, thiourea, and squaramide containing selector units. Based on initial sensing results and 1H NMR binding studies, the most promising selectors are chosen and further optimized. These optimized selectors are attached to SWCNTs and simultaneously tested in a sensor array. The sensors show a very high level of reproducibility between measurements with the same sensor and across different sensors of the same type. Furthermore, the sensors show promising long‐term stability, which renders them suitable for practical applications. Sensor arrays for cyclohexanone and nitromethane are fabricated using single‐walled carbon nanotubes (SWCNTs) that are covalently functionalized with different types of selector units. The thiourea, urea and squaramide‐based selectors are optimized for improved sensing response. The sensors can be easily fabricated and integrated into electronic circuits. Furthermore, they show a very high level of reproducibility and promising long‐term stability.
  PubDate: 2013-05-08T02:10:11.751672-05:
  DOI: 10.1002/adfm.201300131
Exciplex‐Forming Co‐host for Organic Light‐Emitting Diodes with Ultimate Efficiency
- Authors: Young‐Seo Park; Sunghun Lee, Kwon‐Hyeon Kim, Sei‐Yong Kim, Jeong‐Hwan Lee, Jang‐Joo Kim
  Pages: n/a - n/a
  Abstract: Phosphorescent organic light‐emitting diodes (OLEDs) with ultimate efficiency in terms of the external quantum efficiency (EQE), driving voltage, and efficiency roll‐off are reported, making use of an exciplex‐forming co‐host. This exciplex‐forming co‐host system enables efficient singlet and triplet energy transfers from the host exciplex to the phosphorescent dopant because the singlet and triplet energies of the exciplex are almost identical. In addition, the system has low probability of direct trapping of charges at the dopant molecules and no charge‐injection barrier from the charge‐transport layers to the emitting layer. By combining all these factors, the OLEDs achieve a low turn‐on voltage of 2.4 V, a very high EQE of 29.1% and a very high power efficiency of 124 lm W−1. In addition, the OLEDs achieve an extremely low efficiency roll‐off. The EQE of the optimized OLED is maintained at more than 27.8%, up to 10 000 cd m−2. Using an exciplex‐forming co‐host, an organic light‐emitting diode (OLED) with ultimate efficiency is produced. The OLED has a low turn‐on voltage of 2.4 V, a very high external quantum efficiency (EQE) of 29.1%, a very high power efficiency of 124 lm W−1, and an extremely low efficiency roll‐off. The EQE of the optimized OLED is maintained at more than 27.8%, up to 10 000 cd m−2.
  PubDate: 2013-05-06T09:30:33.828496-05:
  DOI: 10.1002/adfm.201300547
Turning ABO3 Antiferroelectrics into Ferroelectrics: Design Rules for Practical Rotation‐Driven Ferroelectricity in Double Perovskites and A3B2O7 Ruddlesden‐Popper Compounds
- Authors: Andrew T. Mulder; Nicole A. Benedek, James M. Rondinelli, Craig J. Fennie
  Pages: n/a - n/a
  Abstract: Ferroic transition metal oxides, which exhibit spontaneous elastic, electrical, magnetic or toroidal order, exhibit functional properties that find use in ultrastable solid‐state memories, sensors and medical imaging technologies. To realize multifunctional behavior, where one order parameter can be coupled to the conjugate field of another order parameter, however, requires a common microscopic origin for the long‐range order. Here, a complete theory is formulated for a novel form of ferroelectricity, whereby a spontaneous and switchable polarization emerges from the destruction of an antiferroelectric state due to octahedral rotations and ordered cation sublattices. A materials design framework is then constructed based on crystal‐chemistry descriptors rooted in group theory, which enables the facile design of artificial oxides with large electric polarizations, P, simultaneous with small energetic switching barriers between +P and ‐P. The theory is validated with first principles density functional calculations on more than 16 perovskite‐structured oxides, illustrating it could be operative in any materials classes exhibiting two‐ or three‐dimensional corner‐connected octahedral frameworks. The principles governing materials selection of the “layered” systems are shown to originate in the lattice dynamics of the A cation displacements stabilized by the pervasive BO6 rotations of single phase ABO3 materials, whereby the latter distortions govern the optical band gaps, magnetic order and critical transition temperatures. This approach provides the elusive route to the practical control of octahedral rotations, and hence a wide range of functional properties, with an applied electric field. A microscopic theory for hybrid improper ferroelectrics is reported, whereby a spontaneous polarization emerges from an antiferroelectric state owing to the combination of octahedral rotations and cation ordering. A materials design framework is constructed based on crystal‐chemistry descriptors rooted in group theory— enabling the design of artificial oxides with large electric polarizations and small energetic switching barriers.
  PubDate: 2013-05-06T09:30:26.911701-05:
  DOI: 10.1002/adfm.201300210
Photoinduced Electron Transfer Between Pyridine Coated Cadmium Selenide Quantum Dots and Single Sheet Graphene
- Authors: Shirui Guo; Duoduo Bao, Srigokul Upadhyayula, Wei Wang, Ali B. Guvenc, Jennifer R. Kyle, Hamed Hosseinibay, Krassimir N. Bozhilov, Valentine I. Vullev, Cengiz S. Ozkan, Mihrimah Ozkan
  Pages: n/a - n/a
  Abstract: Interest in graphene as a two‐dimensional quantum‐well material for energy applications and nanoelectronics has increased exponentially in the last few years. The recent advances in large‐area single‐sheet fabrication of pristine graphene have opened unexplored avenues for expanding from nano‐ to meso‐scale applications. The relatively low level of absorptivity and the short lifetimes of excitons of single‐sheet graphene suggest that it needs to be coupled with light sensitizers in order to explore its feasibility for photonic applications, such as solar‐energy conversion. Red‐emitting CdSe quantum dots are employed for photosensitizing single‐sheet graphene with areas of several square centimeters. Pyridine coating of the quantum dots not only enhances their adhesion to the graphene surface, but also provides good electronic coupling between the CdSe and the two‐dimensional carbon allotrope. Illumination of the quantum dots led to injection of n‐carrier in the graphene phase. Time‐resolved spectroscopy reveals three modes of photoinduced electron transfer between the quantum dots and the graphene occurring in the femtosecond and picosecond time‐domains. Transient absorption spectra provide evidence for photoinduced hole‐shift from the CdSe to the pyridine ligands, thereby polarizing the surface of the quantum dots. That is, photoinduced electrical polarization, which favors the simultaneous electron transfer from the CdSe to the graphene phase. These mechanistic insights into the photoinduced interfacial charge transfer have a promising potential to serve as guidelines for the design and development of composites of graphene and inorganic nanomaterials for solar‐energy conversion applications. Red‐emitting CdSe quantum dots coated with pyridine, self assembled into a layer over a large area single‐sheet graphene, serve as photosensitizers. Pyridine coating of the quantum dots enhances their adhesion to the graphene surface, and provides electronic coupling that is essential for efficient photoinduced electron transfer from the CdSe to the 2D carbon allotrope. A hole shift to the pyridinium ligands accompanies the electron transfer, making this interfacial charge transfer electrostatically favorable.
  PubDate: 2013-05-06T09:30:19.80843-05:0
  DOI: 10.1002/adfm.201203652
Shape‐Memory Microfluidics
- Authors: Aditya Balasubramanian; Robert Morhard, Christopher J Bettinger
  Pages: n/a - n/a
  Abstract: Materials with embedded vascular networks afford rapid and enhanced control over bulk material properties including thermoregulation and distribution of active compounds such as healing agents or stimuli. Vascularized materials have a wide range of potential applications in self‐healing systems and tissue engineering constructs. Here, the application of vascularized materials for accelerated phase transitions in stimuli‐responsive microfluidic networks is reported. Poly(ester amide) elastomers are hygroscopic and exhibit thermo‐mechanical properties (Tg ≈ 37 °C) that enable heating or hydration to be used as stimuli to induce glassy‐rubbery transitions. Hydration‐dependent elasticity serves as the basis for stimuli‐responsive shape‐memory microfluidic networks. Recovery kinetics in shape‐memory microfluidics are measured under several operating modes. Perfusion‐assisted delivery of stimulus to the bulk volume of shape‐memory microfluidics dramatically accelerates shape recovery kinetics compared to devices that are not perfused. The recovery times are 4.2 ± 0.1 h and 8.0 ± 0.3 h in the perfused and non‐perfused cases, respectively. The recovery kinetics of the shape‐memory microfluidic devices operating in various modes of stimuli delivery can be accurately predicted through finite element simulations. This work demonstrates the utility of vascularized materials as a strategy to reduce the characteristic length scale for diffusion, thereby accelerating the actuation of stimuli‐responsive bulk materials. Vascularization in complex organisms plays an important role in reducing the characteristic diffusion length scale of tissue structures with large dimensions. This strategy is recapitulated in synthetic shape‐memory materials. Vascular networks accelerate phase transitions and increase recovery rates in stimuli‐responsive polymers by reducing the characteristic diffusion length scale of the bulk material.
  PubDate: 2013-05-06T09:23:04.726174-05:
  DOI: 10.1002/adfm.201203618
Versatile Graphene‐Promoting Photocatalytic Performance of Semiconductors: Basic Principles, Synthesis, Solar Energy Conversion, and Environmental Applications
- Authors: Wenguang Tu; Yong Zhou, Zhigang Zou
  Pages: n/a - n/a
  Abstract: Graphene‐semiconductor nanocomposites, considered as a kind of most promising photocatalysts, have shown remarkable performance and drawn significant attention in the field of photo‐driven chemical conversion using solar energy, due to the unique physicochemical properties of graphene. The photocatalytic enhancement of graphene‐based nanocomposites is caused by the reduction of the recombination of electron‐hole pairs, the extension of the light absorption range, increase of absorption of light intensity, enhancement of surface active sites, and improvement of chemical stability of photocatalysts. Recent progress in the photocatalysis development of graphene‐based nanocomposites is highlighted and evaluated, focusing on the mechanism of graphene‐enhanced photocatalytic activity, the understanding of electron transport, and the applications of graphene‐based photocatalysts on water splitting, degradation or oxidization of organic contaminants, photoreduction of CO2 into renewable fuels, toxic elimination of heavy metal ions, and antibacterial applications. Recent progress in the photocatalysis development of graphene‐based nanocomposites is highlighted and evaluated. The focus is on the mechanism of graphene‐enhanced photocatalytic activity, the understanding of electron transport, and the applications of graphene‐based photocatalysts for water splitting, degradation or oxidization of organic contaminants, photoreduction of CO2 into renewable fuels, toxic elimination of heavy metal ions, and antibacterial applications.
  PubDate: 2013-05-05T17:23:06.604867-05:
  DOI: 10.1002/adfm.201203547
Relaxing the Conductivity/Transparency Trade‐Off in MOCVD ZnO Thin Films by Hydrogen Plasma
- Authors: Laura Ding; Sylvain Nicolay, Jérôme Steinhauser, Ulrich Kroll, Christophe Ballif
  Pages: n/a - n/a
  Abstract: Increasing the conductivity of polycrystalline zinc oxide films without impacting the transparency is a key aspect in the race to find affordable and high quality material as replacement of indium‐containing oxides. Usually, ZnO film conductivity is provided by a high doping and electron concentration, detrimental to transparency, because of free carrier absorption. Here we show that hydrogen post‐deposition plasma treatment applied to ZnO films prepared by metalorganic low‐pressure chemical vapor deposition allows a relaxation of the constraints of the conductivity/transparency trade‐off. Upon treatment, an increase in electron concentration and Hall mobility is observed. The mobility reaches high values of 58 and 46 cm2V−1s−1 for 2‐μm‐ and 350‐nm‐thick films, respectively, without altering the visible range transparency. From a combination of opto‐electronic measurements, hydrogen is found, in particular, to reduce electron trap density at grain boundaries. After treatment, the values for intragrain or optical mobility are found similar to Hall mobility, and therefore, electron conduction is found to be no longer limited by the phenomenon of grain boundary scattering. This allows to achieve mobilities close to 60 cm2V−1s−1, even in ultra‐transparent films with carrier concentration as low as 1019 cm−3. Enhanced mobility and conductivity are reached in polycrystalline zinc oxide films by hydrogen plasma post‐deposition treatment. Opto‐electrical characterization methods show that electron trap defects at grain boundaries are passivated, and free electron concentration is increased, and that the electron transport is no longer limited by grain boundary scattering, even for non‐intentionally doped films.
  PubDate: 2013-05-02T02:20:38.700957-05:
  DOI: 10.1002/adfm.201203541
High Efficiency White Organic Light‐Emitting Devices Incorporating Yellow Phosphorescent Platinum(II) Complex and Composite Blue Host
- Authors: Shiu‐Lun Lai; Wai‐Yip Tong, Steven C. F. Kui, Mei‐Yee Chan, Chi‐Chung Kwok, Chi‐Ming Che
  Pages: n/a - n/a
  Abstract: A new class of charge neutral, strongly luminescent cyclometalated platinum(II) complexes supported by dianionic tetradentate ligand are synthesized. One of these platinum(II) complexes, Y‐Pt, displays a high photoluminescence quantum yield of 86% and electroluminescence efficacy (ηpower) of up to 52 lm W−1, and is utilized as a yellow phosphorescent dopant in the fabrication of white organic light‐emitting devices (WOLEDs). WOLEDs based on conventional structures with yellow emission from Y‐Pt in combination with blue emission from bis(4,6‐difluorophenyl‐pyridinato‐N,C2′) (picolinate) iridium(III) (FIrpic) show a total ηpower of up to 31 lm W−1. A two‐fold increase in ηpower by utilizing a modified WOLED structure comprising of a composite blue host is realized. With this modified device structure, the total ηpower and driving voltage at a luminance of 1000 cd m−2 can be improved to 61 lm W−1 and 7.5 V (i.e., 10 V for control devices). The performance improvement is attributed to an effectively broaden exciton formation‐recombination zone and alleviation of localized exciton accumulation within the FIrpic‐doped composite host for reduced triplet‐triplet annihilation, yielding blue light‐emission with enhanced intensity. The modified device structure can also adopt a higher concentration of Y‐Pt towards its optimal value, leading to WOLEDs with high efficiency. An effective and simple approach for boosting the performance of white organic light‐emitting devices (WOLEDs) is demonstrated. A modified device structure consisting of a composite blue host that can significantly improve both device efficiency and efficiency roll‐off is developed. With the modified structure, a WOLED exhibiting twofold enhancement in the total efficacy from 31 up to 61 lm W−1 is realized.
  PubDate: 2013-05-02T02:20:34.582369-05:
  DOI: 10.1002/adfm.201300281
Engineered 3D Silk‐Based Metastasis Models: Interactions Between Human Breast Adenocarcinoma, Mesenchymal Stem Cells and Osteoblast‐Like Cells
- Authors: Sarmistha Talukdar; Subhas C. Kundu
  Pages: n/a - n/a
  Abstract: Bone metastasis occurs in 70% of breast cancer patients and is a frequent cause of morbidity in cancer patients. A delicate balance exists in the bone microenvironment, but the functional dynamics underlying the tumor cell‐microenvironment interactions remain poorly understood. 3D in vitro model systems of metastasis can throw new light on this phenomenon. Silk protein fibroin scaffolds, are cytocompatible for 3D cancer cell culture. They are structurally more resistant to protease degradation than other native biomaterials making these matrices suitable for cancer modeling. In this report, human breast adenocarcinoma cells, human osteoblast like cells and mesenchymal stem cells are co‐cultered. Cancer cells and osteoblast‐like cells are found to interact through secreted products. Decreased population of osteoblast‐like cells and mineralization of extracellular matrix are observed as a result of co‐culture. Significantly increased migration of breast cancer cells is observed in the bone‐like constructs than in non‐seeded scaffolds. The co‐culture constructs show significant increase in drug resistance, invasiveness and angiogenicity. Co‐culture of breast cancer cells with osteoblast like cells and mesenchymal stem cells also indicate that the interaction of cancer cells with bone microenvironment varies with spatial organization, presence of osteogenic factors as well as stromal cell type. Here, results show that 3D in vitro co‐culture models is possibly a better system to study and target cancer progression. Bone metastasis depends on the complex interactions between the primary tumor and the metastatic niche. Metastasis models can illuminate this poorly understood phenomenon. Antheraea mylitta silk fibroin scaffold based 3D in vitro co‐cultured tumor model systems can help in understanding the tumor‐stroma interactions during metastasis as well as evaluate chemoresistance.
  PubDate: 2013-05-02T02:20:31.254663-05:
  DOI: 10.1002/adfm.201300312
Visible‐Near Infrared Absorbing Polymers Containing Thienoisoindigo and Electron‐Rich Units for Organic Transistors with Tunable Polarity
- Authors: Gitish K. Dutta; A‐Reum Han, Junghoon Lee, Yiho Kim, Joon Hak Oh, Changduk Yang
  Pages: n/a - n/a
  Abstract: Systematic creation of polymeric semiconductors from novel building blocks is critical for improving charge transport properties in organic field‐effect transistors (OFETs). A series of ultralow‐bandgap polymers containing thienoisoindigo (TIIG) as a thiophene analogue of isoindigo (IIG) is synthesized. The UV‐Vis absorptions of the TIIG‐based polymers (PTIIG‐T, PTIIG‐Se, and PTIIG‐DT) exhibit broad bands covering the visible to near‐infrared range of up to 1600 nm. All the polymers exhibit unipolar p‐channel operations with regard to gold contacts. PTIIG‐DT with centrosymmetric donor exhibits a maximum mobility of 0.20 cm2 V−1 s−1 under gold contacts, which is higher than those of the other polymers containing axisymmetric donors. Intriguingly, OFETs fabricated with aluminum electrodes show ambipolar charge transport with hole and electron mobilities of up to 0.28 (PTIIG‐DT) and 0.03 (PTIIG‐T) cm2 V−1 s−1, respectively. This is a record value for the hitherto reported TIIG‐based OFETs. The finding demonstrates that TIIG‐based polymers can potentially function as either unipolar or ambipolar semiconductors without reliance on the degree of electron affinity of the co‐monomers. A series of ultralow‐bandgap polymers made by copolymerizing various electron‐rich units with a newly conceived thienoisoindigo (TIIG) moiety is presented for organic field‐effect transistors. Investigation of their field‐effect performance indicates that the TIIG‐based polymers can function as either a unipolar p‐type or ambipolar semiconductor via the variation of the electrode metals, independent of the electron affinity of the counterparts.
  PubDate: 2013-05-02T02:20:26.059988-05:
  DOI: 10.1002/adfm.201300536
Flexible Multi‐Colored Electrochromic and Volatile Polymer Memory Devices Derived from Starburst Triarylamine‐Based Electroactive Polyimide
- Authors: Hung‐Ju Yen; Chih‐Jung Chen, Guey‐Sheng Liou
  Pages: n/a - n/a
  Abstract: Flexible multi‐colored electrochromic and volatile memory devices are fabricated from a solution‐processable electroactive aromatic polyimide with starburst triarylamine unit. The polyimide prepared by the chemical imidization was highly soluble in many organic solvents and showed useful levels of thermal stability associated with high glass‐transition temperatures. The polyimide with strong electron‐donating capability possesses static random access memory behavior and longer retention time than other 6FDA‐based polyimides. The differences of the highest‐occupied and lowest unoccupied molecular orbital levels among these polyimides with different electron‐donating moieties are investigated and the effect on the memory behavior is demonstrated. The polymer film shows reversible electrochemical oxidation and electrochromism with high contrast ratio both in the visible range and near‐infrared region, which also exhibits high coloration efficiency, low switching time, and the outstanding stability for long‐term electrochromic operation. The highly stable electrochromism and interesting volatile memory performance are promising properties for the practical flexible electronics applications in the future. Flexible multi‐colored electrochromic (EC) and volatile polymer memory devices are fabricated from starburst triarylamine‐based polyimide. The polyimide possesses static random access memory behavior and longer retention time than other C(CF3)2‐based polyimides. The flexible EC device showed multicolor electrochromism with excellent stability for long‐term EC operation. The characteristics suggest that the novel starburst triarylamine‐containing polyimide has great potential for future flexible electronics.
  PubDate: 2013-05-02T02:20:19.113499-05:
  DOI: 10.1002/adfm.201300569
A Water‐Soluble Mechanochromic Luminescent Pyrene Derivative Exhibiting Recovery of the Initial Photoluminescence Color in a High‐Humidity Environment
- Authors: Yoshimitsu Sagara; Toru Komatsu, Tasuku Ueno, Kenjiro Hanaoka, Takashi Kato, Tetsuo Nagano
  Pages: n/a - n/a
  Abstract: Switching of the luminescence properties of molecular materials in response to mechanical stimulation is of fundamental interest and also has a range of potential applications. Herein, a water‐soluble mechanochromic luminescent pyrene derivative having two hydrophilic dendrons is reported. This pyrene derivative is the first example of a mechanochromic luminescent organic compound that responds to relative humidity. Mechanical stimulation (grinding) of this pyrene derivative in the solid state results in a change of the photoluminescence from yellow to green. Subsequent exposure to water vapor induces recovery of the initial yellow photoluminescence. The color change is reversible through at least ten cycles. It is also demonstrated that this compound can be applied as a mechano‐sensing material in frictional wear testing for grease, owing to its immiscibility in non‐polar solvents and its non‐crystalline behavior. Transmission electron microscope and atomic force microscope observations of samples prepared from dilute aqueous solutions of the pyrene derivative on suitable substrates, together with dynamic light scattering measurements for the compound in aqueous solution, indicate that this amphiphilic dumbbell‐shaped molecule forms micelles in water. A water‐soluble mechanochromic luminescent pyrene derivative with two hydrophilic dendrons is reported. This amphiphilic dumbbell‐shaped molecule forms micelles in water. Mechanical stimulation (grinding) of this pyrene derivative in the solid state results in a change of the photoluminescence from yellow to green. Subsequent exposure to water vapor induces recovery of the initial yellow photoluminescence.
  PubDate: 2013-05-02T02:20:12.838289-05:
  DOI: 10.1002/adfm.201300180
Bio‐Inspired Synthesis of High‐Performance Nanocomposite Catalysts for Hydrogen Oxidation
- Authors: Chang Sun Kong; Hong‐Li Zhang, Ferenc Somodi, Daniel E. Morse
  Pages: n/a - n/a
  Abstract: A biologically inspired synthesis method is presented as a new tool for the design of novel electrochemically active materials, focusing on the advantages for fuel cell development. The need for cost‐effective, high‐performance materials is driving contemporary fuel cell research, with the expectation that advances in synthetic methods will be necessary for commercialization of this energy technology. Highly active electrocatalysts for proton‐exchange‐membrane (PEM) fuel cells are being developed, by combining a kinetically controlled synthesis method of the nanocrystalline metal catalyst with the mesoscale assembly of two morphologically different carbon building blocks of the supporting matrix. These methods provide access to new combinations of porosity, conductivity and electrochemical hydrogen oxidation. The relationships between the porous morphologies of the carbon matrices, the sizes of the platinum nanocrystals and their resulting electrochemical activities are discussed, correlating these with the relevant fuel cell principles. Biologically inspired, kinetically controlled synthesis is used to develop a new family of nanocrystalline Pt‐based catalytic electrodes. A novel carbon‐carbon composite of carbon black and multiwall carbon nanotubes is formed for the control of mesoscale morphology and used as the matrix for nucleation and growth of nanocrystalline Pt, providing access to new combinations of porosity, conductivity and electrochemical hydrogen oxidation.
  PubDate: 2013-04-30T04:20:33.034302-05:
  DOI: 10.1002/adfm.201203882
Light‐Switchable Single‐Walled Carbon Nanotubes Based on Host–Guest Chemistry
- Authors: Zanru Guo; Yujun Feng, Dingwei Zhu, Shuai He, Hanbin Liu, Xiangrong Shi, Jing Sun, Meizhen Qu
  Pages: n/a - n/a
  Abstract: A new type of light‐switchable “smart” single‐walled carbon nanotube (SWNTs) is developed by the reversible host–guest interaction between azobenzene‐terminal PEO (AzoPEO) and pyrene‐labeled host attached on the sidewalls of nanotubes via π–π stacking. The SWNTs hybrids not only are well dispersed in pure water, but also exhibit switchable dispersion/aggregation states upon the alternate irradiation of UV and visible light. Moreover, the SWNTs hybrids dispersion is preliminarily used as coating fluid to form transparent conductive films. The dispersant AzoPEO is removed by the contamination‐free UV treatment, decreasing the resistance of the films. This kind of light‐switchable SWNTs hybrids, possessing a ‘‘green’’ trigger and intact structure of the nanotube, may find potential applications in sensor of biomedicines, device fabrication etc. Additionally, such a reversible host–guest interaction system may open up the possibility to control the dispersion state of SWNTs by other common polymers. A new type of “smart” single‐walled carbon nanotube is created by combining reversible host–guest interaction and noncovalent π–π stacking. The SWNTs hybrids not only are well dispersed in pure water, but they also exhibit switchable dispersion/aggregation states upon the alternate irradiation with UV and visible light in pure water.
  PubDate: 2013-04-30T04:20:26.715732-05:
  DOI: 10.1002/adfm.201300434
Beneficial Contribution of Alloy Disorder to Electron and Phonon Transport in Half‐Heusler Thermoelectric Materials
- Authors: Hanhui Xie; Heng Wang, Yanzhong Pei, Chenguang Fu, Xiaohua Liu, G. Jeffrey Snyder, Xinbing Zhao, Tiejun Zhu
  Pages: n/a - n/a
  Abstract: Electron and phonon transport characteristics determines the potential of thermoelectric materials for power generation or refrigeration. This work shows that, different from most of high performance thermoelectric materials with dominant acoustic phonon scattering, the promising ZrNiSn based half‐Heusler thermoelectric solid solutions exhibit an alloy scattering dominated charge transport. A low deformation potential and a low alloy scattering potential are found for the solid solutions, which is beneficial to maintain a relatively high electron mobility despite of the large effective mass, and can be intrinsic favorable features contributing to the noticeably high power factors of ZrNiSn based alloys. A quantitive description of the different phonon scattering mechanisms suggests that the point defect scattering is the most important mechanism that determines the phonon transport process of the solid solutions. The present results indicate that alloying can be an effective approach for such materials systems to enhance thermoelectric figure of merit ZT by reducing phonon thermal conductivity, while minimizing the deterioration of charge mobility due to the low alloy scatteirng potential. Alloy scattering dominated charge transport is found in the ZrNiSn thermoelectric solid solutions. A low deformation potential and a low alloy scattering potential are derived by a quantitative modeling of electrical transport properties, which is beneficial for a relatively high mobility. These intrinsic favorable features can contribute to the high power factors of the half‐Heusler alloys.
  PubDate: 2013-04-30T04:20:20.677661-05:
  DOI: 10.1002/adfm.201300663
The Formation of Performance Enhancing Pseudo‐Composites in the Highly Active La1–xCaxFe0.8Ni0.2O3 System for IT‐SOFC Application
- Authors: Nagore Ortiz‐Vitoriano; Idoia Ruiz de Larramendi, Stuart N. Cook, Mónica Burriel, Ainara Aguadero, John A. Kilner, Teófilo Rojo
  Pages: n/a - n/a
  Abstract: The La1–xCaxFe0.8Ni0.2O3–δ (0 ≤ x ≤ 0.9) system is investigated for potential application as a cathode material for intermediate temperature solid oxide fuel cells (IT‐SOFCs). A broad range of experimental techniques have been utilized in order to elucidate the characteristics of the entire compositional range. Low A‐site Ca content compositions (x ≤ 0.4) feature a single perovskite solid solution. Compositions with 40% Ca content (x = 0.4) exhibit the highest electrical and ionic conductivities of these single phase materials (250 and 1.9 × 10−3 S cm−1 at 800 °C, respectively), a level competitive with state‐of‐the‐art (La,Sr)(Fe,Co)O3. Between 40 and 50% Ca content (0.4 > x > 0.5) a solubility limit is reached and a secondary, brownmillerite‐type phase appears for all higher Ca content compositions (0.5 ≤ x ≤ 0.9). While typically seen as detrimental to electrochemical performance in cathode materials, this phase brings with it ionic conductivity at operational temperatures. This gives rise to the effective formation of pseudo‐composite materials which feature significantly enhanced performance characteristics, while also providing the closest match in thermal expansion behavior to typical electrolyte materials. This all comes with the advantage of being produced through a simple, single‐step, low‐cost production route without the issues associated with typical composite materials. The highest performing pseudo‐composite material (x = 0.5) exhibits electronic conductivity of 300–350 S cm−1 in the 600–800 °C temperature range while the best polarisation resistance (Rp) values of approximately 0.2 Ω cm2 are found in the 0.5 ≤ x ≤ 0.7 range. A wide compositional range of materials are proposed and applied as cathodes featuring highly competitive performance. Contrary to expectations, the formation of perovskite‐brownmillerite pseudo‐composites with high Ca substitution results in significant improvements to electrochemical performance and excellent thermomechanical compatibility with electrolytes. Furthermore, new insights are gained into the material surface properties controlling oxygen reduction processes.
  PubDate: 2013-04-30T04:20:15.739615-05:
  DOI: 10.1002/adfm.201300481
Controlled Growth of Porous α‐Fe2O3 Branches on β‐MnO2 Nanorods for Excellent Performance in Lithium‐Ion Batteries
- Authors: Xin Gu; Liang Chen, Zhicheng Ju, Huayun Xu, Jian Yang, Yitai Qian
  Pages: n/a - n/a
  Abstract: Hierarchical nanocomposites rationally designed in component and structure, are highly desirable for the development of lithium‐ion batteries, because they can take full advantages of different components and various structures to achieve superior electrochemical properties. Here, the branched nanocomposite with β‐MnO2 nanorods as the back‐bone and porous α‐Fe2O3 nanorods as the branches are synthesized by a high‐temperature annealing of FeOOH epitaxially grown on the β‐MnO2 nanorods. Since the β‐MnO2 nanorods grow along the four‐fold axis, the as‐produced branches of FeOOH and α‐Fe2O3 are aligned on their side in a nearly four‐fold symmetry. This synthetic process for the branched nanorods built by β‐MnO2/α‐Fe2O3 is characterized. The branched nanorods of β‐MnO2/α‐Fe2O3 present an excellent lithium‐storage performance. They exhibit a reversible specific capacity of 1028 mAh g−1 at a current density of 1000 mA g−1 up to 200 cycles, much higher than the building blocks alone. Even at 4000 mA g−1, the reversible capacity of the branched nanorods could be kept at 881 mAh g−1. The outstanding performances of the branched nanorods are attributed to the synergistic effect of different components and the hierarchical structure of the composite. The disclosure of the correlation between the electrochemical properties and the structure/component of the nanocomposites, would greatly benefit the rational design of the high‐performance nanocomposites for lithium ion batteries, in the future. Branched nanorods with α‐Fe2O3 as the branches and β‐MnO2 as the stems are synthesized by the high‐temperature annealing of FeOOH epitaxially grown on the β‐MnO2 nanorods. These branched nanorods present an excellent lithium‐storage performance in terms of reversible capacity, cycling stability, and rate capability.
  PubDate: 2013-04-30T04:10:26.677154-05:
  DOI: 10.1002/adfm.201203779
Bio‐Inspired Superoleophobic Fluorinated Wax Crystalline Surfaces
- Authors: Sasha Pechook; Noga Kornblum, Boaz Pokroy
  Pages: n/a - n/a
  Abstract: A novel, single‐step and single‐component bio‐inspired fabrication method of hierarchical superoleophobic surfaces is presented. The method consists in thermal deposition of self‐assembling ultra‐low‐surface‐energy fluorinated wax on diverse surfaces. The thermal deposition results in crystalline, oriented, three‐dimensional hierarchical structures with high surface roughness and re‐entrant curvature, which in combination with the low surface energy of the fluorinated wax results in high contact angles of low‐surface‐tension liquids and low contact‐angle hysteresis (CAH) values (Δθ). The values achieved for Δθ are below 10° even for ethanol, which exhibits a surface tension as low as 22.4 mN·m−1. In addition to their superoleophobic properties our substrates exhibit extreme superhydrophobic qualities (CAH as low as 2° and contact angle >170°) with exceptional surface stability over many months. The proposed fabrication method may be utilized for a variety of applications where non‐wetting of low‐surface‐tension liquids is required, for example non‐staining surfaces and antifouling. The ease of fabrication and the variety of substrates that can be modified will undoubtedly widen its use. Single step production of bio‐inspired hierarchical fluorinated wax crystalline surfaces, which exhibit superoleophobic characteristics, is reported. The marked surface roughness and re‐entrant curvature, in combination with the low surface energy of the fluorinated wax, results in high contact angles of low‐surface‐tension liquids and low contact‐angle hysteresis values.
  PubDate: 2013-04-30T04:10:21.839616-05:
  DOI: 10.1002/adfm.201203878
Ductile Biodegradable Mg‐Based Metallic Glasses with Excellent Biocompatibility
- Authors: Hai‐Jun Yu; Jun‐Qiang Wang, Xue‐Tao Shi, Dmitri V. Louzguine‐Luzgin, Hong‐Kai Wu, John H. Perepezko
  Pages: n/a - n/a
  Abstract: Magnesium‐based metallic glasses (MMGs) show intriguing potentials for application as implantable biomaterials owing to their disordered atomic structure, good biodegradability, low elastic modulus, high strength, and large elasticity. However, despite of all these advantages, their brittleness is their Achilles’ heel, which severely limits their application as biomedical materials. In the current study, a significantly improved ductility of MMGs under bending and tensile loading through minor alloying with rare‐earth element ytterbium (Yb) at an atomic concentration of 2 and 4% is reported. The enhanced ductility is attributed to the increased density of shear bands close to fracture end and larger plastic zones on the fracture surface. In comparison with that of Yb‐free control, in vitro cell culture study confirms an improved biocompatibility of MMGs alloyed with Yb as determined by MTT, live‐dead, and cytoskeleton staining assays, respectively. Mg‐based metallic glasses have good ductility and excellent cell compatibility when properly alloyed by Yb. The good bending and tensile ductility is likely due to the formation of dense shear bands and large plastic zones. The improved cell compatibility can be attributed to the good corrosion resistance and the reduced release of cations and hydrogen.
  PubDate: 2013-04-26T07:10:24.949145-05:
  DOI: 10.1002/adfm.201203738
Highly Elastic Micropatterned Hydrogel for Engineering Functional Cardiac Tissue
- Authors: Nasim Annabi; Kelly Tsang, Suzanne M. Mithieux, Mehdi Nikkhah, Afshin Ameri, Ali Khademhosseini, Anthony S. Weiss
  Pages: n/a - n/a
  Abstract: Heart failure is a major international health issue. Myocardial mass loss and lack of contractility are precursors to heart failure. Surgical demand for effective myocardial repair is tempered by a paucity of appropriate biological materials. These materials should conveniently replicate natural human tissue components, convey persistent elasticity, promote cell attachment, growth and conformability to direct cell orientation and functional performance. Here, microfabrication techniques are applied to recombinant human tropoelastin, the resilience‐imparting protein found in all elastic human tissues, to generate photocrosslinked biological materials containing well‐defined micropatterns. These highly elastic substrates are then used to engineer biomimetic cardiac tissue constructs. The micropatterned hydrogels, produced through photocrosslinking of methacrylated tropoelastin (MeTro), promote the attachment, spreading, alignment, function, and intercellular communication of cardiomyocytes by providing an elastic mechanical support that mimics their dynamic mechanical properties in vivo. The fabricated MeTro hydrogels also support the synchronous beating of cardiomyocytes in response to electrical field stimulation. These novel engineered micropatterned elastic gels are designed to be amenable to 3D modular assembly and establish a versatile, adaptable foundation for the modeling and regeneration of functional cardiac tissue with potential for application to other elastic tissues. Highly elastic hydrogels containing well‐defined micropatterns are engineered from recombinant human tropoelastin, the resilience‐imparting protein found in all elastic human tissues. These elastic substrates are then used to engineer biomimetic cardiac tissue constructs. The micropatterned hydrogels support the alignment, intercellular communication, and synchronous beating of cardiomyocytes by providing an elastic mechanical support that mimics their dynamic mechanical properties in vivo (scale bar: 50 μm).
  PubDate: 2013-04-26T07:10:18.565668-05:
  DOI: 10.1002/adfm.201300570
Assessing Antisite Defect and Impurity Concentrations in Bi2Te3 Based Thin Films by High‐Accuracy Chemical Analysis
- Authors: Nicola Peranio; Markus Winkler, Michael Dürrschnabel, Jan König, Oliver Eibl
  Pages: n/a - n/a
  Abstract: In Bi2Te3‐based materials charge‐carrier densities are determined by antisite defects and controlling these defects is a key issue for thermoelectric and topological insulator materials. Bi‐Te thin films with high‐quality thermoelectric properties are deposited using a nano‐alloying approach by molecular beam epitaxy (MBE) and sputtering. The in‐plane transport properties are measured at room temperature as a function of charge‐carrier density. High‐accuracy chemical analysis by wavelength‐dispersive X‐ray spectrometry (WDX) is applied for the first time to these Bi2Te3‐based thin films. The acquisition conditions for WDX spectrometry are established using Monte Carlo simulations for the electron trajectories, which guarantees a high lateral resolution and rules out stray radiation generated in the substrate of the films. In contrast to energy‐dispersive X‐ray spectrometry (EDX), which is usually applied, WDX offers unprecedented accuracy for measuring antisite defect concentrations and thus has a high impact on improving the quality of thin films. The charge‐carrier densities are calculated from the WDX results according to the point‐defect model of Miller and Li and the thermopower and electrical conductivity are calculated for different charge‐carrier densities by solving the linearized Boltzmann transport equation. A good quantitative agreement is found for the dependence of the thermopower on stoichiometry, whereas the electrical conductivity is sensitively affected by contaminants. Wavelength‐dispersive X‐ray spectrometry (WDX) is applied to Bi2Te3 thermoelectric thin films for accurate chemical analysis. The antisite densities can thus be measured and this can be used to control the charge‐carrier densities. The transport properties are compared with solutions of the linearized Boltzmann transport equation. For sputtered films, the argon concentration, which is relevant for phonon scattering, is measured.
  PubDate: 2013-04-26T07:10:13.183679-05:
  DOI: 10.1002/adfm.201300606
A Versatile Light‐Switchable Nanorod Memory: Wurtzite ZnO on Perovskite SrTiO3
- Authors: Ashok Bera; Haiyang Peng, James Lourembam, Youde Shen, Xiao Wei Sun, T. Wu
  Pages: n/a - n/a
  Abstract: Integrating materials with distinct lattice symmetries and dimensions is an effective design strategy toward realizing novel devices with unprecedented functionalities, but many challenges remain in synthesis and device design. Here, a heterojunction memory made of wurtzite ZnO nanorods grown on perovskite Nb‐doped SrTiO3 (NSTO) is reported, the electronic properties of which can be drastically reconfigured by applying a voltage and light. Despite of the distinct lattice structures of ZnO and NSTO, a consistent nature of single crystallinity is achieved in the heterojunctions via the low‐temperature solution‐based hydrothermal growth. In addition to a high and persistent photoconductivity, the ZnO/NSTO heterojunction diode can be turned into a versatile light‐switchable resistive switching memory with highly tunable ON and OFF states. The reversible modification of the effective interfacial energy barrier in the concurrent electronic and ionic processes most likely gives rise to the high susceptibility of the ZnO/NSTO heterojunction to external electric and optical stimuli. Furthermore, this facile synthesis route is promising to be generalized to other novel functional nanodevices integrating materials with diverse structures and properties. An optically reconfigurable resistive switching diode is realized by growing ZnO nanorods on Nb‐doped SrTiO3 single crystals in solution at low temperatures. The ZnO nanorods/Nb:SrTiO3 heterojunction forms a high‐quality Schottky diode that shows persistent photoconductivity and light‐controlled resistive switching behaviors with highly tunable ON and OFF memory states.
  PubDate: 2013-04-25T01:20:18.867609-05:
  DOI: 10.1002/adfm.201300509
Coupling of Micromagnetic and Structural Properties Across the Martensite and Curie Temperatures in Miniaturized Ni‐Mn‐Ga Ferromagnetic Shape Memory Alloys
- Authors: A. M. Jakob; M. Hennes, M. Müller, D. Spemann, S. G. Mayr
  Pages: n/a - n/a
  Abstract: Micromagnetic structure evolution in Ni‐Mn‐Ga ferromagnetic shape memory thin films is investigated by means of temperature dependent magnetic force microscopy (TD‐MFM). The center of interest is the magnetic properties of epitaxial Ni‐Mn‐Ga thin films on MgO substrates across thermally induced phase transitions. Experimental results are discussed within the framework of competing magnetic interactions arising in stressed thin ferromagnetic films. Measurements on 14M martensite specimens are supplemented by three‐dimensional micromagnetic simulations. Corresponding calculated MFM micrographs are compared to experimental data. The influence of twin variant dimension and orientation on micromagnetic domain formation and wall structure is depicted from a theoretical point of view. A micromagnetic model system of partial flux closure is proposed and calculated analytically to estimate a stress induced magneto crystalline anisotropy constant in austenite Ni‐Mn‐Ga. Coupling of structural and micromagnetic properties of ferromagnetic Ni‐Mn‐Ga shape memory thin films is investigated by means of three dimensional numerical simulations and temperature dependent magnetic force microscopy measurements across the martensite and Curie temperatures. A stress‐induced magneto‐crystalline anisotropy is detected within the austenite phase, which is quantified based on a model of partial flux closure.
  PubDate: 2013-04-25T01:20:12.683317-05:
  DOI: 10.1002/adfm.201300165
Co‐Percolating Graphene‐Wrapped Silver Nanowire Network for High Performance, Highly Stable, Transparent Conducting Electrodes
- Authors: Ruiyi Chen; Suprem R. Das, Changwook Jeong, Mohammad Ryyan Khan, David B. Janes, Muhammad A. Alam
  Pages: n/a - n/a
  Abstract: Transparent conducting electrodes (TCEs) require high transparency and low sheet resistance for applications in photovoltaics, photodetectors, flat panel displays, touch screen devices and imagers. Indium tin oxide (ITO), or other transparent conductive oxides, have typically been used, and provide a baseline sheet resistance (RS) vs. transparency (T) relationship. However, ITO is relatively expensive (due to limited abundance of Indium), brittle, unstable, and inflexible; moreover, ITO transparency drops rapidly for wavelengths above 1000 nm. Motivated by a need for transparent conductors with comparable (or better) RS at a given T, as well as flexible structures, several alternative material systems have been investigated. Single‐layer graphene (SLG) or few‐layer graphene provide sufficiently high transparency (≈97% per layer) to be a potential replacement for ITO. However, large‐area synthesis approaches, including chemical vapor deposition (CVD), typically yield films with relatively high sheet resistance due to small grain sizes and high‐resistance grain boundaries (HGBs). In this paper, we report a hybrid structure employing a CVD SLG film and a network of silver nanowires (AgNWs): RS as low as 22 Ω/□ (stabilized to 13 Ω/□ after 4 months) have been observed at high transparency (88% at λ = 550 nm) in hybrid structures employing relatively low‐cost commercial graphene with a starting RS of 770 Ω/□. This sheet resistance is superior to typical reported values for ITO, comparable to the best reported TCEs employing graphene and/or random nanowire networks, and the film properties exhibit impressive stability under mechanical pressure, mechanical bending and over time. The design is inspired by the theory of a co‐percolating network where conduction bottlenecks of a 2D film (e.g., SLG, MoS2) are circumvented by a 1D network (e.g., AgNWs, CNTs) and vice versa. The development of these high‐performance hybrid structures provides a route towards robust, scalable and low‐cost approaches for realizing high‐performance TCE. Single layer graphene‐silver nanowire network co‐percolating 2D‐1D hybrid transparent conducting electrodes (TCE) with very low sheet resistance‐optical transparency values (22 Ω/□ (stabilized to 13 Ω/□ after 4 months) at 88% transmittance at λ = 550 nm) are achieved. The development of such a high performance (chemically/mechanically/optically stable) material is inspired by a theoretical prediction of co‐percolating transport in graphene‐wrapped silver nanowire network.
  PubDate: 2013-04-25T01:20:11.578409-05:
  DOI: 10.1002/adfm.201300124
Formation of Functionalized Nanowires by Control of Self‐Assembly Using Multiple Modified Amyloid Peptides
- Authors: Hiroki Sakai; Ken Watanabe, Yuya Asanomi, Yumiko Kobayashi, Yoshiro Chuman, Lihong Shi, Takuya Masuda, Thomas Wyttenbach, Michael T. Bowers, Kohei Uosaki, Kazuyasu Sakaguchi
  Pages: n/a - n/a
  Abstract: Amyloid peptides have great potential as building blocks in the creation of functional nanowires due to their natural ability to self‐assemble into nanofibrillar structures and because they can be easily modified with various functional groups. However, significant modifications of an amyloid peptide generally alter its self‐assembly property, making it difficult to construct functionalized fibrils with a desired structure and function. In this study, a very effective method to overcome this problem is demonstrated by using our structure‐controllable amyloid peptides (SCAPs) terminated with a three‐amino‐acid‐residue cap. The method consists on mixing two or more structurally related amyloid peptides with a fraction of modified SCAPs which co‐assemble into a fibril. This SCAP‐mixing method provides remarkable control over the self‐assembly process both on the small oligomers level and the macroscopic fibrils level. Furthermore, it is shown that the modified peptides imbedded in the resulting fibril can subsequently be functionalized to generate nanowires with the desired properties, highlighting the importance of our SCAP method for nanotechnology applications. Straightforward preparation of various functionalized or inorganic nanowires can be achieved by a very simple method based on mixing structurally related modified amyloid peptides, which allows effective control of self‐assembly. The peptides contain three‐amino‐acid‐residue units that provide remarkable control during the entire self‐assembly process, starting from a small oligomer up to the macroscopic fibril level.
  PubDate: 2013-04-23T04:10:12.84374-05:0
  DOI: 10.1002/adfm.201300577
Bright Blue Solution Processed Triple‐Layer Polymer Light‐Emitting Diodes Realized by Thermal Layer Stabilization and Orthogonal Solvents
- Authors: Roman Trattnig; Leonid Pevzner, Monika Jäger, Raphael Schlesinger, Marco Vittorio Nardi, Giovanni Ligorio, Christos Christodoulou, Norbert Koch, Martin Baumgarten, Klaus Müllen, Emil J. W. List
  Pages: n/a - n/a
  Abstract: The realization of fully solution processed multilayer polymer light‐emitting diodes (PLEDs) constitutes the pivotal point to push PLED technology to its full potential. Herein, a fully solution processed triple‐layer PLED realized by combining two different deposition strategies is presented. The approach allows a successive deposition of more than two polymeric layers without extensively redissolving already present layers. For that purpose a poly(9,9‐dioctyl‐fluorene‐co‐N‐(4‐butylphenyl)‐diphenylamine) (TFB) layer is stabilized by a hard‐bake process as hole transport layer on top of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). As emitting layer, a deep blue emitting pyrene‐triphenylamine copolymer is deposited from toluene solution. To complete the device assembly 9,9‐bis(3‐(5′,6′‐bis(4‐(polyethylene glycol)phenyl)‐[1,1′:4′,1″‐terphenyl]‐2′‐yl)propyl)‐9′,9′‐dioctyl‐2,7‐polyfluorene (PEGPF), a novel polyfluorene‐type polymer with polar sidechains, which acts as the electron transport layer, is deposited from methanol in an orthogonal solvent approach. Atomic force microscopy verifies that all deposited layers stay perfectly intact with respect to morphology and layer thickness upon multiple solvent treatments. Photoelectron spectroscopy reveals that the offsets of the respective frontier energy levels at the individual polymer interfaces lead to a charge carrier confinement in the emitting layer, thus enhancing the exciton formation probability in the device stack. The solution processed PLED‐stack exhibits bright blue light emission with a maximum luminance of 16 540 cd m−2 and a maximum device efficiency of 1.42 cd A−1, which denotes a five‐fold increase compared to corresponding single‐layer devices and demonstrates the potential of the presented concept. A conceptual study on triple‐layer PLEDs fabricated by successive solution based deposition of multiple polymer layers without extensively redissolving of existing layers is shown. Ultraviolet photoemission spectroscopy (UPS) measurements of the triple‐layer assembly reveal a favorable energy level alignment at the respective material interfaces resulting in a charge carrier confinement in the emitting layer, thus enhancing maximum luminance and luminous efficiency values.
  PubDate: 2013-04-22T02:10:14.163621-05:
  DOI: 10.1002/adfm.201300360
Self‐Healing Materials via Reversible Crosslinking of Poly(ethylene oxide)‐Block‐Poly(furfuryl glycidyl ether) (PEO‐b‐PFGE) Block Copolymer Films
- Authors: Markus J. Barthel; Tobias Rudolph, Anke Teichler, Renzo M. Paulus, Jürgen Vitz, Stephanie Hoeppener, Martin D. Hager, Felix H. Schacher, Ulrich S. Schubert
  Pages: n/a - n/a
  Abstract: The application of well‐defined poly(furfuryl glycidyl ether) (PFGE) homopolymers and poly(ethylene oxide)‐b‐poly(furfuryl glycidyl ether) (PEO‐b‐PFGE) block copolymers synthesized by living anionic polymerization as self‐healing materials is demonstrated. This is achieved by thermo‐reversible network formation via (retro) Diels‐Alder chemistry between the furan groups in the side‐chain of the PFGE segments and a bifunctional maleimide crosslinker within drop‐cast polymer films. The process is studied in detail by differential scanning calorimetry (DSC), depth‐sensing indentation, and profilometry. It is shown that such materials are capable of healing complex scratch patterns, also multiple times. Furthermore, microphase separation within PEO‐b‐PFGE block copolymer films is indicated by small angle X‐ray scattering (lamellar morphology with a domain spacing of approximately 19 nm), differential scanning calorimetry, and contact angle measurements. Films of poly(furfuryl glycidyl ether) (PFGE) and poly(ethylene oxide)‐b‐poly(furfuryl glycidyl ether) PEO‐b‐PFGE block copolymers are prepared and reversibly crosslinked by Diels‐Alder chemistry. The self‐healing of damaged surfaces is studied in detail with help of differential scanning calorimetry, depth‐sensing indentation, small angle X‐ray scattering and profilometry.
  PubDate: 2013-04-19T06:10:29.981527-05:
  DOI: 10.1002/adfm.201300469
Investigating the Role of Emissive Layer Architecture on the Exciton Recombination Zone in Organic Light‐Emitting Devices
- Authors: Nicholas C. Erickson; Russell J. Holmes
  Pages: n/a - n/a
  Abstract: An experimental approach to determine the spatial extent and location of the exciton recombination zone in an organic light‐emitting device (OLED) is demonstrated. This technique is applicable to a wide variety of OLED structures and is used to examine OLEDs which have a double‐ (D‐EML), mixed‐ (M‐EML), or graded‐emissive layer (G‐EML) architecture. The location of exciton recombination in an OLED is an important design parameter, as the local optical field sensed by the exciton greatly determines the efficiency and angular distribution of far‐field light extraction. The spatial extent of exciton recombination is an important parameter that can strongly impact exciton quenching and OLED efficiency, particularly under high excitation. A direct measurement of the exciton density profile is achieved through the inclusion of a thin, exciton sensitizing strip in the OLED emissive layer which locally quenches guest excitons and whose position in the emissive layer can be translated across the device to probe exciton formation. In the case of the G‐EML device architecture, an electronic model is developed to predict the location and extent of the exciton density profile by considering the drift, diffusion, and recombination of charge carriers within the device. The impact of emissive layer architecture on the exciton recombination zone in organic light‐emitting devices is investigated. A technique to measure the location and spatial extent of exciton recombination in organic light‐emitting devices is demonstrated. The technique relies on the Förster energy transfer of an exciton from a luminescent guest to a dilute sensitizing molecule included in narrow strips within the emissive layer. It is found that the recombination zone depends strongly on emissive layer composition and architecture.
  PubDate: 2013-04-19T06:10:23.935673-05:
  DOI: 10.1002/adfm.201300101
Extracellular Matrix Control of Collagen Mineralization In Vitro
- Authors: Alexander J. Lausch; Bryan D. Quan, Jason W. Miklas, Eli D. Sone
  Pages: n/a - n/a
  Abstract: Collagen biomineralization is a complex process and the controlling factors, at the molecular level, are still not well understood. A particularly high level of spatial control over collagen mineralization is evident in the anchorage of teeth to the jawbone by the periodontal ligament. Here, unmineralized ligament collagen fibrils become mineralized at an extremely sharp mineralization front in the root of the tooth. A model of collagen biomineralization based on demineralized cryosections of mouse molars in the bone socket is presented. When exposed to metastable calcium and phosphate‐containing solutions, mineral re‐deposits selectively into the natively mineralized tissues with high fidelity, demonstrating that the extracellular matrix retains sufficient information to control the rate of mineralization at the tissue level. While solutions of simulated bodily fluid produce amorphous calcium phosphate within the tissue section, a more highly supersaturated solution stabilized with polyaspartic acid produce oriented, crystalline calcium phosphate with diffraction patterns consistent with hydroxyapatite. The model thus replicates both spatial control of mineral deposition, as well as the matrix‐mineral relationships of natively mineralized collagen fibrils, and can be used to elucidate roles of specific biomolecules in the highly controlled process of collagen biomineralization. This knowledge will be critical in the design of collagen‐based scaffolds for tissue engineering of hard‐soft tissue interfaces. A novel model of collagen biomineralization is presented, based on deposition of mineral from metastable calcium and phosphate‐containing solutions into demineralized sections of mouse periodontal tissues. Mineral deposits selectively into natively mineralized tissues of the tooth root with high fidelity, mimicking the pattern of mineralization in vivo, and demonstrating that the extracellular matrix of these tissues retains sufficient information to control collagen mineralization.
  PubDate: 2013-04-19T06:10:15.531215-05:
  DOI: 10.1002/adfm.201203760
Interactive Growth Effects of Rare‐Earth Nanoparticles on Nanorod Formation in YBa2Cu3Ox Thin Films
- Authors: F. Javier Baca; Timothy J. Haugan, Paul N. Barnes, Terry G. Holesinger, Boris Maiorov, Rongtao Lu, Xiang Wang, Joshua N. Reichart, Judy Z. Wu
  Pages: n/a - n/a
  Abstract: The controlled growth of self‐assembled second‐phase nanostructures has been shown to be an essential tool for enhancing properties of several composite oxide thin film systems. Here, the role of Y2O3 nanoparticles on the growth of BaZrO3 (BZO) nanorods is investigated in order to understand the mechanisms governing their self‐assembly in YBa2Cu3O7–x (YBCO) thin films and to more fully control the resulting defect landscape. By examining the microstructure and current‐carrying capacity of BZO‐doped YBCO films, it is shown that the nanorod growth dynamics are significantly enhanced when compared to films double‐doped with BZO and Y2O3 nanoparticles. The average nanorod length and associated critical current densities are found to increase at a significantly higher rate in the absence of Y2O3 nanoparticles when the growth temperature is increased. Using microstructural data from transmission electron microscopy studies and the response in critical current density, the interactive effects of multiple dopants that must be considered to fully control the defect landscape in oxide thin films are shown. Average BaZrO3 nanorod lengths are shown to increase at a significantly higher rate in YBa2Cu3O7–x, compared to composite films double‐doped with Y2O3 nanoparticles and BaZrO3, when the growth temperature is increased. From this, the significance of interactive effects between dopants is demonstrated; this can provide additional control over nanostructured composite films.
  PubDate: 2013-04-19T02:23:26.058036-05:
  DOI: 10.1002/adfm.201203660
Exploring the Energy Storage Mechanism of High Performance MnO2 Electrochemical Capacitor Electrodes: An In Situ Atomic Force Microscopy Study in Aqueous Electrolyte
- Authors: Xinyong Tao; Jun Du, Yong Sun, Shulan Zhou, Yang Xia, Hui Huang, Yongping Gan, Wenkui Zhang, Xiaodong Li
  Pages: n/a - n/a
  Abstract: The basic microstructure‐dependent charge storage mechanisms of nanostructured MnO2 are investigated via dynamic observation of the growth and in situ probing the mechanical properties by using in situ AFM in conjunction with in situ nanoindentation. The progressive nucleation followed by three‐dimensional growth yields pulsed current deposited porous nanostructured γ‐MnO2, which exhibits a high specific capacitance of 437 F/g and a remarkable cycling performance with >96% capacitance retention after 10 000 cycles. The proton intercalation induced expansion of MnO2 can be self‐accommodated by the localized compression and reduction of the porosity. More coincidentally, the proton intercalation induced softening is favorable for the elastic deformation of MnO2. This self‐adaptive capability of nanostructured MnO2 could generate high structural reliability during cycling. These discoveries offer important mechanistic insights for the design of advanced electrochemical capacitors. The basic microstructure‐dependent energy storage mechanisms of nanostructured MnO2 are investigated via dynamic observation of the growth and in situ probing the mechanical properties by using in situ AFM. A series of dramatic evolutions of nanostructured MnO2 involving progressive nucleation, three‐dimensional growth, reversible expansion, proton intercalation induced softening, and self‐accommodation phenomena can be correlated to its remarkable energy storage performance.
  PubDate: 2013-04-19T02:23:21.812965-05:
  DOI: 10.1002/adfm.201300359
Three‐Dimensionally Ordered Macroporous Polymeric Materials by Colloidal Crystal Templating for Reversible CO2 Capture
- Authors: Hongkun He; Mingjiang Zhong, Dominik Konkolewicz, Karin Yacatto, Timothy Rappold, Glenn Sugar, Nathaniel E. David, Jeff Gelb, Naomi Kotwal, Arno Merkle, Krzysztof Matyjaszewski
  Pages: n/a - n/a
  Abstract: The design and preparation of porous materials with controlled structures and functionalities is crucial to a variety of absorption‐ or separation‐relevant applications, including CO2 capture. Here, novel functional polymeric materials with three‐dimensionally ordered macroporous (3DOM) structures are prepared by using colloidal crystals as templates using relatively simple, rapid, and inexpensive approaches. These ordered structures are used for the reversible CO2 capture from ambient air by humidity swing. Typically, the colloidal crystal template is synthesized from polymer latex particles of poly(methyl methacrylate) (PMMA) or polystyrene (PS). To maintain the functionality of the material, it is important to prevent the porous structure collapsing, which can occur by the hydrolysis of the ester bonds in conventional crosslinkers under basic conditions. This hydrolysis can be prevented by using a water‐soluble crosslinker containing two quaternary ammonium moieties, which can be used to prepare stable porous crosslinked polymers with the monomer (vinylbenzyl)trimethylammonium chloride (VBTMACl) and using a PMMA‐based colloidal crystal template. The hydroxide‐containing monomer and dicationic crosslinker are synthesized from their chloride precursors, avoiding the ion‐exchange step which causes shrinkage of the pores. An analysis of different methods for infiltrating the monomer solution into the colloidal crystal template shows that infiltration using capillary forces leads to fewer defects than infiltration under a partial vacuum. In addition, functional macroporous films with micrometer thickness are prepared from a template of PS‐based colloidal crystals in a thin film. In general, the colloidal crystal templated materials showed improved CO2 absorption/desorption rates and swing sizes compared to a commercially available material with similar functional groups. This work could easily be extended to create a new generation of ordered macroporous polymeric materials with tunable functionalities for other applications. Novel functionalized three‐dimensionally ordered macroporous (3DOM) polymeric materials with well‐interconnected and chemically stable structures are created through templating with latex colloidal crystals. These materials display improved CO2 air capture performance compared to a commercially available resin. A versatile strategy is developed to prepare functional porous polymers by colloidal crystal templating, which opens up the use of 3DOM materials for CO2 capture.
  PubDate: 2013-04-19T02:23:15.094104-05:
  DOI: 10.1002/adfm.201300401
Nondestructive Characterization of Graphene Defects
- Authors: Thuc Hue Ly; Dinh Loc Duong, Quang Huy Ta, Fei Yao, Quoc An Vu, Hye Yun Jeong, Sang Hoon Chae, Young Hee Lee
  Pages: n/a - n/a
  Abstract: An effective method is reported for oxidizing graphene/copper film in which air oxidation of the underlying copper film occurs through the grain boundary lines of graphene without oxidizing graphene. This oxidation is realized by partially immersing the graphene/copper film in sodium chloride solution. Electrons generated during etching of the graphene/copper film in electrolyte diffuse into the film in contact with air, which eventually enhances air oxidation of copper through the graphene layer. While the graphene layer acts as a protective layer against oxidation of the copper film, oxidation of the underlying Cu film near graphene grain boundary lines is observed by optical microscopy. This observation could be attributed to the selective diffusion of oxygen radicals through isolated defects and graphene grain boundaries. The process involves no appreciable oxidation of the graphene layer including the graphene grain boundary, as confirmed by use of detailed Raman and X‐ray photoelectron spectroscopy. An effective method for nondestructive characterization of graphene defects is shown by enhancing oxidation of a Cu film underneath graphene through the graphene grain boundaries in air by electron injection supplied from electrochemical reaction of the graphene/Cu film. This process involves no appreciable oxidation of the graphene layer or the graphene grain boundary, as confirmed by detailed Raman and X‐ray photoelectron spectroscopy.
  PubDate: 2013-04-19T02:23:09.346368-05:
  DOI: 10.1002/adfm.201300493
Silk Hydrogels as Soft Substrates for Neural Tissue Engineering
- Authors: Amy M. Hopkins; Laura De Laporte, Federico Tortelli, Elise Spedden, Cristian Staii, Timothy J. Atherton, Jeffrey A. Hubbell, David L. Kaplan
  Pages: n/a - n/a
  Abstract: There is great need for soft biomaterials that match the stiffness of human tissues for tissue engineering and regeneration. Hydrogels are frequently employed for extracellular matrix functionalization and to provide appropriate mechanical cues. It is challenging, however, to achieve structural integrity and retain bioactive molecules in hydrogels for complex tissue formation that may take months to develop. This work aims to investigate mechanical and biochemical characteristics of silk hydrogels for soft tissue engineering, specifically for the nervous system. The stiffness of 1 to 8% silk hydrogels, measured by atomic force microscopy, is 4 to 33 kPa. The structural integrity of silk gels is maintained throughout embryonic chick dorsal root ganglion (cDRG) explant culture over 4 days whereas fibrin and collagen gels decrease in mass over time. Neurite extension of cDRGs cultured on 2 and 4% silk hydrogels exhibit greater growth than softer or stiffer gels. Silk hydrogels release
  PubDate: 2013-04-18T07:10:42.321487-05:
  DOI: 10.1002/adfm.201300435
Sensitization of Er3+ Infrared Photoluminescence Embedded in a Hybrid Organic‐Inorganic Copolymer containing Octahedral Molybdenum Clusters
- Authors: Yann Molard; Christophe Labbé, Julien Cardin, Stéphane Cordier
  Pages: n/a - n/a
  Abstract: Luminescent hybrid copolymers are obtained by copolymerizing in bulk methylmethacrylate with a methacrylic acid (MAC) solution containing [n‐Bu4N]2[Mo6Br8(MAC)6], and aliquots of an Er(TMHD)3 complex (TMHD for 2,2,6,6‐tetramethyl‐3,5‐heptanedione) solution. This leads to novel homogeneous and transparent hybrid materials in which the Er3+ infrared luminescence at 1.55 μm, a standard wavelength for telecommunication applications, is up to six time more intense in the presence of Mo6 clusters when samples are irradiated at 476.5 nm. This work demonstrates the outstanding potential of Mo6 clusters, compounds obtained by high‐temperature solid‐state synthesis, in the design of functional hybrid materials via soft chemistry routes. Integrating Er3+ ions in a hybrid Mo6‐PMMA hybrid matrix enables the sensitization of its IR photoluminescence upon irradiation anywhere in the octahedral metallic cluster absorption band. Meanwhile it causes a noticeable decrease of the Mo6 red luminescence. Mo6 clusters being covalently linked to polymer strands, the hybrid material shows excellent aging behavior, leaving promising perspectives for telecom applications.
  PubDate: 2013-04-18T07:10:35.658723-05:
  DOI: 10.1002/adfm.201300417
Maximizing the Energy Density of Dielectric Elastomer Generators Using Equi‐Biaxial Loading
- Authors: Jiangshui Huang; Samuel Shian, Zhigang Suo, David R. Clarke
  Pages: n/a - n/a
  Abstract: Dielectric elastomer generators (DEGs) for harvesting electrical energy from mechanical work have been demonstrated but the energy densities achieved are still small compared with theoretical predictions. In this study, significant improvements in energy density (560 J/kg with a power density of 280 W/kg and an efficiency of 27%) are achieved using equi‐biaxial stretching, a mechanical loading configuration that maximizes the capacitance changes. The capacitance of dielectric elastomers subjected to equi‐biaxial stretches is demonstrated to be proportional to the fourth power of the stretch. Quantification of the individual energy contributions indicates that attaining higher conversion efficiencies is limited by viscous losses within the acrylic elastomer, suggesting that still higher conversion efficiencies with other elastomers should be attainable with our novel mechanical loading design. A thin sheet of acrylic elastomer, coated with black carbon conductive grease on both sides, is equi‐biaxially stretched by applying radial forces to its circumference. In its unstretched state, the elastomer thickness is 0.5 mm, and the electroded radius is 2.0 cm corresponding to a mass of dielectric elastomer of 0.60 g.
  PubDate: 2013-04-18T07:10:31.192137-05:
  DOI: 10.1002/adfm.201300402
Flexible Nonvolatile Transistor Memory Devices Based on One‐Dimensional Electrospun P3HT:Au Hybrid Nanofibers
- Authors: Hsuan‐Chun Chang; Cheng‐Liang Liu, Wen‐Chang Chen
  Pages: n/a - n/a
  Abstract: A novel flexible nonvolatile flash transistor memory devices on polyethylene naphthalate (PEN) substrate using 1D electrospun nanofiber of poly(3‐hexylthiophene) (P3HT):gold nanoparticles (Au NPs) hybrid as the channel is presented. The Au NPs are functionalized with self‐assembled monolayer (SAM) of para‐substituted amino (Au‐NH2), methyl (Au‐CH3) or trifluoromethyl (Au‐CF3) tail groups on the benzenethiol moiety. They are employed as localized charge traps across the nanofiber channel and program/erase the device towards low conductance (OFF)/high conductance (ON) states under the applied electrical field. With the low operation voltage of ±5 V, the hybrid nanofiber transistor memories exhibit a 3.5–10.6 V threshold voltage shifting and at least 104 s data retention, with a minimum effect on ≈100 programmed/erased stress endurances. The dipoles of the SAM probably modify the work function of the Au NPs associated with the P3HT nanofiber channel and manifest the degree of negative threshold voltage shifting in an order of Au‐NH2 > Au‐CH3 > Au‐CF3. The devices remain reliable and stable even under the bending conditions (radius: 5–30 mm) or 1000 repetitive bending cycles. The hybrid nanofiber can be used to obtain high‐performance digital nanoscale memories for flexible high density data storage devices. Novel Flexible Nonvolatile Transistor Memory Devices based on the electrospun nanofiber of poly(3‐hexylthiophene):surface‐modified gold nanoparticles show a low voltage operation (±5 V), large threshold voltage shift (3.5–10.6 V), long retention times (>104 s) and good endurance properties (>100 cycles) regardless of mechanical bending stress.
  PubDate: 2013-04-18T07:10:24.597079-05:
  DOI: 10.1002/adfm.201300283
Efficient Light‐Emitting Electrochemical Cells (LECs) Based on Ionic Iridium(III) Complexes with 1,3,4‐Oxadiazole Ligands
- Authors: Jing Zhang; Li Zhou, Hameed A. Al‐Attar, Kuizhan Shao, Li Wang, Dongxia Zhu, Zhongmin Su, Martin R. Bryce, Andrew P. Monkman
  Pages: n/a - n/a
  Abstract: Eight new iridium(III) complexes 1‐8, with 1,3,4‐oxadiazole (OXD) derivatives as the cyclometalated C^N ligand and/or the ancillary N^N ligands are synthesized and their electrochemical, photophysical, and solid‐state light‐emitting electrochemical cell (LEC) properties are investigated. Complexes 1, 2, 7 and 8 are additionally characterized by single crystal X‐ray diffraction. LECs based on complexes 1‐8 are fabricated with a structure indium tin oxide (ITO)/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/cationic iridium complex:ionic liquid/Al. LECs of complexes 1–6 with OXD derivatives as the cyclometalated ligands and as the ancillary ligand show yellow luminescence (λmax = 552–564 nm). LECs of complexes 7 and 8 with cyclometalated C^N phenylpyridine ligands and an ancillary N^N OXD ligand show red emission (λmax 616–624 nm). Using complex 7 external quantum efficiency (EQE) values of >10% are obtained for devices (210 nm emission layer) at 3.5 V. For thinner devices (70 nm) high brightness is achieved: red emission for 7 (8528 cd m−2 at 10 V) and yellow emission for 1 (3125 cd m−2 at 14 V). 2,5‐Diaryl‐1,3,4‐oxadiazole derivatives are shown to be versatile cyclometalating C^N and coordinating N^N ligands for ionic iridium complexes that are fabricated into light‐emitting electrochemical cells (LECs). High brightness red emission from a LEC at a driving voltage of 10 V is observed.
  PubDate: 2013-04-18T07:10:17.803186-05:
  DOI: 10.1002/adfm.201300344
Humidity‐Sensitive Polypyrrole Films for Electro‐Active Polymer Actuators
- Authors: Hidenori Okuzaki; Takayoshi Kuwabara, Keiichi Funasaka, Tomooki Saido
  Pages: n/a - n/a
  Abstract: Electrochemically synthesized polypyrrole (PPy) films undergo rapid and intensive bending in response to sorption of water vapor, which is applied to a new type of motor capable of transducing chemical free energy change of sorption directly into mechanical work. Furthermore, the PPy film contracts in air under an electric field, which is associated with desorption of water vapor due to Joule heating. This paper features an overview of our comprehensive study on the humidity‐sensitive PPy films and applications to electro‐active polymer actuators. Upon application of 3 V, the film generates contractile stress of 9.8 MPa which is 4 orders of magnitude larger than the gram‐force of its own weight (7.6 mgf) and is nearly 30 times that of skeletal muscle in animals (0.35 MPa). The work capacity increases with the applied voltage and reaches 48 kJ m−3 at 3 V, while the degree of contraction is about 1%. The ‘origami’ actuator fabricated by folding the PPy film exhibits a significant expansion as large as 147% at 2 V, which lies in the electrically induced changes in the elastic modulus of the humidity‐sensitive PPy film. Utilizing the origami technique, a biomorphic origami robot is fabricated, which can move with a caterpillar‐like motion by repeated expansion and contraction at a velocity of 2 cm min−1. Humidity‐sensitive polypyrrole films can provide an insight to the development of an origami robot fabricated by folding the polypyrrole film. The principle lies in the electrically induced changes in the elastic modulus caused by desorption of water vapor due to Joule heating, which is responsible for amplifying a contraction of the film to more than a 100‐fold expansion.
  PubDate: 2013-04-18T07:10:04.913632-05:
  DOI: 10.1002/adfm.201203883
Quantitative Recovery of Magnetic Nanoparticles from Flowing Blood: Trace Analysis and the Role of Magnetization
- Authors: Christoph M. Schumacher; Inge K. Herrmann, Stephanie B. Bubenhofer, Sabrina Gschwind, Ann‐Marie Hirt, Beatrice Beck‐Schimmer, Detlef Günther, Wendelin J. Stark
  Pages: n/a - n/a
  Abstract: Magnetic nanomaterials find increasing application as separation agents to rapidly isolate target compounds from complex biological media (i.e., blood purification). The responsiveness of the used materials to external magnetic fields (i.e., their saturation magnetization) is one of the most critical parameters for a fast and thorough separation. In the present study, magnetite (Fe3O4) and non‐oxidic cementite (Fe3C) based carbon‐coated nanomagnets are characterized in detail and compared regarding their separation behavior from human whole blood. A quantification approach for iron‐based nanomaterials in biological samples with strong matrix effects (here, salts in blood) based on platinum spiking is shown. Both materials are functionalized with polyethyleneglycol (PEG) to improve cytocompatibility (confirmed by cell toxicity tests) and dispersability. The separation performance is tested in two setups, namely under stationary and different flow‐conditions using fresh human blood. The results reveal a superior separation behavior of the cementite based nanomagnets and strongly suggest the use of nanomaterials with high saturation magnetizations for magnetic retention under common blood flow conditions such as in veins. The responsiveness to external magnetic fields is a critical parameter for the successful application of nanomagnets in biomedicine as fast and complete magnetic separations are essential. Here, the superior separation performance of strongly magnetizable carbon‐coated cementite nanoparticles over magnetite nanomagnets from stagnant/flowing human blood is shown. Beyond, a robust quantification approach for iron‐based nanomaterials in iron‐rich matrices is presented.
  PubDate: 2013-04-17T03:40:34.11621-05:0
  DOI: 10.1002/adfm.201300696
The Exfoliation of Graphene in Liquids by Electrochemical, Chemical, and Sonication‐Assisted Techniques: A Nanoscale Study
- Authors: Zhen Yuan Xia; Sergio Pezzini, Emanuele Treossi, Giuliano Giambastiani, Franco Corticelli, Vittorio Morandi, Alberto Zanelli, Vittorio Bellani, Vincenzo Palermo
  Pages: n/a - n/a
  Abstract: The different exfoliation routes of graphite to produce graphene by sonication in solvent, chemical oxidation and electrochemical oxidation are compared. The exfoliation process and roughening of a flat graphite substrate is directly visualized at the nanoscale by scanning probe and electron microscopy. The etching damage in graphite and the properties of the exfoliated sheets are compared by Raman spectroscopy and X‐ray diffraction analysis. The results show the trade‐off between exfoliation speed and preservation of graphene quality. A key step to achieve efficient exfoliation is to couple gas production and mechanical exfoliation on a macroscale with non‐covalent exfoliation and preservation of graphene properties on a molecular scale. The different exfoliation routes of graphite to produce graphene by sonication in solvent, chemical oxidation, and electrochemical oxidation are compared. The results obtained show the trade‐off between exfoliation speed and preservation of graphene quality. A key step to achieve efficient exfoliation is to couple gas production and mechanical exfoliation on a macroscale with non‐covalent exfoliation and preservation of graphene properties on a molecular scale.
  PubDate: 2013-04-17T03:40:29.384149-05:
  DOI: 10.1002/adfm.201203686
Probing Bias‐Dependent Electrochemical Gas‐Solid Reactions in (LaxSr1‐x)CoO3‐δ Cathode Materials
- Authors: Amit Kumar; Francesco Ciucci, Donovan Leonard, Stephen Jesse, Mike Biegalski, Hans Christen, Eva Mutoro, Ethan Crumlin, Yang Shao‐Horn, Albina Borisevich, Sergei V. Kalinin
  Pages: n/a - n/a
  Abstract: Spatial variability of bias‐dependent electrochemical processes on a (La0.5Sr0.5)2CoO4±δ modified (LaxSr1–x)CoO3–δ surface is studied using first‐order reversal curve method in electrochemical strain microscopy (ESM). The oxygen reduction/evolution reaction (ORR/OER) is activated at voltages as low as 3–4 V with respect to bottom electrode. The degree of bias‐induced transformation as quantified by ESM hysteresis loop area increases with applied bias. The variability of electrochemical activity is explored using correlation analysis and the ORR/OER is shown to be activated in grains at relatively low biases, but the final reaction rate is relatively small. At the same time, at grain boundaries the onset of reaction process corresponds to larger voltages, but limiting reactivity is much higher. The reaction mechanism in ESM of mixed electronic‐ionic conductor is further analyzed. These studies both establish the framework for probing bias‐dependent electrochemical processes in solids and demonstrate rich spectrum of electrochemical transformations underpinning catalytic activity in cobaltites. Spatial variability of bias‐dependent electrochemical processes on a (La0.5Sr0.5)2CoO4±δ modified (LaxSr1–x)CoO3–δ surface is studied using first‐order reversal curve method in electrochemical strain microscopy. Reversible oxygen reduction/evolution reaction (ORR/OER) is activated at voltages as low as 3–4 V. The degree of bias‐induced transformation increases with applied bias. ORR/OER is shown to be activated in grains at relatively low biases and larger voltages at grain boundaries.
  PubDate: 2013-04-17T03:40:23.025812-05:
  DOI: 10.1002/adfm.201202401
In Situ Atomic Force Microscopy as a Tool for Investigating Interactions and Assembly Dynamics in Biomolecular and Biomineral Systems
- Authors: James J. De Yoreo; Sungwook Chung, Raymond W. Friddle
  Pages: n/a - n/a
  Abstract: Atomic force imaging and spectroscopy provide unique tools for investigating molecular interactions and dynamics in biomolecular and biomineral systems in situ. Herein, three recent examples of methods used to gain mechanistic insights into the self‐assembly of protein matrices and biomolecular controls over mineral formation are reviewed. Studies of S‐layer protein assembly reveal the complex nature of the nucleation and growth pathway, demonstrate the importance of kinetic traps in determining that pathway and provide quantification of the energy barriers controlling formation rates. Investigations of citrate and polypeptide modification of calcium oxalate monohydrate growth combined with molecular dynamics simulations (MD) demonstrate the importance of stereochemical matching at atomic steps on the crystal surface and establish a direct relationship between the step edge binding energies and shape modification. Measurements of step kinetics lead to detailed atomic‐scale models that include both thermodynamic and kinetic effects, including time‐dependent phenomena related to the multi‐stage binding dynamics of polypeptide chains. Dynamic force spectroscopy measurements of binding between amelogenin peptide segments and hydroxyapatite (HAP) crystal faces, again combined with MD simulations, establish an energetic rationale for the observed c‐axis elongation characteristic of HAP in tooth enamel, based on determinations of the peptide‐HAP binding free energy. These examples demonstrate the deep level of understanding that can be obtained by applying in situ AFM imaging and force spectroscopy to biomolecular and biomineral systems. Atomic force imaging and spectroscopy provide unique tools for investigating molecular interactions and dynamics in biomolecular and biomineral systems in situ. Three recent examples are presented that illustrate the use of these methods to gain mechanistic insights into biomolecular controls over mineral formation and the self‐assembly of protein matrices, such as the S‐layer protein membrane shown here.
  PubDate: 2013-04-17T03:40:14.64236-05:0
  DOI: 10.1002/adfm.201203424
Self‐Defensive Biomaterial Coating Against Bacteria and Yeasts: Polysaccharide Multilayer Film with Embedded Antimicrobial Peptide
- Authors: G. Cado; R. Aslam, L. Séon, T. Garnier, R. Fabre, A. Parat, A. Chassepot, J.‐C. Voegel, B. Senger, F. Schneider, Y. Frère, L. Jierry, P. Schaaf, H. Kerdjoudj, M.‐H. Metz‐Boutigue, F. Boulmedais
  Pages: n/a - n/a
  Abstract: Prevention of pathogen colonization of medical implants is a major medical and financial issue since infection by microorganisms constitutes one of the most serious complications after surgery or critical care. Immobilization of antimicrobial molecules on biomaterials surfaces is an efficient approach to prevent biofilm formation. Herein, the first self‐defensive coating against both bacteria and yeasts is reported where the release of the antimicrobial peptide is triggered by enzymatic degradation of the film due to the pathogens themselves. Biocompatible and biodegradable polysaccharide multilayer films based on functionalized hyaluronic acid by cateslytin (CTL), an endogenous host‐defensive antimicrobial peptide, and chitosan (HA‐CTL‐C/CHI) were deposited on a planar surface with the aim of designing both antibacterial and antifungal coating. After 24 h of incubation, HA‐CTL‐C/CHI films fully inhibit the development of Gram‐positive Staphylococcus aureus bacteria and Candida albicans yeasts, which are common and virulent pathogens agents encountered in care‐associated diseases. Hyaluronidase, secreted by the pathogens, leads to the film degradation and the antimicrobial action of the peptide. Furthermore, the limited fibroblasts adhesion, without cytotoxicity, on HA‐CTL‐C/CHI films highlights a medically relevant application to prevent infections on catheters or tracheal tubes where fibrous tissue encapsulation is undesirable. Polysaccharide multilayer films based on antimicrobial peptide functionalized hyaluronic acid as polyanion and chitosan as polycation, are deposited on planar surfaces with the aim of designing a self‐defensive coating against both bacteria and yeasts.
  PubDate: 2013-04-16T02:10:22.789556-05:
  DOI: 10.1002/adfm.201300416
A Method for Fabricating an Ultrathin Multilayer Film Composed of Poly(p‐phenylenevinylene) and Reduced Graphene Oxide on a Plastic Substrate for Flexible Optoelectronic Applications
- Authors: Boon‐Hong Wee; Jong‐Dal Hong
  Pages: n/a - n/a
  Abstract: The photoconductive properties of a uniform ultrathin multilayer film composed of alternating poly(p‐phenylene vinylene) (PPV) and reduced graphene oxide (RGO) layers, fabricated on a poly(ethylene terephthalate) (PET) sheet are reported. The assembly of the two electron‐rich layer components on the temperature‐sensitive substrate is realized using a layer‐by‐layer‐deposition technique under mild conditions and HI/H2O vapor treatment at 100 °C. This protocol is established to simultaneously convert the layer components to their conjugated counterparts, PPV and RGO in the multilayer films, whose total thicknesses shrinks to 50% of their original values due to lattice contraction. Furthermore, the surface roughness decreases significantly, in contrast to the results obtained from general chemical treatments. The PET sheets coated with (PPV/RGO)15 films exhibit a photocurrent of 115 μA at an illumination intensity of 1.1 mW and a photoresponsivity of 111.1 mA W−1 at an illumination intensity of 0.5 mW; these are among the best values yet achieved in carbon‐based materials. The establishment of a method for fabricating (PPV/RGO) films on a temperature‐sensitive transparent flexible sheet is crucial for the development of organic‐based portable electronic devices. A novel method for the fabrication of a uniform ultrathin multilayer film composed of alternating poly(p‐phenylene vinylene) and reduced graphene oxide layers on a plastic substrate is reported for flexible optoelectronic applications. The ultrathin photodiode exhibits extraordinary optoelectric characteristics, including photocurrent and photoresponsivity; these are among the best values achieved in carbon‐based materials.
  PubDate: 2013-04-16T02:10:15.661498-05:
  DOI: 10.1002/adfm.201300224
Development of a Seedless Floating Growth Process in Solution for Synthesis of Crystalline ZnO Micro/Nanowire Arrays on Graphene: Towards High‐Performance Nanohybrid Ultraviolet Photodetectors
- Authors: Jianwei Liu; Rongtao Lu, Guowei Xu, Judy Wu, Prem Thapa, David Moore
  Pages: n/a - n/a
  Abstract: A seedless solution process is developed for controllable growth of crystalline ZnO micro/nanowire arrays directly on single‐layer graphene sheets made in chemical vapor deposition (CVD). In particular, the alignment of the ZnO micro/nanowires correlates well with the density of the wires, which is determined by both the sample configuration in solution and the graphene surface cleaning. With increasing wire density, the ZnO micro/nanowire array alignment may be varied from horizontal to vertical by increasing the physical confinement. Ultraviolet photodetectors based on the vertically aligned ZnO micro/nanowires on graphene show high responsivity of 1.62 A W−1 per volt, a 500% improvement over epitxial ZnO sensors, a 300% improvement over ZnO nanoparticle sensors, and a 40% improvement over the previous best results for nanowire/graphene hybrid sensors. This seedless, floating growth process could be scaled up for large scale growth of oriented ZnO micro/nanowires on graphene at low costs. A seedless solution process is developed to control the morphologies and orientation of crystalline ZnO micro/nanowires on graphene sheets by face‐down floating or face‐up in the solution. The UV detectors based on vertically aligned ZnO micro/nanowires on graphene show high responsivity and fast response.
  PubDate: 2013-04-15T04:10:16.128699-05:
  DOI: 10.1002/adfm.201300468
Crystalline CoFeB/Graphite Interfaces for Carbon Spintronics Fabricated by Solid Phase Epitaxy
- Authors: P. K. Johnny Wong; Elmer van Geijn, Wen Zhang, Anton A. Starikov, T. Lan Anh Tran, Johnny G. M. Sanderink, Martin H. Siekman, Geert Brocks, Paul J. Kelly, Wilfred G. van der Wiel, Michel P. de Jong
  Pages: n/a - n/a
  Abstract: Structurally ordered interfaces between ferromagnetic electrodes and graphene or graphite are of great interest for carbon spintronics, since they allow spin‐filtering due to k‐vector conservation. By solid phase epitaxy of amorphous/nanocrystalline CoFeB at elevated temperatures, the feasibility of fabricating crystalline interfaces between a 3d ferromagnetic alloy and graphite is demonstrated, without suffering from the unwetting problem that was commonly seen in many previous studies with 3d transition metals. The films fabricated on graphite in this way are found to have a strong body‐centered‐cubic (110) texture, albeit without a unique, well‐defined in‐plane epitaxial relationship with the substrate lattice. Using various X‐ray spectroscopic techniques, it is shown that boron suppresses the formation of CoFe‐O during CoFeB deposition, and then diffuses out of the CoFe lattice. Segregation of B occurred exclusively to the film surface upon in situ annealing, and not to the interface between CoFeB and graphite. This is favorable for obtaining a high spin polarization at the hybrid CoFe/graphite crystalline interface. The Co and Fe spin moments in the crystalline film, determined by X‐ray magnetic circular dichroism, are found to be bulk‐like, while their orbital moments show an unusual giant enhancement which has yet to be understood. Solid phase epitaxy of the amorphous alloy CoFeB is used to fabricate crystalline ferromagnet/graphite interfaces, which are of great interest for carbon spintronics but hardly achievable with conventional thin film deposition techniques. The heterointerface features a strong body‐centred‐cubic (110) texture and is free from boron accumulation upon crystallization, favorable for obtaining a high spin polarization at the CoFe/graphite interface.
  PubDate: 2013-04-15T04:10:06.409625-05:
  DOI: 10.1002/adfm.201203460
A Negative Conductivity Material Makes a dc Invisibility Cloak Hide an Object at a Distance
- Authors: Fan Yang; Zhong Lei Mei, Xin Yu Yang, Tian Yu Jin, Tie Jun Cui
  Pages: n/a - n/a
  Abstract: The theoretical design and the first experimental verification of an exterior dc invisibility cloak that can hide an object from dc detection at a distance are presented. Based on the transformation optics theory, the exterior dc cloak requires negative conductivity material to create folded geometry, which will cancel the real geometry of detected object in distance and make it invisible. Negative conductivities are designed and realized using active devices, together with resistor networks, to generate the equivalent conductivity materials required by the exterior dc cloak. An experimental sample of the dc cloak is fabricated on the printed circuit board and the measured result has excellent agreement with numerical and circuit simulations, showing very good cloaking performance at a distance. Closed dc cloaks can render a conducting object invisible to external detection and distortions of probing currents due to the embedded object can be limited to inside the cloak. Using negative conductivities, exterior dc cloaks that make a nearby object invisible to the outside detectors can be realized.
  PubDate: 2013-04-11T07:11:06.948604-05:
  DOI: 10.1002/adfm.201300226
Novel All‐Natural Microcapsules from Gelatin and Shellac for Biorelated Applications
- Authors: Ashok R. Patel; Caroline Remijn, Ana‐isabel Mulet Cabero, Patricia C.M. Heussen, Jack W.M. Seijen ten Hoorn, Krassimir P. Velikov
  Pages: n/a - n/a
  Abstract: The generation of novel all‐natural biopolymeric microcapsules fabricated using natural biopolymers, protein (gelatin) and resin (shellac), is reported. These novel microcapsules are generated using a simple extrusion method wherein the gelatin‐shellac mixture is dropped in an acidic medium resulting in an instantaneous solidification of aqueous drops into solid spherical microcapsules that retain their shape on air‐drying. The formation of the microcapsules is basically due to the strong interactions between two oppositely charged polymers (as confirmed from isothermal titration calorimetry and infrared spectroscopy) and the instant precipitation of acid‐resistant shellac. These novel microcapsules prepared without the help of any cross‐linkers or harsh solvent are extensively characterized and several biorelated applications for pharmaceuticals (encapsulation and release of bioactive molecules), foods (loading of colorants and flavors), sensors (encapsulation of pH sensitive dye), and biotechnology (enzyme immobilization) fields are further demonstrated. Novel all‐natural microcapsules fabricated from biopolymers, protein (gelatin) and resin (shellac), using a simple extrusion method. Furthermore, a range of biorelated applications including encapsulation and release of bioactives, loading of colorants and pH sensitive dye, temperature‐triggered flavor release, and enzyme immobilization are successfully demonstrated.
  PubDate: 2013-04-11T07:10:56.625194-05:
  DOI: 10.1002/adfm.201300320
Nanosphere Lithography as a Versatile Method to Generate Surface‐Imprinted Polymer Films for Selective Protein Recognition
- Authors: Júlia Bognár; Júlia Szűcs, Zsanett Dorkó, Viola Horváth, Róbert E. Gyurcsányi
  Pages: n/a - n/a
  Abstract: A versatile approach based on nanosphere lithography is proposed to generate surface‐imprinted polymers for selective protein recognition. A layer of 750 nm diameter latex bead‐protein conjugate is deposited onto the surface of gold‐coated quartz crystals followed by the electrosynthesis of a poly(3,4‐ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) film with thicknesses on the order of the bead radius. The removal of the polymer bead‐protein conjugates, facilitated by using a cleavable protein‐nanosphere linkage is shown to result in 2D arrays of periodic complementary size cavities. Here it is demonstrated by nanogravimetric measurements that the imprinting proceeds further at molecular level and the protein (avidin) coating of the beads generates selective recognition sites for avidin on the surface of the PEDOT/PSS film. The binding capacity of such surface‐imprinted polymer films is ca. 6.5 times higher than that of films imprinted with unmodified beads. They also exhibit excellent selectivity against analogues of avidin, i.e., extravidin, streptavidin, and neutravidin, the latter being in fact undetectable. This methodology, if coupled with properly oriented conjugation of the macromolecular template to the nanoparticles, offers the possibility of site‐directed imprinting. A versatile approach based on nanosphere lithography is proposed to generate surface‐imprinted polymers for selective protein recognition. Nanogravimetric measurements demonstrate that the protein (avidin) coating of the nanospheres generates selective recognition sites for avidin on the surface of the PEDOT/PSS film. This methodology coupled with oriented conjugation of the macromolecular template to the nanospheres offers the possibility of site‐directed imprinting.
  PubDate: 2013-04-11T07:10:51.413628-05:
  DOI: 10.1002/adfm.201300113
Domain Wall Conduction and Polarization‐Mediated Transport in Ferroelectrics
- Authors: R. K. Vasudevan; W. Wu, J. R. Guest, A. P. Baddorf, A. N. Morozovska, E. A. Eliseev, N. Balke, V. Nagarajan, P. Maksymovych, S. V. Kalinin
  Pages: n/a - n/a
  Abstract: Nanometer‐scale electronic transport in engineered interfaces in ferroelectrics, such as domains and topological defects, has emerged as a topic of broad interest due to potential applications in information storage, sensors and photovoltaic devices. Scanning probe microscopy (SPM) methods led to rapid growth in the field by enabling correlation of the unique functional properties with microstructural features in the aforementioned highly localized phenomena. In addition to conduction localized at interfaces, polarization‐mediated control of conduction through domains in nanoscale ferroelectrics suggests significant potential for use in memristor technologies. In parallel with experiment, theory based on thermodynamic Landau‐Ginzburg‐Devonshire (LGD) framework has seen rapid development, both rationalizing the observations, and hinting at possibilities for local, deterministic control of order parameters. These theories can successfully account for static interface conductivity at charged, nominally uncharged and topologically protected domain walls. Here, recent experimental and theoretical progress in SPM‐motivated studies on domain wall conduction in both standard and improper ferroelectrics are reviewed. SPM studies on transport through ferroelectrics reveal that both domains and topological defects in oxides can be exploited as individual elements for use in functional nanoscale devices. Future prospects of the field are discussed. Nanometer‐scale electronic transport in engineered interfaces in ferroelectrics, such as domains and topological defects, has emerged as a topic of broad interest. Here, the use of scanning probe microscopies to access topological defects and directly measure their unique properties is reviewed. It is found, through Landau‐Ginzburg‐Devonshire theory, that observation of enhanced conduction at domain walls can be attributed to segregation of carriers at charged walls. Furthermore, the potential distribution around a curved or tilted wall can be highly assymetric, even for nominally uncharged walls due to strain and flexo‐electric couplings, suggesting novel routes for control of conduction spatially in a wide variety of ferroics.
  PubDate: 2013-04-11T07:10:42.993047-05:
  DOI: 10.1002/adfm.201300085
Fluid Drag Reduction with Shark‐Skin Riblet Inspired Microstructured Surfaces
- Authors: Gregory D. Bixler; Bharat Bhushan
  Pages: n/a - n/a
  Abstract: Engineering marvels found throughout living nature continually provide inspiration to researchers solving technical challenges. For example, skin from fast‐swimming sharks intrigue researchers since its low‐drag riblet microstructure is applicable to many low drag and self‐cleaning (antifouling) applications. An overview of shark skin related studies that have been conducted in both open channel (external) and closed channel (internal) flow experiments is presented. Significant work has been conducted with the open channel flow, and less with closed channel flow. The results provide design guidance when developing novel low drag and self‐cleaning surfaces for applications in the medical, marine, and industrial fields. Experimental parameters include riblet geometry, continuous and segmented configurations, fluid velocity (laminar and turbulent flow), fluid viscosity (water, oil, and air), closed channel height dimensions, wettability, and scalability. The results are discussed and conceptual models are shown suggesting the effect of viscosity, coatings, and the interaction between vortices and riblet surfaces. Engineering marvels found throughout living nature provide inspiration to researchers solving technical challenges. For example, the skins of fast‐swimming sharks intrigue researchers because their riblet microstructures lead to low drag, self‐cleaning, and antifouling properties. An overview of shark skin related studies that have been conducted in both open and closed channel flow experiments is presented. Adapted with permission.24 Copyright 2012, Royal Society of Chemistry.
  PubDate: 2013-04-11T07:10:36.137294-05:
  DOI: 10.1002/adfm.201203683
Microscopy of Graphene Growth, Processing, and Properties
- Authors: Peter Sutter; Eli Sutter
  Pages: n/a - n/a
  Abstract: The growth and properties of two‐dimensional (2D) materials–graphene as well as related monolayer systems, such as hexagonal boron nitride–on metals are topics of high scientific and technological interest. Real‐time low‐energy electron microscopy (LEEM) can provide unique insight into the fundamental growth mechanisms of 2D materials on metal substrates. In combination with in situ spectroscopic measurements, LEEM can greatly facilitate the search for synthesis and processing protocols that produce 2D materials with desired properties for applications. Here, progress is reviewed in understanding the scalable growth of high‐quality graphene on metals, novel processing strategies based on selective chemical reactions at the graphene/metal interface, and important materials properties (structure, electronic properties, work function, etc.) by surface microscopy and complementary methods, using graphene/ruthenium as a model system. The body of work shows that in situ microscopy can be used as a powerful tool for achieving and probing a wide range of functionalities in 2D materials. Real‐time surface microscopy and in situ spectroscopy can provide unique insight into graphene and other 2D materials on metal substrates. Here, the power of in situ microscopy in realizing and probing important functionalities in 2D materials is illustrated by reviewing recent progress in understanding scalable graphene growth on metals, processing by selective chemistry at the graphene/metal interface, and important properties such as band structure, work function, etc.
  PubDate: 2013-04-11T07:10:28.672078-05:
  DOI: 10.1002/adfm.201203426
Amphiphilic Polymeric Nanocarriers with Luminescent Gold Nanoclusters for Concurrent Bioimaging and Controlled Drug Release
- Authors: Dongyun Chen; Zhentao Luo, Najun Li, Jim Yang Lee, Jianping Xie, Jianmei Lu
  Pages: n/a - n/a
  Abstract: Multifunctional theranostic systems with good biocompatibility, strong clinical imaging capability, and target specificity are the desired features of future medicine. Here, the design of a theranostic nanocomposite capable of simultaneous targeting and imaging of the cancer cells is presented. It releases its drug payload by a controlled release mechanism. The nanocomposite contains luminescent gold nanocluster (L‐AuNC) photostable and biocompatible diagnostic probes conjugated to a folic acid (FA)‐modified pH‐responsive amphiphilic polymeric system for controlled drug release. The nanocomposite uses a core‐satellite structure to encapsulate hydrophobic drugs and releases the drug payload in mildly acidic endosomal/lysosomal compartments by the action of the pH‐labile linkages in the polymer. In vivo studies show the selective accumulation of the FA‐conjugated nanocomposite in tumor tissues by folate‐receptor‐mediated endocytosis. These findings demonstrate the potential of the nanocomposite as a nontoxic, folate‐targeting, pH‐responsive drug carrier that is useful for the early detection and therapy of folate‐overexpressing cancerous cells. A facile self‐assembly method is used to integrate orange luminescent gold nanoclusters (L‐AuNCs), which are cancer cell targeting agents, a hydrophobic cancer drug, and a pH‐responsive amphiphilic polymer into a nanocomposite theranostic system. The nanocomposite can selectively target the affected cells and unload the cancer drug via a controlled release mechanism. The release of the drug can be continuously monitored by the luminescence of the co‐delivered imaging probes (L‐AuNCs).
  PubDate: 2013-04-11T07:10:22.472721-05:
  DOI: 10.1002/adfm.201300411
Regulation of Stem Cell Fate in a Three‐Dimensional Micropatterned Dual‐Crosslinked Hydrogel System
- Authors: Oju Jeon; Eben Alsberg
  Pages: n/a - n/a
  Abstract: Micropatterning technology is a powerful tool for controlling the cellular microenvironment and investigating the effects of physical parameters on cell behaviors, such as migration, proliferation, apoptosis, and differentiation. Although there have been significant developments in regulating the spatial and temporal distribution of physical properties in various materials, little is known about the role of the size of micropatterned regions of hydrogels with different crosslinking densities on the response of encapsulated cells. In this study, a novel alginate hydrogel system is engineered that can be micropatterned three‐dimensionally to create regions that are crosslinked by a single mechanism or dual mechanisms. By manipulating micropattern size while keeping the overall ratio of single‐ to dual‐crosslinked hydrogel volume constant, the physical properties of the micropatterned alginate hydrogels are spatially tunable. When human adipose‐derived stem cells (hASCs) are photoencapsulated within micropatterned hydrogels, their proliferation rate is a function of micropattern size. Additionally, micropattern size dictates the extent of osteogenic and chondrogenic differentiation of photoencapsulated hASC. The size of 3D micropatterned physical properties in this new hydrogel system introduces a new design parameter for regulating various cellular behaviors, and this dual‐crosslinked hydrogel system provides a new platform for studying proliferation and differentiation of stem cells in a spatially controlled manner for tissue engineering and regenerative medicine applications. Three‐dimensional micropatterning of dual‐crosslinked oxidized, methacrylated alginate (OMA)/8‐arm PEG amine (PEG) hydrogels containing human adipose‐derived stem cells (hASCs) is employed to regulate hASC fate. The micropatterned dual‐crosslinked OMA/PEG hydrogels for hASC encapsulation yield relatively uniform cell clusters, which scale in size with micropatterned dimensions.
  PubDate: 2013-04-11T07:10:14.431406-05:
  DOI: 10.1002/adfm.201300529
Materials and Fabrication Processes for Transient and Bioresorbable High‐Performance Electronics
- Authors: Suk‐Won Hwang; Dae‐Hyeong Kim, Hu Tao, Tae‐il Kim, Stanley Kim, Ki Jun Yu, Bruce Panilaitis, Jae‐Woong Jeong, Jun‐Kyul Song, Fiorenzo G. Omenetto, John. A. Rogers
  Pages: n/a - n/a
  Abstract: Materials and fabrication procedures are described for bioresorbable transistors and simple integrated circuits, in which the key processing steps occur on silicon wafer substrates, in schemes compatible with methods used in conventional microelectronics. The approach relies on an unusual type of silicon on insulator wafer to yield devices that exploit ultrathin sheets of monocrystalline silicon for the semiconductor, thin films of magnesium for the electrodes and interconnects, silicon dioxide and magnesium oxide for the dielectrics, and silk for the substrates. A range of component examples with detailed measurements of their electrical characteristics and dissolution properties illustrate the capabilities. In vivo toxicity tests demonstrate biocompatibility in sub‐dermal implants. The results have significance for broad classes of water‐soluble, “transient” electronic devices. Materials, designs, and integration techniques are presented for a class of water‐soluble electronics capable of fabrication using wafer‐based processes. The active components exploit biocompatible and bioresorbable materials that are capable of dissolution in biofluids. Characterization of the electronic properties of the devices, their kinetics for dissolution, and preliminary evaluations in animal models highlight key aspects of the materials and concepts.
  PubDate: 2013-04-11T07:10:08.049062-05:
  DOI: 10.1002/adfm.201300127
BiSbTe‐Based Nanocomposites with High ZT: The Effect of SiC Nanodispersion on Thermoelectric Properties
- Authors: Jianhui Li; Qing Tan, Jing‐Feng Li, Da‐Wei Liu, Fu Li, Zong‐Yue Li, Minmin Zou, Ke Wang
  Pages: n/a - n/a
  Abstract: Thermoelectric materials have potential applications in energy harvesting and electronic cooling devices, and bismuth antimony telluride (BiSbTe) alloys are the state‐of‐the‐art thermoelectric materials that have been widely used for several decades. It is demonstrated that mixing SiC nanoparticles into the BiSbTe matrix effectively enhances its thermoelectric properties; a high dimensionless figure of merit (ZT) value of up to 1.33 at 373 K is obtained in Bi0.3Sb1.7Te3 incorporated with only 0.4 vol% SiC nanoparticles. SiC nanoinclusions possessing coherent interfaces with the Bi0.3Sb1.7Te3 matrix can increase the Seebeck coefficient while increasing the electrical conductivity, in addition to its effect of reducing lattice thermal conductivity by enhancing phonon scattering. Nano‐SiC dispersion further endows the BiSbTe alloys with better mechanical properties, which are favorable for practical applications and device fabrication. A high figure of merit (ZT) up to 1.33 at 373 K is achieved by incorporating a tiny number of SiC particles to a traditional Bi0.3Sb1.7Te3 thermoelectric material. The existence of SiC nanoinclusions in the p‐type Bi0.3Sb1.7Te3 thermoelectric matrix reduces the electrical resistivity and increases the Seebeck coefficient, which leads to the remarkable ZT enhancement.
  PubDate: 2013-04-10T03:10:21.601737-05:
  DOI: 10.1002/adfm.201300146
Highly Efficient and Bendable Organic Solar Cells with Solution‐Processed Silver Nanowire Electrodes
- Authors: Myungkwan Song; Dae Sung You, Kyounga Lim, Sujin Park, Sunghoon Jung, Chang Su Kim, Dong‐Ho Kim, Do‐Geun Kim, Jongk‐Kuk Kim, Juyun Park, Yong‐Cheol Kang, Jinhee Heo, Sung‐Ho Jin, Jong Hyun Park, Jae‐Wook Kang
  Pages: n/a - n/a
  Abstract: Highly efficient and bendable organic solar cells (OSCs) are fabricated using solution‐processed silver nanowire (Ag NW) electrodes. The Ag NW films were highly transparent (diffusive transmittance ≈ 95% at a wavelength of 550 nm), highly conductive (sheet resistance ≈ 10 Ω sq−1), and highly flexible (change in resistance ≈ 1.1 ± 1% at a bending radius of ≈200 μm). Power conversion efficiencies of ≈5.80 and 5.02% were obtained for devices fabricated on Ag NWs/glass and Ag NWs/poly(ethylene terephthalate) (PET), respectively. Moreover, the bendable devices fabricated using the Ag NWs/PET films decrease slightly in their efficiency (to ≈96% of the initial value) even after the devices had been bent 1000 times with a radius of ≈1.5 mm. Highly bendable and efficient organic solar cells are developed using solution‐processed silver nanowires. The electrode and solar cell characterizations are also presented with the devices showing high performance and flexibility.
  PubDate: 2013-04-10T03:10:16.36439-05:0
  DOI: 10.1002/adfm.201202646
Catalytic Saloplastics: Alkaline Phosphatase Immobilized and Stabilized in Compacted Polyelectrolyte Complexes
- Authors: Patricia Tirado; Andreas Reisch, Emilie Roger, Fouzia Boulmedais, Loïc Jierry, Philippe Lavalle, Jean‐Claude Voegel, Pierre Schaaf, Joseph B. Schlenoff, Benoît Frisch
  Pages: n/a - n/a
  Abstract: Novel biochemically active compact polyelectrolyte complexes (CoPECs) are obtained through a simple coprecipitation and compaction procedure. As shown for the system composed of poly(acrylic acid) (PAA) and poly(allylamine) (PAH) as polyelectrolytes and alkaline phosphatase (ALP) as enzyme, the enzyme can be firmly immobilized into these materials. The ALP not only remains active in these materials, but the matrix also enhances the specific activity of the enzyme while protecting it from deactivation at higher temperature. The presence of the matrix allows fine control and substantial enhancement of reaction rates by varying the salt concentration of the contacting solution or temperature. The excellent reusability together with the ease of co‐immobilizing other useful components, such as magnetic particles, allowing facile handling of the CoPECs, makes these materials interesting candidates for variable scaffolds for the immobilization of enzymes for small‐ and large‐scale enzyme‐catalyzed processes. Alkaline phosphatase (ALP) is immobilized in compact polyelectrolyte complexes (CoPECs). The materials obtained in this way retain the biocatalytic activity of the enzyme, protect it from elevated temperature, and allow its fine tuning by salt concentration and temperature. Co‐immobilization of magnetic particles yields easily handleable magnetic materials.
  PubDate: 2013-04-10T03:10:03.555884-05:
  DOI: 10.1002/adfm.201300117
Real‐Time Microscopy of Reorientation Driven Nucleation and Growth in Pentacene Thin Films on Silicon Dioxide
- Authors: Abdullah Al‐Mahboob; Yasunori Fujikawa, Toshio Sakurai, Jerzy T. Sadowski
  Pages: n/a - n/a
  Abstract: The role of molecular reorientation processes in the self‐assembly of anisotropic molecules, such as pentacene (Pn) is studied utilizing a unique capability of low‐energy electron microscopy (LEEM) for the real‐time investigation of the film growth. In Pn film on SiO2, a layer‐by‐layer growth is observed, albeit different from the expected Volmer–Weber growth mode typical for the systems with lower adhesion (weak interfacial interaction). The observed growth mechanism is also different than conventional concept of layer‐by‐layer, or Frank van der Merwe growth. In the Pn/SiO2 system the nucleation density decreases in each consecutive layer, at least up to four monolayers. This growth mechanism is hereafter named inverse Stranski‐Krastanov growth. Furthermore, in this growth system the second layer islands nucleate preferentially at the domain boundaries formed by the interconnections of the bottom (first layer) domains. The top layer overgrows bottom layer with its own, initial in‐plane crystal orientation, regardless of the in‐plane orientations in underlying Pn domains. The dark‐field LEEM imaging allows us to distinguish between Pn domains having different azimuthal direction of molecular tilt. LEEM intensity versus start voltage (LEEM I–V) curves taken in the vicinity of mirror potential from the first and second layer Pn islands show that the surface potential of the second layer is higher by about 0.05 eV than that of the first layer, while the surface potentials for the epitaxial and non‐epitaxial parts of the second layer island are identical. Nucleation and growth processes in organic systems such as a pentacene film on silicon dioxide, are often complicated due to anisotropy in both, the molecular shape and the crystal structure. As in the diffusive state, the pentacene molecule is in the lying‐down configuration, interfacial interaction is different from that at the island's step edge. The strength of interfacial interaction in the diffusion state determines the energy barrier for molecule reorientation, such that the stronger interaction increases the relative stability of diffusing molecules.
  PubDate: 2013-04-09T03:10:58.593273-05:
  DOI: 10.1002/adfm.201203427
Single‐Crystal Organic Nanowire Electronics by Direct Printing from Molecular Solutions
- Authors: Kyung S. Park; Boram Cho, Jangmi Baek, Jae K. Hwang, Haiwon Lee, Myung M. Sung
  Pages: n/a - n/a
  Abstract: A one‐step process to generate single‐crystal organic nanowire arrays using a direct printing method (liquid‐bridge‐mediated nanotransfer molding) is reported that enables the simultaneous synthesis, alignment, and patterning of nanowires from molecular ink solutions. Using this method, many single‐crystal organic nanowires can easily be synthesized by self‐assembly and crystallization of organic molecules within the nanoscale channels of molds, and these nanowires can then be directly transferred to specific positions on substrates to generate nanowire arrays by a direct printing process. The position of the nanowires on complex structures is easy to adjust, because the mold is movable on the substrates before the polar liquid layer, which acts as an adhesive lubricant, is dried. Repeated application of the direct printing process can be used to produce organic nanowire‐integrated electronics with two‐ or three‐dimensional complex structures on large‐area flexible substrates. This efficient manufacturing method is used to fabricate high‐performance organic nanowire field‐effect transistors that are integrated into device arrays, inverters, and phototransistors on flexible plastic substrates. Single‐crystal organic nanowire arrays are generated by a direct printing method that enables the simultaneous synthesis, alignment, and patterning of nanowires. Repeated application of the direct printing process can be used to produce high‐performance organic nanowire‐integrated electronics with two‐ or three‐dimensional complex structures.
  PubDate: 2013-04-09T03:10:52.277545-05:
  DOI: 10.1002/adfm.201203540
Multifunctional Graphene–PEDOT Microelectrodes for On‐Chip Manipulation of Human Mesenchymal Stem Cells
- Authors: Yu‐Sheng Hsiao; Chiung‐Wen Kuo, Peilin Chen
  Pages: n/a - n/a
  Abstract: All‐solution‐processed multifunctional organic bioelectronics composed of reduced graphene oxide (rGO) and dexamethasone 21‐phosphate disodium salt (DEX)‐loaded poly(3,4‐ethylenedioxythiophene) (PEDOT) microelectrode arrays on indium tin oxide glass are reported. They can be used to manipulate the differentiation of human mesenchymal stem cells (hMSCs). In the devices, the rGO material functions as an adhesive coating to promote the adhesion and alignment of hMSC cells and to accelerate their osteogenic differentiation. The poly(L‐lysine‐graft‐ethylene glycol) (PLL‐g‐PEG)‐coated PEDOT electrodes serve as electroactive drug‐releasing electrodes. In addition, the corresponding three‐zone parallel devices operate as efficient drug‐releasing components through spatial‐temporal control of the release of the drug DEX from the PEDOT matrix. Such devices can be used for long‐term cell culturing and controlled differentiation of hMSCs through electrical stimulation. The multifunctional organic bioelectronic interfaces composed of integrated reduced graphene oxide (rGO) and drug dexamethasone 21‐phosphate disodium salt (DEX)‐loaded poly(3,4‐ethylenedioxythiophene) (PEDOT) microelectrode arrays are reported. They can be used to manipulate the attachment, orientation, and differentiation of human mesenchymal stem cells (hMSCs) for long‐term cell culturing through electrical stimulation.
  PubDate: 2013-04-09T03:10:50.191976-05:
  DOI: 10.1002/adfm.201203631
Homogeneous CoO on Graphene for Binder‐Free and Ultralong‐Life Lithium Ion Batteries
- Authors: Xiao‐lei Huang; Ru‐zhi Wang, Dan Xu, Zhong‐li Wang, Heng‐guo Wang, Ji‐jing Xu, Zhong Wu, Qing‐chao Liu, Yu Zhang, Xin‐bo Zhang
  Pages: n/a - n/a
  Abstract: Ultralong cycle life, high energy, and power density rechargeable lithium‐ion batteries are crucial to the ever‐increasing large‐scale electric energy storage for renewable energy and sustainable road transport. However, the commercial graphite anode cannot perform this challenging task due to its low theoretical capacity and poor rate‐capability performance. Metal oxides hold much higher capacity but still are plagued by low rate capability and serious capacity degradation. Here, a novel strategy is developed to prepare binder‐free and mechanically robust CoO/graphene electrodes, wherein homogenous and full coating of β‐Co(OH)2 nanosheets on graphene, through a novel electrostatic induced spread growth method, plays a key role. The combined advantages of large 2D surface and moderate inflexibility of the as‐obtained β‐Co(OH)2/graphene hybrid enables its easy coating on Cu foil by a simple layer‐by‐layer stacking process. Devices made with these electrodes exhibit high rate capability over a temperature range from 0 to 55 °C and, most importantly, maintain excellent cycle stability up to 5000 cycles even at a high current density. Homogeneous β‐Co(OH)2 on graphene is synthesized using a simple and effective electrostatic induced spread growth method, which ensures the facile fabrication of a binder‐free and mechanically robust CoO/graphene electrode by means of a layer‐by‐layer stacking process. When employed as an anode in Li‐ion batteries, a high rate capability and excellent cycle stability up to 5000 cycles are successfully obtained.
  PubDate: 2013-04-09T03:10:47.757678-05:
  DOI: 10.1002/adfm.201203777
Leaf‐Like Graphene Oxide with a Carbon Nanotube Midrib and Its Application in Energy Storage Devices
- Authors: Ziyang Guo; Jie Wang, Fei Wang, DanDan Zhou, Yongyao Xia, Yonggang Wang
  Pages: n/a - n/a
  Abstract: Graphene oxide (GO) has recently attracted a great deal of attention because of its heterogeneous chemical and electronic structures and its consequent exhibition of a wide range of potential applications, such as plastic electronics, optical materials, solar cells, and biosensors. However, its insulating nature also limits its application in some electronic and energy storage devices. In order to further widen the applications of GO, it is necessary to keep its inherent characteristics while improving its conductivity. Here, a novel leaf‐like GO with a carbon nanotube (CNT) midrib is developed using vapor growth carbon fiber (VGCF) through the conventional Hummers method. The CNT midrib provides a natural electron diffusion path for the leaf‐like GO, and therefore, this leaf‐like GO with a CNT midrib displays excellent performance when applied in energy storage devices, including Li‐O2 batteries, Li‐ion batteries, and supercapacitors. A novel leaf‐like graphene oxide (GO) with a carbon nanotube (CNT) midrib is prepared from vapor growth carbon fiber (VGCF) using the conventional Hummers method. The CNT midrib provides a natural electron diffusion path for this leaf‐like GO, and therefore, this leaf‐like GO with a CNT midrib displays excellent performance when applied in energy storage devices, including Li‐O2 batteries, Li‐ion batteries, and supercapacitors.
  PubDate: 2013-04-09T03:10:42.221225-05:
  DOI: 10.1002/adfm.201300130
Template‐Free Vibrational Indentation Patterning (VIP) of Micro/Nanometer‐Scale Grating Structures with Real‐Time Pitch and Angle Tunability
- Authors: Se Hyun Ahn; Jong G. Ok, Moon Kyu Kwak, Kyu‐Tae Lee, Jae Yong Lee, L. Jay Guo
  Pages: n/a - n/a
  Abstract: A template‐free, high‐throughput patterning technique named vibrational indentation‐driven patterning (VIP), which achieves continuous, period‐tunable fabrication of micro/nanometer‐scale grating structures, is reported. In VIP, a tilted edge of a hard material vertically vibrating at high frequency makes periodic indentations onto a moving substrate of any material softer than the tool, thereby continuously creating grating patterns at high speed. By modulating the tool vibration frequency, substrate feeding rate, and the tool tilting angle, the period‐variable chirped gratings and angle‐tunable blazed gratings can be easily achieved; they can be utilized in various optoelectronics and photonics applications. As an example, an infrared polarizer directly fabricated from the VIP‐created blazed grating is demonstrated. A template‐free, high‐throughput patterning technique—vibrational indentation‐driven patterning (VIP)—realizes continuous, period‐tunable fabrication of micro/nanometer‐scale gratings by vertical indentations of a vibrating flat tool edge on a moving substrate. By modulating the tool vibration, substrate feeding rate, and the tool tilting angle, the period‐variable chirped gratings and angle‐tunable blazed gratings can be easily achieved.
  PubDate: 2013-04-09T03:10:36.87303-05:0
  DOI: 10.1002/adfm.201300293
Engineering of Facets, Band Structure, and Gas‐Sensing Properties of Hierarchical Sn2+‐Doped SnO2 Nanostructures
- Authors: Hongkang Wang; Kunpeng Dou, Wey Yang Teoh, Yawen Zhan, Tak Fu Hung, Feihu Zhang, Jiaqiang Xu, Ruiqin Zhang, Andrey L. Rogach
  Pages: n/a - n/a
  Abstract: Hierarchical SnO2 nanoflowers, assembled from single‐crystalline SnO2 nanosheets with high‐index (11$ \bar 3 $) and (10$ \bar 2 $) facets exposed, are prepared via a hydrothermal method using sodium fluoride as the morphology controlling agent. Formation of the 3D hierarchical architecture comprising of SnO2 nanosheets takes place via Ostwald ripening mechanism, with the growth orientation regulated by the adsorbate fluorine species. The use of Sn(II) precursor results in simultaneous Sn2+ self‐doping of SnO2 nanoflowers with tunable oxygen vacancy bandgap states. The latter further results in the shifting of semiconductor Fermi levels and extended absorption in the visible spectral range. With increased density of states of Sn2+‐doped SnO2 selective facets, this gives rise to enhanced interfacial charge transfer, that is, high sensing response, and selectivity towards oxidizing NO2 gas. The better gas sensing performance over (10$ \bar 2 $) compared to (11$ \bar 3 $) faceted SnO2 nanostructures is elucidated by surface energetic calculations and Bader analyses. This work highlights the possibility of simultaneous engineering of surface energetics and electronic properties of SnO2 based materials. Flower‐like hierarchical SnO2 nanostructures are assembled from single‐crystalline SnO2 nanosheets with high‐index (11$ \bar 3 $) and (10$ \bar 2 $) exposed facets. Sn2+ self‐doping leads to formation of tunable oxygen vacancy bandgap states and extended absorption in the visible spectral range. This work highlights the possibility of simultaneous engineering of surface energetics and electronic properties of SnO2 based materials.
  PubDate: 2013-04-09T03:10:32.91152-05:0
  DOI: 10.1002/adfm.201300303
Hindering the Oxidation of Silicene with Non‐Reactive Encapsulation
- Authors: Alessandro Molle; Carlo Grazianetti, Daniele Chiappe, Eugenio Cinquanta, Elena Cianci, Grazia Tallarida, Marco Fanciulli
  Pages: n/a - n/a
  Abstract: The chemical stability of buckled silicene, i.e., the silicon counterpart of graphene, is investigated then resulting in a low reactivity with O2 when dosing up to 1000 L and in a progressive oxidation under ambient conditions. The latter drawback is addressed by engineering ad hoc Al‐ and Al2O3‐based encapsulations of the silicene layer. This encapsulation design can be generally applied to any silicene configuration, irrespective of the support substrate, and it leads to the fabrication of atomically sharp and chemically intact Al/silicene and Al2O3/silicene interfaces that can be functionally used for ex situ characterization as well as for gated device fabrication. The chemical stability of epitaxial silicene results in a low reactivity with O2 when dosing up to 1000 L and in a progressive oxidation under ambient conditions. Non‐destructive Al‐ and Al2O3‐based encapsulation approaches that can be exploited for ex situ characterization of silicene and for gated silicene devices (irrespective of the support substrate) are engineered.
  PubDate: 2013-04-09T03:10:27.165343-05:
  DOI: 10.1002/adfm.201300354
Tuning of the Photovoltaic Parameters of Molecular Donors by Covalent Bridging
- Authors: Dora Demeter; Victorien Jeux, Philippe Leriche, Philippe Blanchard, Yoann Olivier, Jérôme Cornil, Riccardo Po, Jean Roncali
  Pages: n/a - n/a
  Abstract: The synthesis of donor‐acceptor molecules involving triarylamines and dicyanovinyl blocks is described. Optical and electrochemical results show that rigidification of the acceptor part of the molecule by a covalent bridge leads to a ca. 0.20 eV increase of the band gap due to a parallel increase of the lowest unoccupied molecular orbital level. A preliminary evaluation of these compounds as donor materials in organic solar cells shows that although this structural modification reduces the light‐harvesting properties of the donor molecule, it nevertheless induces an increase of the efficiency of the resulting solar cells due to a simultaneous improvement of the open‐circuit voltage and fill factor. Covalent bridging of the dicyanovinyl group with the adjacent thiophene ring of small push‐pull molecular donors based on triarylamines leads to a significant improvement of the efficiency of the resulting organic solar cells due to the simultaneous increase of the fill factor and open‐circuit voltage.
  PubDate: 2013-04-09T03:10:18.565576-05:
  DOI: 10.1002/adfm.201300427
A Low‐Toxic Multifunctional Nanoplatform Based on Cu9S5@mSiO2 Core‐Shell Nanocomposites: Combining Photothermal‐ and Chemotherapies with Infrared Thermal Imaging for Cancer Treatment
- Authors: Guosheng Song; Qian Wang, Yang Wang, Gang Lv, Chun Li, Rujia Zou, Zhigang Chen, Zongyi Qin, Keke Huo, Ronggui Hu, Junqing Hu
  Pages: n/a - n/a
  Abstract: Copper chalcogenides have been demonstrated to be a promising photothermal agent due to their high photothermal conversion efficiency, synthetic simplicity, and low cost. However, the hydrophobic and less biocompatible characteristics associated with their synthetic processes hamper widely biological applications. An alternative strategy for improving hydrophilicity and biocompatibility is to coat the copper chalcogenide nanomaterials with silica shell. Herein, the rational preparation design results in successful coating mesoporous silica (mSiO2) on as‐synthesized Cu9S5 nanocrystals, forming Cu9S5@mSiO2‐PEG core‐shell nanostructures. As‐prepared Cu9S5@mSiO2‐PEG core‐shell nanostructures show low cytotoxicity and excellent blood compatibility, and are effectively employed for photothermal ablation of cancer cells and infrared thermal imaging. Moreover, anticancer drug of doxorubicin (DOX)‐loaded Cu9S5@mSiO2‐PEG core‐shell nanostructures show pH sensitive release profile and are therefore beneficial to delivery of DOX into cancer cells for chemotherapy. Importantly, the combination of photothermal‐ and chemotherapies demonstrates better effects of therapy on cancer treatment than individual therapy approaches in vitro and in vivo. A multifunctional nanoplatform based on the Cu9S5@mSiO2‐PEG core‐shell nanocomposites demonstrates an excellent biocompatibility and can be used for combining photothermal‐ and chemotherapies with infrared thermal imaging of cancer treatment.
  PubDate: 2013-04-09T03:10:08.481183-05:
  DOI: 10.1002/adfm.201203317
Large‐Area Flexible Core–Shell Graphene/Porous Carbon Woven Fabric Films for Fiber Supercapacitor Electrodes
- Authors: Xiao Li; Xiaobei Zang, Zhen Li, Xinming Li, Peixu Li, Pengzhan Sun, Xiao Lee, Rujing Zhang, Zhenghong Huang, Kunlin Wang, Dehai Wu, Feiyu Kang, Hongwei Zhu
  Pages: n/a - n/a
  Abstract: New porous materials are of great importance in many technological applications. Here, the direct synthesis of multi‐layer graphene and porous carbon woven composite films by chemical vapor deposition on Ni gauze templates is reported. The composite films integrate the dual advantages of graphene and porous carbon, having not only the excellent electrical properties and flexibility of graphene but also the porous characteristics of amorphous carbon. The multi‐layer graphene/porous carbon woven fabric film creates a new platform for a variety of applications, such as fiber supercapacitors. The designed composite film has a capacitance of 20 μF/cm2, which is close to the theoretical value and a device areal capacitance of 44 mF/cm2. Direct synthesis of multi‐layer graphene and porous carbon woven composite films by chemical vapor deposition is reported. The composite films integrate the dual advantages of graphene and porous carbon, having not only the excellent electrical properties and flexibility of graphene but also the porous characteristics of amorphous carbon, creating a new platform for a variety of applications, such as supercapacitors.
  PubDate: 2013-04-08T04:10:25.978278-05:
  DOI: 10.1002/adfm.201300464
Low Roll‐Off and High Efficiency Orange Organic Light Emitting Diodes with Controlled Co‐Doping of Green and Red Phosphorescent Dopants in an Exciplex Forming Co‐Host
- Authors: Sunghun Lee; Kwon‐Hyeon Kim, Daniel Limbach, Young‐Seo Park, Jang‐Joo Kim
  Pages: n/a - n/a
  Abstract: An exciplex forming co‐host is introduced in order to fabricate orange organic light‐emitting diodes (OLEDs) with high efficiency, low driving voltage and an extremely low efficiency roll‐off, by the co‐doping of green and red emitting phosphorescence dyes in the host. The orange OLEDs achieves a low turn‐on voltage of 2.4 V, which is equivalent to the triplet energy gap of the phosphorescent‐green emitting dopant, and a very high external quantum efficiency (EQE) of 25.0%. Moreover, the OLEDs show low efficiency roll‐off with an EQE of over 21% at 10 000 cdm−2. The device displays a very good orange color (CIE of (0.501, 0.478) at 1000 cdm−2) with very little color shift with increasing luminance. The transient electroluminescence of the OLEDs indicate that both energy transfer and direct charge trapping takes place in the devices. An orange organic light emitting diode (OLED) with controlled co‐doping of green and red phosphorescent dopants achieves a low turn‐on voltage of 2.4 V and a very high EQE of 25.0%. The OLED demonstrates an external quantum efficiency over 21% at 10 000 cdm−2. The orange OLED displays a very good orange color (Commission Internationaled'Eclairage of (0.501, 0.478) at 1000 cdm−2) with very little color shift with increasing luminance.
  PubDate: 2013-04-08T04:10:20.645137-05:
  DOI: 10.1002/adfm.201300187
A New Type of Efficient CO2 Adsorbent with Improved Thermal Stability: Self‐Assembled Nanohybrids with Optimized Microporosity and Gas Adsorption Functions
- Authors: Tae Woo Kim; In Young Kim, Tae Sung Jung, Chang Hyun Ko, Seong‐Ju Hwang
  Pages: n/a - n/a
  Abstract: A new type of efficient CO2 absorbent with improved thermal stability is synthesized via self‐assembly between 2D inorganic nanosheets and two kinds of 0D inorganic nanoclusters. In these self‐assembled nanohybrids, the nanoclusters of CdO and Cr2O3 are commonly interstratified with layered titanate nanosheets, leading to the formation of highly microporous pillared structure with increased basicity of pore wall. The co‐pillaring of basic CdO with Cr2O3 is fairly effective at increasing a proportion of micropores and reactivity for CO2 molecules and at improving the thermal stability of the resulting porous structure. Of prime importance is that the present inorganic‐pillared nanohybrids show highly efficient CO2 adsorption capacity, which is much superior to those of many other absorbents and compatible to those of CO2 adsorbing metal−organic framework (MOF) compounds. Taking into account an excellent thermal stability of the present nanohybrids, these materials are very promising CO2 adsorbents usable at elevated temperature. This is the first example of efficient CO2 adsorbent from pillared materials. The co‐pillaring of basic metal oxide nanoclusters employed in this study can provide a very powerful way of developing thermally stable CO2 adsorbents from many known pillared systems. An effective way to synthesize efficient CO2 adsorbents from pillared materials is developed on the basis of an electrostatically derived self‐assembly between exfoliated 2D nanosheets and two kinds of guest nanoclusters. The co‐pillaring of basic CdO with Cr2O3 makes it possible to improve the microporosity and basicity of the resulting pore structure and to enhance the CO2 adsorption function and thermal stability of pillared materials.
  PubDate: 2013-04-08T04:10:18.278259-05:
  DOI: 10.1002/adfm.201300071
Tuning Molecular Adhesion via Material Anisotropy
- Authors: Wenliang Zhang; Yuan Lin, Jin Qian, Weiqiu Chen, Huajian Gao
  Pages: n/a - n/a
  Abstract: Cell adhesion with extracellular matrix depends on the collective behaviors of a large number of receptor‐ligand bonds at the compliant cell‐matrix interface. While most biological tissues and structures, including cells and extracellular matrices, exhibit strongly anisotropic material properties, existing studies on molecular adhesion via receptor‐ligand bonds have been largely limited to isotropic materials. Here the effects of transverse isotropy, a common form of material anisotropy in biological systems, in modulating the adhesion behavior of a cluster of receptor‐ligand bonds are investigated. The results provide a theoretical basis to understand cell adhesion on anisotropic extracellular matrices and to explore the possibility of controlling cell adhesion via anisotropy design in material properties. The combined analysis and simulations show that the orientation of material anisotropy strongly affects the apparent softness felt by the adhesive bonds, thereby altering their ensemble lifetime by several orders of magnitude. An implication of this study is that distinct cellular behaviors can be achieved through remodeling of material anisotropy in either extracellular matrix or cytoskeleton. Comparison between different loading conditions, together with the effects of material anisotropy, yields a rich array of out‐of‐equilibrium behaviors in the molecular interaction between reactant‐bearing soft surfaces, with important implications on the mechanosensitivity of cells. The collective behaviors of multiple receptor‐ligand bonds between a cell and a transversely isotropic matrix are investigated using a two‐level model in which the reversible processes of stochastic bond rupture and rebinding at molecular scale and the elastic response of transversely isotropic materials to bond force distributions at continuum scale are integrated.
  PubDate: 2013-04-08T03:40:20.922488-05:
  DOI: 10.1002/adfm.201300069
Near‐Field Enhanced Plasmonic‐Magnetic Bifunctional Nanotubes for Single Cell Bioanalysis
- Authors: Xiaobin Xu; Huifeng Li, Dihan Hasan, Rodney S. Ruoff, Alan X. Wang, D. L. Fan
  Pages: n/a - n/a
  Abstract: Near‐field enhanced bifunctional plasmonic‐magnetic (PM) nanostructures consisting of silica nanotubes with embedded solid nanomagnets and uniformly dual‐surface‐coated plasmonic Ag nanoparticles (NPs) are rationally synthesized. The solid embedded sections of nanotubes provide single‐molecule sensitivity with an enhancement factor up to 7.2 × 109 for surface‐enhanced Raman scattering (SERS). More than 2× SERS enhancement is observed from the hollow section compared to the solid section of the same nanotube. The substantial SERS enhancement on the hollow section is attributed to the dual‐sided coating of Ag NPs as well as the near‐field optical coupling of Ag NPs across the nanotube walls. Experimentation and modeling are carried out to understand the dependence of SERS enhancement on the NP sizes, junctions, and the near field effects. By tuning the aspect ratio of the embedded nanomagnets, the magnetic anisotropy of nanotubes can be readily controlled to be parallel or vertical to the long directions for nano‐manipulation. Leveraging the bifunctionality, a nanotube is magnetically maneuvered to a single living mammalian cell amidst many and its membrane composition is analyzed via SERS spectroscopy. Unique near‐field enhanced plasmonic‐magnetic bifunctional nanotubes are fabricated and their plasmonic properties are investigated by both experimentation and theoretical modeling. By leveraging the bifunctionality, a nanotube can be precisely transported to a single living Chinese hamster ovary (CHO) cell amidst many and its membrane chemistry (lipid and protein) is revealed with surface‐enhanced Raman scattering (SERS) spectroscopy.
  PubDate: 2013-04-08T03:40:16.86445-05:0
  DOI: 10.1002/adfm.201203822
Scanning Probe Microscopy of Nanocomposite Membranes and Dynamic Organization
- Authors: Gabriel A. Montaño; Peter G. Adams, Xiaoyin Xiao, Peter M. Goodwin
  Pages: n/a - n/a
  Abstract: Nanocomposite membrane assemblies are a class of materials that incorporate inorganic/organic nanoscale materials, such as fullerenes and gold nanoparticles or nanostructured materials with bio‐inspired amphiphilic structures composed of molecules such as lipids or block copolymers. One of the intrigues of such materials is the potential to develop programmable membrane assemblies that mimic biological membrane complexity, dynamics and function. Due to the nanoscale nature of the assemblies, it becomes necessary to understand interactions between these materials with nanoscale resolution. Although many techniques are able to provide information as to the overall organization of membrane‐based assemblies, only scanning probe microscopy (SPM) methods allow for a direct visualization of stochastic processes under environmentally relevant conditions. Here, an overview of nanocomposite membrane and thin film architecture investigations is presented with an emphasis on using in situ atomic force microscopy (AFM) in combination with fluorescence microscopy/spectroscopy techniques to understand organization and dynamics, in relation to activities and capabilities at the Center for Integrated Nanotechnologies. Scanning probe microscopy allows for high‐resolution characterization and manipulation of nanocomposite membranes. In this Feature Article, several examples are shown and their characterization/manipulation are discussed including lipid/nanoparticle composites and grafted mixed polymer systems shown.
  PubDate: 2013-04-08T03:40:08.152777-05:
  DOI: 10.1002/adfm.201203429
Unlocking the Latent Antimicrobial Potential of Biomimetically Synthesized Inorganic Materials
- Authors: Matthew B. Dickerson; Wanda J. Lyon, William E. Gruner, Peter A. Mirau, Michael L. Jespersen, Yunnan Fang, Kenneth H. Sandhage, Rajesh R. Naik
  Pages: n/a - n/a
  Abstract: Inspired by biomineralization, biomimetic approaches utilize biomolecules and synthetic analogs to produce materials of controlled chemistry, morphology, and function under relatively benign conditions. A common characteristic of biological and biomimetic mineral‐forming processes is the generation of mineral/biomolecule nanocomposites. In this work, it is demonstrated that a facile chemical reaction may be utilized to halogenate the nitrogen‐containing moieties of the organics entrapped within bio‐inorganic composites to yield halamine compounds. This process provides rapid and potent bactericidal activity to biomimetically and biologically produced materials that otherwise lack such functionality. Additionally, bio‐inorganic composites containing the chlorinated peptide protamine are effective in rapidly neutralizing Bacillus spores (≥99.97% reduction in colony forming units within 10 min). The straightforward nature of the described process, and the efficacy of halamine compounds in neutralizing biological and chemical agents, provide new applicability to biogenic and biomimetic materials. Bio‐inspired synthesis techniques are capable of producing bio‐inorganic composite materials under benign reaction conditions. A facile method for the antimicrobial functionalization of such composites is described. The nitrogen moieties associated with the biopolymers entrapped within the hybrid material are chlorinated in situ to yield halamine compounds. These halamine‐charged materials exhibit potent bactericidal and sporicidal activity.
  PubDate: 2013-04-05T07:10:21.544007-05:
  DOI: 10.1002/adfm.201202851
Niobium Nanowire Yarns and their Application as Artificial Muscles
- Authors: Seyed M. Mirvakili; Alexey Pazukha, William Sikkema, Chad W. Sinclair, Geoffrey M. Spinks, Ray H. Baughman, John D. W. Madden
  Pages: n/a - n/a
  Abstract: Metal nanowires are twisted to form yarns that are strong (0.4 to 1.1 GPa), pliable, and more conductive (3 × 106 S m−1) than carbon nanotube yarns. Niobium nanowire fibers are extracted by etching a copper‐niobium nano‐composite material fabricated using the severe plastic deformation process. When impregnated with paraffin wax, the niobium (Nb) nanowire yarns produce fast rotational actuation as the wax is heated. The heated wax expands, untwisting the yarn, which then re‐twists upon cooling. Normalized to yarn length, 12 deg mm−1 of torsional rotation was achieved along with twist rates in excess of 1800 rpm. Tensile modulus of 19 ± 5 GPa was measured for the Nb yarns, which is very similar to those of carbon multiwalled nanotubes. Niobium nanowires are twisted to form strong (0.4 to 1.1 GPa) and highly conductive (3 × 106 S m−1) yarns. Impregnation with paraffin wax produces linear and large torsional actuation in response to heating and cooling.
  PubDate: 2013-04-05T07:10:19.463999-05:
  DOI: 10.1002/adfm.201203808
Structure‐Properties Relationship in Iron Oxide‐Reduced Graphene Oxide Nanostructures for Li‐Ion Batteries
- Authors: Seung‐Ho Yu; Donato E. Conte, Seunghwan Baek, Dong‐Chan Lee, Seung‐Keun Park, Kyung Jae Lee, Yuanzhe Piao, Yung‐Eun Sung, Nicola Pinna
  Pages: n/a - n/a
  Abstract: Non‐aqueous sol‐gel routes involving the reaction of metal oxide precursors in organic solvents (e.g., benzyl alcohol) at moderate temperature and pressure, offer advantages such as high purity, high reproducibility and the ability to control the crystal growth without the need of using additional ligands. In this paper, a study carried out on a series of iron oxide/reduced graphene oxide composites is presented to elucidate a structure‐properties relationship leading to an improved electrochemical performance of such composites. Moreover, it is demonstrated that the easy production of the composites in a variety of temperature and composition ranges, allows a fine control over the final particles size, density and distribution. The materials obtained are remarkable in terms of the particle's size homogeneity and dispersion onto the reduced graphene oxide surface. Moreover, the synthesis method used to obtain the graphene oxide clearly affects the performances of the final composites through the control of the restacking of the reduced graphene oxide sheets. It is shown that a homogeneous and less defective reduced graphene oxide enables good electrochemical performances even at high current densities (over 500 mAh/g delivered at current densities as high as 1600 mA/g). The electrochemical properties of improved samples reach the best compromise between specific capacity, rate capability and cycle stability reported so far. A series of iron oxide/reduced graphene oxide composites are synthesized in one‐step by a simple microwave‐assisted non‐aqueous sol‐gel route in just few minutes. The precise characterization and comparison between the sample studied allow elucidation of structure–property relationships and the improvement of electrochemical performance targeting Li‐Ion battery applications.
  PubDate: 2013-04-05T07:10:15.939561-05:
  DOI: 10.1002/adfm.201300190
Giant Temperature Coefficient of Resistance in Carbon Nanotube/Phase‐Change Polymer Nanocomposites
- Authors: Gustavo E. Fernandes; Jin Ho Kim, Ashok K. Sood, Jimmy Xu
  Pages: n/a - n/a
  Abstract: The temperature coefficient of resistance of a carbon nanotube nanocomposite with the non‐conductive phase‐change hydrogel Poly(N‐isopropylacrylamide) is studied. This nanocomposite is found to achieve the largest reported temperature coefficient of resistance, ≈−10%/°C, observed in carbon nanotube‐polymer nanocomposites to date. The giant temperature coefficients of resistance results from a volume‐phase‐transition that is induced by the humidity present in the surrounding atmosphere and that enhances the temperature dependence of the resistivity via direct changes in the tunneling resistance that electrons experience in moving between nearby carbon nanotubes. The bolometric photoresponses of this new material are also studied. The nanocomposite's enhanced responses to temperature and humidity give it great potential for sensor applications and uncooled infrared detection. A carbon nanotube phase‐change nanocomposite with a hydrogel achieves giant temperature coefficients of resistance resulting from a phase‐transition that directly changes the tunneling potential that electrons experience in moving between nearby nanotubes. The bolometric photoresponses of this material are studied and its giant responses to temperature and humidity give it great potential for sensor applications and uncooled infrared detection.
  PubDate: 2013-04-05T07:10:11.941467-05:
  DOI: 10.1002/adfm.201300208
Room‐Temperature Nanosoldering of a Very Long Metal Nanowire Network by Conducting‐Polymer‐Assisted Joining for a Flexible Touch‐Panel Application
- Authors: Jinhwan Lee; Phillip Lee, Ha Beom Lee, Sukjoon Hong, Inhwa Lee, Junyeob Yeo, Seung Seob Lee, Taek‐Soo Kim, Dongjin Lee, Seung Hwan Ko
  Pages: n/a - n/a
  Abstract: As an alternative to the brittle and expensive indium tin oxide (ITO) transparent conductor, a very simple, room‐temperature nanosoldering method of Ag nanowire percolation network is developed with conducting polymer to demonstrate highly flexible and even stretchable transparent conductors. The drying conducting polymer on Ag nanowire percolation network is used as a nanosoldering material inducing strong capillary‐force‐assisted stiction of the nanowires to other nanowires or to the substrate to enhance the electrical conductivity, mechanical stability, and adhesion to the substrate of the nanowire percolation network without the conventional high‐temperature annealing step. Highly bendable Ag nanowire/conducting polymer hybrid films with low sheet resistance and high transmittance are demonstrated on a plastic substrate. The fabricated flexible transparent electrode maintains its conductivity over 20 000 cyclic bends and 5 to 10% stretching. Finally, a large area (A4‐size) transparent conductor and a flexible touch panel on a non‐flat surface are fabricated to demonstrate the possibility of cost‐effective mass production as well as the applicability to the unconventional arbitrary soft surfaces. These results suggest that this is an important step toward producing intelligent and multifunctional soft electric devices as friendly human/electronics interface, and it may ultimately contribute to the applications in wearable computers. A very simple, room‐temperature nanosoldering of a Ag nanowire percolation network by conducting‐polymer‐assisted nanowire joining is developed to demonstrate highly flexible, and even stretchable, transparent conductors. Furthermore, a large area (A4‐size) transparent conductor and a flexible touch panel on a non‐flat surface are fabricated to demonstrate the possibility of cost‐effective mass production and the applicability to the unconventional arbitrary soft, non‐flat surfaces.
  PubDate: 2013-04-05T04:10:22.668379-05:
  DOI: 10.1002/adfm.201203802
In Situ Observations and Tuning of Physical and Chemical Phenomena on the Surfaces of Strongly Correlated Oxides
- Authors: Zheng Gai; S. V. Kalinin, An‐Ping Li, Jian Shen, A. P. Baddorf
  Pages: n/a - n/a
  Abstract: The characteristic aspect of strongly correlated oxides systems is the strong coupling between the structural, electronic and magnetic properties. A small change in one property can produce a large change in another. Controllable surface tuning provides the opportunity to study how structural, electronic, and magnetic properties respond to the broken symmetry and opens avenues for exploration of completely new physical properties. The extreme sensitivity of properties to external chemical and physical stimuli makes in situ characterization a requirement for controlled tuning of complex correlated materials. This paper reviews some recent progress in in situ observations and tuning of physical and chemical phenomena on the surfaces of strongly correlated oxides thin films and crystals, including atomic‐level structural studies, control, and tuning of the physical properties. Controllable surface tuning provides the opportunity to study how structural, electronic, and magnetic properties respond to the broken symmetry and opens avenues for exploration of completely new physical properties. This paper reviews some recent progress in in situ observations and tuning of physical and chemical phenomena on the surfaces of strongly correlated oxides thin films and crystals, including atomic‐level structural studies, control, and tuning of the physical properties.
  PubDate: 2013-04-05T04:10:17.824554-05:
  DOI: 10.1002/adfm.201203425
Solution‐pH‐Modulated Rectification of Ionic Current in Highly Ordered Nanochannel Arrays Patterned with Chemical Functional Groups at Designed Positions
- Authors: Cheng‐Yong Li; Feng‐Xiang Ma, Zeng‐Qiang Wu, Hong‐Li Gao, Wen‐Ting Shao, Kang Wang, Xing‐Hua Xia
  Pages: n/a - n/a
  Abstract: A new ionic current rectification device responsive to a broad range of pH stimuli is established using highly ordered nanochannels of porous anodic alumina membrane with abrupt surface charge discontinuity. The asymmetric surface charge distribution is achieved by patterning the nanochannels with surface amine functional groups at designed positions using a two‐step anodization process. Due to the protonation/deprotonation of the patterned amine and the remaining intrinsic hydroxyl groups upon solution pH variation, the nanochannel‐array‐based device is able to regulate ion transport selectivity and has ionic current rectification properties. The rectification ratio of the device is mainly determined by the nanochannel size, and the rectification ratio is less sensitive to the patterned length of the amine groups when the nanochannels size is defined. Thus, the isoelectric point of nanochannels can be easily estimated to be the pH value with a unit rectification ratio. The present ionic device is promising for biosensing, molecular transport and separation, and drug delivery in confined environments. A new ionic current rectification device responsive to a broad range of pH stimuli is established using highly ordered nanochannels of porous anodic alumina membrane with abrupt surface charge discontinuity. Due to the protonation/deprotonation of the patterned amine and the remaining intrinsic hydroxyl groups, the nanochannel‐array‐based device is able to regulate ion transport selectivity and has ionic current rectification properties.
  PubDate: 2013-04-04T02:10:12.766546-05:
  DOI: 10.1002/adfm.201300315
Superior Mass Transfer Properties of Technical Zeolite Bodies with Hierarchical Porosity
- Authors: Laurent Gueudré; Maria Milina, Sharon Mitchell, Javier Pérez‐Ramírez
  Pages: n/a - n/a
  Abstract: Mass transfer in zeolite crystals can be enhanced by the introduction of a hierarchical network of auxiliary mesopores. To fully exploit pore engineering in the design of more efficient industrial catalysts, the benefit needs to be demonstrated over technically relevant forms. Here, the influence of shaping on the adsorption and diffusion properties of hierarchical ZSM‐5 is assessed by studying the gravimetric uptake of 2,2‐dimethylbutane over powders and millimeter‐sized bodies. Formed by extrusion or granulation with clay binders, the latter display a complex trimodal network of micro‐, meso‐, and macropores. The enhanced intracrystalline diffusivity due to the interconnected mesopores is preserved in the macroscopic bodies, independent of the shaping method or binder applied. Furthermore the superior overall diffusivity is retained in the hierarchical bodies compared to their conventional (purely microporous) counterparts, despite the significant extracrystalline resistance to mass transfer. The connective participation of mesopores, leading to a 6 times improved effective diffusivity in hierarchical with respect to conventional zeolite powders, is revealed by the distinct dependence on the adsorbate concentration and the relationship with the mesopore surface area. Analysis of the thermodynamic parameters derived from the adsorption isotherm proves a sensitive method to detect binder‐zeolite interactions induced upon shaping. The first systematic study of the impact of shaping on the beneficial effects of intracrystalline mesopores on mass transfer in hierarchical ZSM‐5 is presented. Using gravimetric adsorption studies of a bulky alkane probe, the enhancing effect of the developed mesopores can be preserved in multicomponent millimeter‐sized granules and extrudates.
  PubDate: 2013-04-04T02:10:08.043639-05:
  DOI: 10.1002/adfm.201203557
Two‐Dimensional Molybdenum Trioxide and Dichalcogenides
- Authors: Sivacarendran Balendhran; Sumeet Walia, Hussein Nili, Jian Zhen Ou, Serge Zhuiykov, Richard B. Kaner, Sharath Sriram, Madhu Bhaskaran, Kourosh Kalantar‐zadeh
  Pages: n/a - n/a
  Abstract: In the quest to discover the properties of planar semiconductors, two‐dimensional molybdenum trioxide and dichalcogenides have recently attracted a large amount of interest. This family, which includes molybdenum trioxide (MoO3), disulphide (MoS2), diselenide (MoSe2) and ditelluride (MoTe2), possesses many unique properties that make its compounds appealing for a wide range of applications. These properties can be thickness dependent and may be manipulated via a large number of physical and chemical processes. In this Feature Article, a comprehensive review is delivered of the fundamental properties, synthesis techniques and applications of layered and planar MoO3, MoS2, MoSe2, and MoTe2 along with their future prospects. In the quest for alternative two‐dimensional semiconductors, layered molybdenum trioxide and dichalcogenides are gaining significant scientific interest. This Feature Article delivers a comprehensive review of the fundamental properties, synthesis techniques, and applications of layered MoO3, MoS2, MoSe2, and MoTe2 as well as exploring their future prospects in the field of two‐dimensional semiconductors.
  PubDate: 2013-04-02T02:40:53.281191-05:
  DOI: 10.1002/adfm.201300125
Universality of Polarization Switching Dynamics in Ferroelectric Capacitors Revealed by 5D Piezoresponse Force Microscopy
- Authors: Yunseok Kim; Xiaoli Lu, Stephen Jesse, Dietrich Hesse, Marin Alexe, Sergei V. Kalinin
  Pages: n/a - n/a
  Abstract: Ferroelectric polarization switching is sensitively affected by phenomena on multiple length scales, giving rise to complex voltage‐ and time‐controlled behaviors. Here, spatially resolved switching dynamics in ferroelectric nanocapacitors are explored as a function of voltage pulse time and magnitude. A remarkable persistence of formal macroscopic scaling laws for polarization switching based on classical models down to nanoscale volumes is observed. These observations illustrate the persistence of the return point memory in the material and allow the thermodynamic parameters of defects controlling switching to be estimated. Ferroelectric polarization switching is sensitively affected by phenomena on multiple length scales, giving rise to complex voltage‐ and time‐controlled behavior. Spatially resolved switching dynamics in ferroelectric nanocapacitors are explored as a function of voltage pulse time and magnitude. A remarkable persistence of formal macroscopic scaling laws for polarization switching based on classical models down to nanoscale volumes is observed.
  PubDate: 2013-04-02T02:40:45.740954-05:
  DOI: 10.1002/adfm.201300079
Origin of Reduced Bimolecular Recombination in Blends of Conjugated Polymers and Fullerenes
- Authors: D. H. K. Murthy; Armantas Melianas, Zheng Tang, Gytis Juška, Kȩstutis Arlauskas, Fengling Zhang, Laurens D. A. Siebbeles, Olle Inganäs, Tom J. Savenije
  Pages: n/a - n/a
  Abstract: Bimolecular charge carrier recombination in blends of a conjugated copolymer based on a thiophene and quinoxaline (TQ1) with a fullerene derivative ((6,6)‐phenyl‐C71‐butyric acidmethyl ester, PC71BM) is studied by two complementary techniques. TRMC (time‐resolved microwave conductance) monitors the conductance of photogenerated mobile charge carriers locally on a timescale of nanoseconds, while using photo‐CELIV (charge extraction by linearly increasing voltage) charge carrier dynamics are monitored on a macroscopic scale and over tens of microseconds. Despite these significant differences in the length and time scales, both techniques show a reduced Langevin recombination with a prefactor ζ close to 0.05. For TQ1:PC71BM blends, the ζ value is independent of temperature. On comparing TRMC data with electroluminescence measurements it is concluded that the encounter complex and the charge transfer state have very similar energetic properties. The ζ value for annealed poly(3‐hexylthiophene) (P3HT):(6,6)‐phenyl‐C61‐butyric acid methyl ester (PC61BM) is approximately 10−4, while for blend systems containing an amorphous polymer ζ values are close to 1. These large differences can be related to the extent of charge delocalization of opposite charges in an encounter complex. Insight is provided into factors governing the bimolecular recombination process, which forms a major loss mechanism limiting the efficiency of polymer solar cells. The measured bimolecular recombination rate of electrons and holes in a polymer:fullerene blend is 20 times smaller than the Langevin rate. The reduced rate is explained in terms of dissociation of electron‐hole encounter complexes into free charge carriers rather than decay to the ground state.
  PubDate: 2013-04-02T02:40:39.846076-05:
  DOI: 10.1002/adfm.201203852
Bright White Scattering from Protein Spheres in Color Changing, Flexible Cuttlefish Skin
- Authors: Lydia M. Mäthger; Stephen L. Senft, Meng Gao, Sinan Karaveli, George R. R. Bell, Rashid Zia, Alan M. Kuzirian, Patrick B. Dennis, Wendy J. Crookes‐Goodson, Rajesh R. Naik, George W. Kattawar, Roger T. Hanlon
  Pages: n/a - n/a
  Abstract: Throughout nature, elegant biophotonic structures have evolved into sophisticated arrangements of pigments and structural reflectors that manipulate light in the skin, cuticles, feathers and fur of animals. Not many spherical biophotonic structures are known and those described are often angle dependent or spectrally tuned. White light scattering by the flexible skin of cuttlefish (Sepia officinalis) is examined and how the unique structure and composition of leucophore cells serve as physiologically passive reflectors approximating the optical properties of a broadband Lambertian surface is investigated. Leucophores are cells that contain thousands of spherical microparticles called leucosomes that consist of sulfated glycoproteins or proteoglycans and reflectin. A leucophore containing ≈12 000 leucosome microspheres is characterized three‐dimensionally by electron microscopy and the average refractive index of individual leucosomes is measured by holographic microscopy to be 1.51 ± 0.02. Modeling of the ultrastructural data and spectral measurements with Lorenz‐Mie theory and Monte Carlo simulations suggest that leucophore whiteness is produced by incoherent scattering based upon a randomly ordered system. These soft, compliant, glycosylated proteinacious spheres may provide a template for bio‐inspired approaches to efficient light scattering in materials science and optical engineering. White light scattering cells in the flexible skin of cuttlefish (Sepia officinalis) are described. How the structure and composition of leucophores serve as passive reflectors approximating optical properties of a broadband Lambertian surface is investigated. The cuttlefish system may provide a template for bio‐inspired approaches to efficient light scattering in materials science and optical engineering.
  PubDate: 2013-04-02T02:40:35.423321-05:
  DOI: 10.1002/adfm.201203705
Polarization Dynamics in Ferroelectric Capacitors: Local Perspective on Emergent Collective Behavior and Memory Effects
- Authors: Rama Vasudevan; Daniel Marincel, Stephen Jesses, Yunseok Kim, Amit Kumar, Sergei Kalinin, Susan Trolier‐McKinstry
  Pages: n/a - n/a
  Abstract: Functional properties of ferroelectric materials depend both on the residual domain states and on the mobility of domain walls in response to the applied electric and stress fields. This paper reviews the use of multidimensional scanning probe microscopy to assess these factors in the time‐ and voltage domains, with an emphasis on the manner in which domain walls respond collectively to stimuli. It is found that in many PbZr1‐xTixO3‐based capacitors, domain wall motion is correlated over length scales that exceed the domain and grain sizes by orders of magnitude, suggesting emergent collective electromechanical behavior. The role of mechanical boundary conditions and field history on the domain wall contributions and the stability of the ferroelectric domain state are discussed. Functional properties of ferroelectric materials depend both on the residual domain states and on the mobility of domain walls in response to the applied electric and stress fields. Through advanced spectroscopic techniques on model ferroelectric capacitors, it is possible to determine the domain nucleation sites and domain wall velocity as a function of pulse amplitude and widths, in a single experiment.
  PubDate: 2013-04-02T02:40:28.798416-05:
  DOI: 10.1002/adfm.201203422
Scanning Near‐Field Microwave Microscopy of VO2 and Chemical Vapor Deposition Graphene
- Authors: Alexander Tselev; Nickolay V. Lavrik, Andrei Kolmakov, Sergei V. Kalinin
  Pages: n/a - n/a
  Abstract: Near‐field scanning microwave microscopy (SMM) is a near‐field technique, which enables probing local electric properties of materials, i.e., complex permittivity. Recently, this technique was incorporated into a commercially available atomic force microscope (AFM), providing a new powerful imaging mode in the suite of AFM techniques. AFM probe‐surface distance control allows routine acquisition of near‐field microwave images with a lateral resolution better than 100 nm, which was previously unattainable. In this paper, work performed with an AFM‐based SMM system at the Center for Nanophase Materials Sciences at ORNL is reviewed. As an introduction, a brief general overview of the near‐field microwave microscopy is provided followed by a description of the SMM system. Application of the technique to studies of metal‐insulator phase transition in single‐crystalline nanoplatelets of vanadium dioxide is illustrated. Further, the capabilities of SMM in its application to imaging of conductivity inhomogeneities in single‐ and few‐layer graphene samples grown via different chemical vapor deposition (CVD) routes is demonstrated. The imaging of graphene illustrates the specific nature of contrast in the SMM, where the signal is described by complex numbers. To facilitate the interpretation of the contrast, a simple graphical scheme inspired by standard Nyquist plots is proposed. Near‐field scanning microwave microscopy is currently capable of routine imaging of dielectric constant and conductivity with a spatial resolution below 100 nm. The technique is illustrated with two examples: studies of metal‐insulator phase transition in single‐crystalline nanoplatelets of vanadium dioxide and imaging of conductivity inhomogeneities in single‐ and few‐layer graphene grown by chemical vapor deposition.
  PubDate: 2013-04-02T02:40:22.309754-05:
  DOI: 10.1002/adfm.201203435
Red, Green, and Blue Light‐Emitting Polyfluorenes Containing a Dibenzothiophene‐S,S‐Dioxide Unit and Efficient High‐Color‐Rendering‐Index White‐Light‐Emitting Diodes Made Therefrom
- Authors: Lei Yu; Jie Liu, Sujun Hu, Ruifeng He, Wei Yang, Hongbin Wu, Junbiao Peng, Ruidong Xia, Donal D. C. Bradley
  Pages: n/a - n/a
  Abstract: A series of blue (B), green (G) and red (R) light‐emitting, 9,9‐bis(4‐(2‐ethyl‐hexyloxy)phenyl)fluorene (PPF) based polymers containing a dibenzothiophene‐S,S‐dioxide (SO) unit (PPF‐SO polymer), with an additional benzothiadiazole (BT) unit (PPF‐SO‐BT polymer) or a 4,7‐di(4‐hexylthien‐2‐yl)‐benzothiadiazole (DHTBT) unit (PPF‐SO‐DHTBT polymer) are synthesized. These polymers exhibit high fluorescence yields and good thermal stability. Light‐emitting diodes (LEDs) using PPF‐SO25, PPF‐SO15‐BT1, and PPF‐SO15‐DHTBT1 as emission polymers have maximum efficiencies LEmax = 7.0, 17.6 and 6.1 cd A−1 with CIE coordinates (0.15, 0.17), (0.37, 0.56) and (0.62, 0.36), respectively. 1D distributed feedback lasers using PPF‐SO30 as the gain medium are demonstrated, with a wavelength tuning range 467 to 487 nm and low pump energy thresholds (≥18 nJ). Blending different ratios of B (PPF‐SO), G (PPF‐SO‐BT) and R (PPF‐SO‐DHTBT) polymers allows highly efficient white polymer light‐emitting diodes (WPLEDs) to be realized. The optimized devices have an attractive color temperature close to 4700 K and an excellent color rendering index (CRI) ≥90. They are relatively stable, with the emission color remaining almost unchanged when the current densities increase from 20 to 260 mA cm−2. The use of these polymers enables WPLEDs with a superior trade‐off between device efficiency, CRI, and color stability. White polymer light‐emitting diodes with an attractive power efficiency‐color rendering index (CRI)‐color stability trade‐off are successfully realized by blending red‐green‐blue (RGB) light‐emitting polymers based on 9,9‐bis(4‐(2‐ethyl‐hexyloxy)phenyl)fluorene copolymerized with dibenzothiophene‐S,S‐dioxide (PPF‐SO, blue), and benzothiadiazole (PPF‐SO‐BT, green) or dithienylbenzothiadiazole (PPF‐SO‐DHTBT, red) units.
  PubDate: 2013-04-02T02:40:14.686093-05:
  DOI: 10.1002/adfm.201203675
Multifunctional Up‐Converting Nanocomposites with Smart Polymer Brushes Gated Mesopores for Cell Imaging and Thermo/pH Dual‐Responsive Drug Controlled Release
- Authors: Xiao Zhang; Piaoping Yang, Yunlu Dai, Ping'an Ma, Xuejiao Li, Ziyong Cheng, Zhiyao Hou, Xiaojiao Kang, Chunxia Li, Jun Lin
  Pages: n/a - n/a
  Abstract: Multifunctional nanocarriers based on the up‐conversion luminescent nanoparticles of NaYF4:Yb3+/Er3+ core (UCNPs) and thermo/pH‐coupling sensitive polymer poly[(N‐isopropylacrylamide)‐co‐(methacrylic acid)] (P(NIPAm‐co‐MAA)) gated mesoporous silica shell are reported for cancer theranostics, including fluorescence imaging, and for controlled drug release for therapy. The as‐synthesized hybrid nanospheres UCNPs@mSiO2‐P(NIPAm‐co‐MAA) show bright green up‐conversion fluorescence under 980 nm laser excitation and the thermo/pH‐sensitive polymer is active as a “valve” to moderate the diffusion of the embedded drugs in‐and‐out of the pore channels of the silica container. The anticancer drug doxorubicin hydrochloride (DOX) can be absorbed into UCNPs@mSiO2‐P(NIPAm‐co‐MAA) nanospheres and the composite drug delivery system (DDS) shows a low level of leakage at low temperature/high pH values but significantly enhanced release at higher temperature/lower pH values, exhibiting an apparent thermo/pH controlled “on‐off” drug release pattern. The as‐prepared UCNPs@mSiO2‐P(NIPAm‐co‐MAA) hybrid nanospheres can be used as bioimaging agents and biomonitors to track the extent of drug release. The reported multifunctional nanocarriers represent a novel and versatile class of platform for simultaneous imaging and stimuli‐responsive controlled drug delivery. Multifunctional nanocarriers based on up‐conversion luminescent nanoparticles of NaYF4:Yb3+/Er3+ core (UCNPs) and thermo/pH‐coupling sensitive polymer poly[(N‐isopropylacrylamide)‐co‐(methacrylic acid)] brushes gated mesoporous silica shell are reported. They have applications in cancer theranostics, including fluorescence imaging, and for controlled drug release for therapy.
  PubDate: 2013-03-27T03:40:42.188034-05:
  DOI: 10.1002/adfm.201300136
Organic Light‐Emitting Diodes with 30% External Quantum Efficiency Based on a Horizontally Oriented Emitter
- Authors: Sei‐Yong Kim; Won‐Ik Jeong, Christian Mayr, Young‐Seo Park, Kwon‐Hyeon Kim, Jeong‐Hwan Lee, Chang‐Ki Moon, Wolfgang Brütting, Jang‐Joo Kim
  Pages: n/a - n/a
  Abstract: High‐efficiency phosphorescent organic light‐emitting diodes (OLEDs) doped with Ir(ppy)2(acac) [bis(2‐phenylpyridine)iridium(III)‐acetylacetonate] in an exciplex forming co‐host have been optically analyzed. This emitter has a preferred orientation with the horizontal to vertical dipole ratio of 0.77:0.23 as compared to 0.67:0.33 in the isotropic case. Theoretical analysis based on the orientation factor (Θ, the ratio of the horizontal dipoles to total dipoles) and the photoluminescence quantum yield (qPL) of the emitter predicts that the maximum external quantum efficiency (EQE) of the OLEDs with this emitter is about 30%, which matches very well with the experimental data, indicating that the electrical loss of the OLEDs is negligible and the device structure can be utilized as a platform to demonstrate the validity of optical modeling. Based on the results, the maximum EQE achievable for a certain emitting dye in a host can be predicted by just measuring qPL and Θ in a neat film on glass without the need to fabricate devices, which offers a universal plot of the maximum EQE as a function of qPL and Θ. High‐efficiency phosphorescent organic light‐emitting diodes (OLEDs) doped with Ir(ppy)2(acac) in an exciplex forming co‐host have a preferred horizontal emitter orientation. Based on optical analysis a maximum efficiency of the OLEDs of about 30% is calculated, which matches very well with the experimental data. Furthermore, a simple method to predict the maximum efficiency achievable with a certain emitting dye in a host matrix is suggested.
  PubDate: 2013-03-27T03:40:38.709152-05:
  DOI: 10.1002/adfm.201300104
Hierarchical Structuring in Block Copolymer Nanocomposites through Two Phase‐Separation Processes Operating on Different Time Scales
- Authors: Elina Ploshnik; Karol M. Langner, Amit Halevi, Meirav Ben‐Lulu, Axel H. E. Müller, Johannes G. E. M. Fraaije, G. J. Agur Sevink, Roy Shenhar
  Pages: n/a - n/a
  Abstract: Tailoring the size and surface chemistry of nanoparticles allows one to control their position in a block copolymer, but this is usually limited to one‐dimensional distribution across domains. Here, the hierarchical assembly of poly(ethylene oxide)‐stabilized gold nanoparticles (Au‐PEO) into hexagonally packed clusters inside mesostructured ultrathin films of polystyrene‐block‐poly(methyl methacrylate) (PS‐b‐PMMA) is described. A close examination of the structural evolution at different nanoparticle filling fractions and PEO ligand molecular weights suggests that the mechanism leading to this structure‐within‐structure is the existence of two phase separation processes operating on different time scales. The length of the PEO ligand is shown to influence not only the interparticle distances but also the phase separation processes. These conclusions are supported by novel mesoscopic simulations, which provide additional insight into the kinetic and thermodynamic factors that are responsible for this behavior. Combining block copolymers with nanoparticles that are highly incompatible with one block and only slightly incompatible with the other leads to hierarchical nanoparticle structures. While the block copolymer domains evolve, the nanoparticles segregate to the least incompatible domain. Then, they phase separate from it, forming hexagonally packed arrays within its confines, where interparticle distance is determined by the ligand length.
  PubDate: 2013-03-27T03:40:33.495118-05:
  DOI: 10.1002/adfm.201300091
Honeycomb‐Like Organized TiO2 Photoanodes with Dual Pores for Solid‐State Dye‐Sensitized Solar Cells
- Authors: Sung Hoon Ahn; Won Seok Chi, Dong Jun Kim, Sung Yeon Heo, Jong Hak Kim
  Pages: n/a - n/a
  Abstract: A solid‐state dye‐sensitized solar cell (ssDSSC) with 7.4% efficiency at 100 mW/cm2 is reported. This efficiency is one of the highest observed for N719 dye. High performance is achieved via a honeycomb‐like, organized mesoporous TiO2 photoanode with dual pores, high porosity, good interconnectivity, and excellent light scattering properties. The TiO2 photoanodes are prepared without any TiCl4 treatment via a one‐step, direct self‐assembly of hydrophilically preformed TiO2 nanocrystals and poly(vinyl chloride)‐g‐poly(oxyethylene methacrylate) (PVC‐g‐POEM) graft copolymer as a titania source and a structure‐directing agent, respectively. Upon controlling the secondary forces between the polymer/TiO2 hybrid and the solvent by varying the amounts of HCl/H2O mixture or toluene, honeycomb‐like structures are generated to improve light scattering properties. Such multifunctional nanostructures with dual pores provide good pore‐filling of solid polymer electrolyte with large volume, enhanced light harvesting and reduced charge recombination, as confirmed by reflectance spectroscopy, incident photon‐to‐electron conversion efficiency (IPCE), and electrochemical impedance spectroscopy (EIS) analysis. Meso/macroscopic, honeycomb‐like organized TiO2 photoanodes with dual pores, high porosity, good interconnectivity, and excellent light scattering properties result in high efficiency solid‐state dye‐sensitized solar cells (7.4% at 100 mW/cm2). This value is one of the highest observed for N719 dye.
  PubDate: 2013-03-27T03:40:27.64541-05:0
  DOI: 10.1002/adfm.201203851
Plasmonic Electrically Functionalized TiO2 for High‐Performance Organic Solar Cells
- Authors: Di Zhang; Wallace C. H. Choy, Fengxian Xie, Wei E. I. Sha, Xinchen Li, Baofu Ding, Kai Zhang, Fei Huang, Yong Cao
  Pages: n/a - n/a
  Abstract: Optical effects of the plasmonic structures and the materials effects of the metal nanomaterials have recently been individually studied for enhancing performance of organic solar cells (OSCs). Here, the effects of plasmonically induced carrier generation and enhanced carrier extraction of the carrier transport layer (i.e., plasmonic‐electrical effects) in OSCs are investigated. Enhanced charge extraction in TiO2 as a highly efficient electron transport layer by the incorporation of metal nanoparticles (NPs) is proposed and demonstrated. Efficient device performance is demonstrated by using Au NPs incorporated TiO2 at a plasmonic wavelength (560–600 nm), which is far longer than the originally necessary UV light. By optimizing the concentration ratio of the Au NPs in the NP‐TiO2 composite, the performances of OSCs with various polymer active layers are enhanced and efficiency of 8.74% is reached. An integrated optical and electrical model, which takes into account plasmonic‐induced hot carrier tunneling probability and extraction barrier between TiO2 and the active layer, is introduced. The enhanced charge extraction under plasmonic illumination is attributed to the strong charge injection of plasmonically excited electrons from NPs into TiO2. The mechanism favors trap filling in TiO2, which can lower the effective energy barrier and facilitate carrier transport in OSCs. Plasmonic‐induced carrier extraction enhancements (plasmonic‐electrical effects) in organic solar cells (OSCs) are investigated. Using a nanoparticle (NP)‐TiO2 composite, an enhanced efficiency of 8.74% is reached. The device can efficiently operate at plasmonic wavelengths far longer than the original UV region. The enhancement is attributed to the plasmonic‐induced charge injection process. This mechanism favors trap filling in TiO2, which facilitates carrier transport in OSCs.
  PubDate: 2013-03-27T03:40:24.426812-05:
  DOI: 10.1002/adfm.201203776
Surface‐Confined Heterometallic Molecular Dyads: Merging the Optical and Electronic Properties of Fe, Ru, and Os Terpyridyl Complexes
- Authors: Tarkeshwar Gupta; Prakash Chandra Mondal, Anup Kumar, Yekkoni Lakshmanan Jeyachandran, Michael Zharnikov
  Pages: n/a - n/a
  Abstract: Molecular assemblies of surface‐confined heterometallic molecular dyads (SURHMDs) composed of optically rich and redox‐active Fe(pytpy)2·2PF6 (Fe‐PT), Ru(pytpy)2·2PF6 (Ru‐PT) and Os(pytpy)2·2PF6 (Os‐PT) pytpy = 4′‐(4‐pyridyl)‐2,2′:6′,2″‐terpyridyl] complexes are fabricated via bottom‐up approach on SiOx based solid supports. Pairing of the two different metal‐organic complexes at a single platform results in significant enlargement of the optical window (λ = 400–800 nm), which can be of interest for potential applications. The use of the Cu‐based linker ensures intramolecular electronic communication between these complexes. In addition, SURHMDs are electrochemically stable under large numbers of read‐write cycles (103) and exhibit multiple redox states at relatively low potentials (
  PubDate: 2013-03-27T03:40:18.230965-05:
  DOI: 10.1002/adfm.201203404
Time‐Resolved Evolution of Short‐ and Long‐Range Order During the Transformation of Amorphous Calcium Carbonate to Calcite in the Sea Urchin Embryo
- Authors: Chantel C. Tester; Ching‐Hsuan Wu, Minna R. Krejci, Laura Mueller, Alex Park, Barry Lai, Si Chen, Chengjun Sun, Mahaling Balasubramanian, Derk Joester
  Pages: n/a - n/a
  Abstract: Use of amorphous precursors is a widespread strategy in biomineralization. In sea urchin embryos, controlled transformation of amorphous calcium carbonate (ACC) to calcite results in smoothly curving and branching single crystals. However, the mechanism of the disorder‐to‐order transformation remains poorly understood. Here, the use of strontium as a probe in X‐ray absorption spectroscopy (XAS) greatly facilitates investigation of the evolution of order. In pulse‐chase experiments, embryos incorporate Sr2+ from Sr‐enriched seawater into small volumes of the growing endoskeleton. During the chase, the Sr‐labeled mineral matures under physiological conditions. Based on Sr K‐edge spectra of cryo‐frozen whole embryos, it is proposed that the transformation occurs in three stages. The initially deposited calcium carbonate has short‐range order resembling synthetic hydrated ACC. Within 3 h, the short‐range order of calcite is established. Between 3 h and 24 h, the short‐range order does not change, while long‐range order increases. These results refute the notion that organisms imprint the local order of the final crystal on ACC. Furthermore, it is proposed that the intermediate is more similar to disordered calcite than to anhydrous ACC. Pulse‐chase experiments in conjunction with heavy element labeling have great potential to improve understanding of phase transformations during biomineralization. Strontium is used as a probe to investigate the structural transformation of amorphous calcium carbonate in sea urchin larval spicules. Sr K‐edge X‐ray absorption spectroscopy reveals that crystallization occurs in three stages: 1) hydrated amorphous calcium carbonate, 2) disordered calcite, and 3) mature calcite.
  PubDate: 2013-03-27T03:40:11.64977-05:0
  DOI: 10.1002/adfm.201203400
A Core‐Shell Nanoporous Pt‐Cu Catalyst with Tunable Composition and High Catalytic Activity
- Authors: Xingbo Ge; Luyang Chen, Jianli Kang, Takeshi Fujita, Akihiko Hirata, Wei Zhang, Jianhua Jiang, Mingwei Chen
  Pages: n/a - n/a
  Abstract: Composition‐controlled fabrication of bimetallic catalysts is of significance in electrochemical energy conversion and storage. A novel nanoporous Pt‐Cu bimetallic catalyst with a Pt skin and a Pt‐Cu core, fabricated by electrochemically dealloying a bulk Pt‐Cu binary alloy using a potential‐controlled approach, is reported. The Pt/Cu ratio of the dealloyed nanoporous catalyst can be readily adjusted in a wide composition range by only controlling dealloying potential. The electro‐catalytic performance of the nanoporous Pt‐Cu catalyst shows evident dependence on Pt/Cu ratio although the surfaces of all the nanoporous catalysts are characterized to be covered by pure Pt. With optimal compositions, the dealloyed nanoporous Pt‐Cu catalyst possesses enhanced electrocatalytic activities toward oxygen reduction reaction and formic acid oxidation in comparison with the commercial Pt/C catalyst. Using a potential‐controlled dealloying approach, a core‐shell nanoporous Pt‐Cu bimetallic catalyst with a widely tunable Pt/Cu ratio is fabricated. The nanoporous Pt‐Cu catalyst consists of a Pt‐Cu alloy core and a pure Pt skin, showing high catalytic activities toward the oxygen reduction reaction and formic electro‐oxidation.
  PubDate: 2013-03-26T03:10:29.484738-05:
  DOI: 10.1002/adfm.201300114
Controllable Synthesis of Mesoporous TiO2 Hollow Shells: Toward an Efficient Photocatalyst
- Authors: Ji Bong Joo; Ilkeun Lee, Michael Dahl, Geon Dae Moon, Francisco Zaera, Yadong Yin
  Pages: n/a - n/a
  Abstract: TiO2 hollow shells with well‐controlled crystallinity, phase, and porosity are desirable in many applications. In photocatalysis in particular, they can provide high active surface area, reduced diffusion resistance, and improved accessibility to reactants. Here, the results from studies of the causes for the failure of a prior etching and calcination scheme to make such shells and on a newly‐developed simple yet robust process for producing uniform mesoporous TiO2 shells with precisely controllable crystallinity and phase are reported. The key finding is that base etching of the SiO2@TiO2 core‐shell particles leads to the formation of sodium titanate species, which, if not removed, promote substantial crystal growth during calcination and destroy the structural integrity of the TiO2 shells. A simple acid treatment of the base‐etched samples may convert the sodium titanates into protonated titanates, which not only prevent the formation of the impurity phases, but also help to maintain the structural integrity of the shell and allow precise control of the TiO2 phase and crystallinity. This new development affords convenient optimization of the structure of the hollow TiO2 shells toward efficient photocatalysts, which outperform the commercial P25‐TiO2 in the photocatalytic decomposition of organic dye molecules. A simple yet effective method is developed for the synthesis of hollow mesoporous TiO2 nanoshells with well‐controlled crystallinity and phase. It is now convenient to optimize their structures for enhanced performance in photocatalysis.
  PubDate: 2013-03-26T02:40:48.232492-05:
  DOI: 10.1002/adfm.201300255
Cross‐Sectional Scanning Tunneling Microscopy Applied to Complex Oxide Interfaces
- Authors: Te Yu Chien; Jak Chakhalian, John W. Freeland, Nathan P. Guisinger
  Pages: n/a - n/a
  Abstract: Understanding interfacial science is critical to many modern technologies. It is very common in solid‐state physics for electronic properties to show novel phenomena when combining various dissimilar materials at atomically abrupt interfaces. For example, semiconductor interfaces have provided the foundation of modern electronic devices for several decades. Now with advances in growth and synthesis, controllable high quality complex oxide heterojunctions can be routinely fabricated. Since complex oxide materials exhibit a wide variety of functionalities owing to their strong coupling to the electron, lattice, orbital and spin degrees of freedom, these materials display a wide spectrum of interesting functionalities. Combining dissimilar complex oxides at interfaces allows one to explore and create intriguing phenomena that are not attainable in the lone bulk constituents. However, the key challenge has been the direct characterization of these interfaces at the nanoscale in order to understand the physical properties found at complex oxide interfaces. This requires the development of new experimental approaches. In this paper, we review the utilization of cross‐sectional scanning tunneling microscopy/spectroscopy as a direct probe of these oxide interfaces at the nanoscale. This technique provides valuable insight to both structural and electronic properties of these unique systems and enables understanding of the detailed electronic structure (e.g., local electronic density of states (LDOS), charge transfer, band bending, etc.) at oxide interfaces, which is of key interest to both fundamental and applied science. Cross‐sectional scanning tunneling microscopy and specstropy (XSTM/S) for complex oxides has recently been developed. XSTM/S is an ideal tool to direct probe the electronic properties at interfaces of dissimilar complex oxdies. The understanding of the emerging phenomena at complex oxide interfaces could be pushed further with the nanometer‐scale electronic information obtained by XSTM/S.
  PubDate: 2013-03-26T02:40:45.734824-05:
  DOI: 10.1002/adfm.201203430
Life Beyond Diffraction: Opening New Routes to Materials Characterization with Next‐Generation Optical Near‐Field Approaches
- Authors: P. James Schuck; Alexander Weber‐Bargioni, Paul D. Ashby, D. Frank Ogletree, Adam Schwartzberg, Stefano Cabrini
  Pages: n/a - n/a
  Abstract: Near‐field optical microscopies and spectroscopies seek to investigate materials by combining the best aspects of optical characterization and scan‐probe microscopy techniques. In principle, this provides access to chemical, morphological, physical and dynamical information at nanometer length scales that is impossible to access by other means. But a number of challenges, particularly on the scan‐probe front, have limited the widespread application of near‐field investigations. This work describes how recent probe engineering and technique innovation have addressed many of these challenges. This Feature Article begins with a short overview of the field, providing perspective and motivation for these developments and highlighting some key improvements. This is followed by a more in‐depth description of the near‐field advances developed at the Molecular Foundry, a national nanoscience User Facility–advances that provide groundwork for generally‐applicable nano‐optical studies. Finally, a discussion is provided of what progress is still needed in order to realize the ultimate objective of translating all optical measurements to the nanoscale. Near‐field optical microscopies and spectroscopies provide potential access to chemical, morphological, physical, and dynamical information at nanometer length scales—information difficult to probe by other means. Recent innovations by the nano‐optics community and at the Molecular Foundry are reported, which address many of the longstanding “nanopectroscopic imaging” challenges and lay the groundwork for unprecedented nano‐optical studies of material properties.
  PubDate: 2013-03-26T02:40:39.145107-05:
  DOI: 10.1002/adfm.201203432
Light Emission in the Unipolar Regime of Ambipolar Organic Field‐Effect Transistors
- Authors: W. S. Christian Roelofs; Willem H. Adriaans, René A. J. Janssen, Martijn Kemerink, Dago M. de Leeuw
  Pages: n/a - n/a
  Abstract: Light emission from ambipolar organic field‐effect transistors (OFETs) is often observed when they are operated in the unipolar regime. This is unexpected, the light emission should be completely suppressed, because in the unipolar regime only one type of charge carrier is accumulated. Here, an electroluminescent diketopyrrolopyrrole copolymer is investigated. Local potential measurements by scanning Kelvin probe microscopy reveal a recombination position that is unstable in time due to the presence of injection barriers. The electroluminescence and electrical transport have been numerically analyzed. It is shown that the counterintuitive unipolar light emission is quantitatively explained by injection of minority carriers into deep tail states of the semiconductor. The density of the injected minority carriers is small. Hence they are relatively immobile and they recombine close the contact with accumulated majority carriers. The unipolar light output is characterized by a constant efficiency independent of gate bias. It is argued that light emission from OFETs predominantly originates from the unipolar regime when the charge transport is injection limited. Light emission from ambipolar organic field‐effect transistors is observed when they are operated in the unipolar regime. This counterintuitive unipolar light emission is quantitatively explained by injection of minority carriers into deep tail states of the semiconductor. The density of the injected minority carriers is small; they are relatively immobile and recombine close the contact with accumulated majority carriers.
  PubDate: 2013-03-26T02:40:33.843121-05:
  DOI: 10.1002/adfm.201203568
Manipulating the Hysteresis in Poly(vinyl alcohol)‐Dielectric Organic Field‐Effect Transistors Toward Memory Elements
- Authors: Tzung‐Da Tsai; Jer‐Wei Chang, Ten‐Chin Wen, Tzung‐Fang Guo
  Pages: n/a - n/a
  Abstract: The origins of hysteresis in organic field‐effect transistors (OFETs) and its applications in organic memory devices is investigated. It is found that the orientations of the hydroxyl groups in poly(vinyl alcohol) (PVA) gate dielectrics are correlated with the hysteresis of transfer characteristics in pentacene‐based OFETs under the forward and backward scan. The applied gate bias partially aligns the orientations of the hydroxyl groups perpendicular to the substrate as characterized by reflective absorption Fourier transform infrared spectroscopy (RA‐FTIR), in which the field‐induced surface dipoles at the pentacene/PVA interface trap charges and cause the hysteresis. Treating PVA with an anhydrous solvent eliminates the residual moisture in the dielectrics layer, allowing for more effective control of the induced dipoles by the applied gate bias. OFETs of dehydrated‐PVA dielectrics present a pronounced shift of the threshold voltage (ΔVTh) of 35.7 V in transfer characteristics, higher than that of 18.5 V for untreated devices and results in sufficient dynamic response for applications in memory elements. This work highlights the usage of non‐ferroelectric gate dielectrics to fabricate OFET memory elements by manipulating the molecular orientations in the dielectrics layer. The use of non‐ferroelectric gate dielectrics to fabricate organic field‐effect transistor memory elements by manipulating the molecular dipoles in the dielectric layer is highlighted. The applied gate bias partially aligns the orientations of the hydroxyl groups perpendicular to the substrate at the pentacene/dielectrics interface, which trap charges and cause the hysteresis.
  PubDate: 2013-03-26T02:40:28.014866-05:
  DOI: 10.1002/adfm.201203694
Superior Micro‐Supercapacitors Based on Graphene Quantum Dots
- Authors: Wen‐Wen Liu; Ya‐Qiang Feng, Xing‐Bin Yan, Jiang‐Tao Chen, Qun‐Ji Xue
  Pages: n/a - n/a
  Abstract: Graphene quantum dots (GQDs) have attracted tremendous research interest due to the unique properties associated with both graphene and quantum dots. Here, a new application of GQDs as ideal electrode materials for supercapacitors is reported. To this end, a GQDs//GQDs symmetric micro‐supercapacitor is prepared using a simple electro‐deposition approach, and its electrochemical properties in aqueous electrolyte and ionic liquid electrolyte are systematically investigated. The results show that the as‐made GQDs micro‐supercapacitor has superior rate capability up to 1000 V s−1, excellent power response with very short relaxation time constant (τ0 = 103.6 μs in aqueous electrolyte and τ0 = 53.8 μs in ionic liquid electrolyte), and excellent cycle stability. Additionally, another GQDs//MnO2 asymmetric supercapacitor is also built using MnO2 nanoneedles as the positive electrode and GQDs as the negative electrode in aqueous electrolyte. Its specific capacitance and energy density are both two times higher than those of GQDs//GQDs symmetric micro‐supercapacitor in the same electrolyte. The results presented here may pave the way for a new promising application of GQDs in micropower suppliers and microenergy storage devices. Graphene quantum dots (GQDs)‐based micro‐supercapacitors are prepared using a simple eletrodeposition approach and their electrochemical properties in aqueous and ionic liquid electrolytes are studied. The GQDs‐based micro‐supercapacitors exhibit superior rate capability, high power response capability, and excellent cyclic stability
  PubDate: 2013-03-26T02:40:25.104659-05:
  DOI: 10.1002/adfm.201203771
Flexible Films Derived from Electrospun Carbon Nanofibers Incorporated with Co3O4 Hollow Nanoparticles as Self‐Supported Electrodes for Electrochemical Capacitors
- Authors: Fang Zhang; Changzhou Yuan, Jiajia Zhu, Jie Wang, Xiaogang Zhang, Xiong Wen (David) Lou
  Pages: n/a - n/a
  Abstract: Flexible porous films are prepared from electrospun carbon nanofibers (CNFs) embedded with Co3O4 hollow nanoparticles (NPs) and are directly applied as self‐supported electrodes for high‐performance electrochemical capacitors. Uniform Co3O4 hollow NPs are well dispersed and/or embedded into each CNF with desirable electrical conductivity. These Co3O4‐CNFs intercross each other and form 3D hierarchical porous hybrid films. Benefiting from intriguing structural features, the unique binder‐free Co3O4 hollow NPs/CNF hybrid film electrodes exhibit high specific capacitance (SC), excellent rate capability and cycling stability. As an example, the flexible hybrid film with loading of 35.9 wt% Co3O4 delivers a SC of 556 F g−1 at a current density of 1 A g−1, and 403 F g−1 even at a very high current density of 12 A g−1. Remarkably, almost no decay in SC is found after continuous charge/discharge cycling for 2000 cycles at 4 A g−1. This exceptional electrochemical performance makes such novel self‐supported Co3O4‐CNFs hybrid films attractive for high‐performance electrochemical capacitors. Flexible porous films derived from electrospun carbon nanofibers incorporated with Co3O4 hollow nanoparticles are efficiently fabricated and directly applied as advanced self‐supported hybrid film electrodes. The exhibit very high specific capacitance and excellent electrochemical stability at high current densities.
  PubDate: 2013-03-26T02:40:20.31392-05:0
  DOI: 10.1002/adfm.201203844
Carboxylates versus Fluorines: Boosting the Emission Properties of Commercial BODIPYs in Liquid and Solid Media
- Authors: Gonzalo Durán‐Sampedro; Antonia R. Agarrabeitia, Luis Cerdán, M. Eugenia Pérez‐Ojeda, Angel Costela, Inmaculada García‐Moreno, Ixone Esnal, Jorge Bañuelos, Iñigo López Arbeloa, María J. Ortiz
  Pages: n/a - n/a
  Abstract: A new and facile strategy for the development of photonic materials is presented that fufills the conditions of being efficient, stable, and tunable laser emitters over the visible region of spectrum, with the possibility of being easily processable and cost‐effective. This approach uses poly(methyl methacrylate) (PMMA) as a host for new dyes with improved efficiency and photostability synthesized. Using a simple protocol, fluorine atoms in the commercial (4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene) (F‐BODIPY) by carboxylate groups. The new O‐BODIPYs exhibit enhanced optical properties and laser behavior both in the liquid and solid phases compared to their commercial analogues. Lasing efficiencies up to 2.6 times higher than those recorded for the commercial dyes are registered with high photostabilities since the laser output remain at 80% of the initial value after 100 000 pump pulses in the same position of the sample at a repetition rate of 30 Hz; the corresponding commercial dye entirely loses its laser action after only 12 000 pump pulses. Distributed feedback laser emission is demonstrated with organic films incorporating new O‐BODIPYs deposited onto quartz substrates engraved with appropriated periodical structures. These dyes exhibit laser thresholds up to two times lower than those of the corresponding parent dyes with lasing intensities up to one order of magnitude higher. The development of new O‐BODIPYs, synthesized via the replacement of fluorine atoms by carboxylate groups in commercial (4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene) (F‐BODIPYs), is a successful strategy to obtain optimized laser dyes. Poly(methyl methacrylate) (PMMA) doped with these new derivatives leads to laser materials that are economically affordable and have optimized emission properties in the visible spectral region.
  PubDate: 2013-03-26T02:40:13.391996-05:
  DOI: 10.1002/adfm.201300198
Electron Transport at the Nanometer‐Scale Spatially Revealed by Four‐Probe Scanning Tunneling Microscopy
- Authors: An‐Ping Li; Kendal W. Clark, X.‐G. Zhang, Arthur P. Baddorf
  Pages: n/a - n/a
  Abstract: Electron transport at the nanometer‐scale is the key to novel applications of nanomaterials in electronic and energy technologies. Due to the restricted dimensionality, one of the distinctive characteristics of nano‐systems is their transport properties critically depend on structural details. Therefore, an important requirement for transport research of a specific nanomaterial system is to examine structures and properties in a coherent manner. In this regard, four‐probe scanning tunneling microscopy (STM), which combines four independently controllable STMs with a scanning electron microscope (SEM) in the same cryogenic environment, is uniquely useful for probing electron transport on multiple length‐scales and revealing how transport is coupled to the electronic and structural properties down to the atomic scale for individual nanomaterials. By utilizing this unique tool, extensive research has been undertaken to explore aspects of nanotransport, which include (a) intertwined electronic and structural phase transitions in surface supported two‐dimensional structures, (b) effects of atomic defects and interwire coupling on the electronic and transport properties of ultra thin quantum wire systems, (c) grain boundary resistances in copper nanowires with one‐to‐one correspondence to the grain boundary structure, (d) defect scattering effects in two‐dimensional electron gas systems, and (e) evaluation of transport behaviors of individual semiconductor nano‐junctions and nanodevices. In this paper, transport measurement techniques are first introduced with a four‐probe STM and then the recent progress on its applications is reviewed with a focus on the spatially resolved electron transport at the nanometer‐scale. The goal is to stimulate further advancement and utilization of techniques capable of characterizing materials properties at the nanometer‐scale to facilitate the exploration of the great promise of nanoscience and nanotechnology. Electrical transport measurement methods with a four‐probe scanning tuneling microscope and recent progress on its applications in nanomaterials with a focus on probing structure‐transport relationships at the nanometer‐scale are described. A broad range of nanomaterials are covered, including surface supported quasi‐1D and 2D electronic systems, semiconducting and metallic nanowires, carbon nanotubes, and graphene. The effects of atomic defects, grain boundaries, interfaces, and electronic interactions are discussed.
  PubDate: 2013-03-26T02:40:08.859111-05:
  DOI: 10.1002/adfm.201203423
Graphene at the Atomic‐Scale: Synthesis, Characterization and Modification
- Authors: Erin V. Iski; Esmeralda N. Yitamben, Li Gao, Nathan P. Guisinger
  Pages: n/a - n/a
  Abstract: Graphene is nature's ideal two‐dimensional conductor and is comprised of a single sheet of hexagonally packed carbon atoms. Since the first electrical measurements made on graphene, researchers have been trying to exploit the unique properties of this material for a variety of applications that span numerous scientific and engineering disciplines. In order to fully realize the potential of graphene, large scale synthesis of high quality graphene and the ability to control the electronic properties of this material on a nanometer length‐scale are necessary and remain key challenges. This article will review the efforts at the Center for Nanoscale Materials that focus on the atomic‐scale characterization and modification of graphene via scanning tunneling microscopy and its synthesis on various materials (SiC, Cu(111), Cu foil, etc.). These fundamental studies explore growth dynamics, film quality, and the role of defects. The chemical modification of graphene following exposure to atomic hydrogen will also be covered, while additional emphasis will be made on graphene's unique structural properties. Atomic‐scale characterization of graphene synthesis on various materials (SiC, Cu(111), Cu foil, etc.) via scanning tunneling microscopy provides fundamental exploration of growth dynamics, film quality, and the role of defects. The chemical modification of graphene following exposure to atomic hydrogen and molecular assembly are also explored.
  PubDate: 2013-03-25T03:20:22.240557-05:
  DOI: 10.1002/adfm.201203421
Template‐Free Sol‐Gel Preparation of Superhydrophobic ORMOSIL Films for Double‐Wavelength Broadband Antireflective Coatings
- Authors: Xin‐Xiang Zhang; Shuang Cai, Dan You, Liang‐Hong Yan, Hai‐Bing Lv, Xiao‐Dong Yuan, Bo Jiang
  Pages: n/a - n/a
  Abstract: A double‐layer double‐wavelength antireflective (AR) coating with 100% transmittance at both 1064 nm and 532 nm, which is very important in high power laser systems, is designed using thin film design software (TFCalc). The refractive indices for the bottom and top layers of the designed AR coating are about 1.30 and 1.14. A simple, template‐free sol‐gel route is proposed to prepare the superhydrophobic ORMOSIL (organically modified silicate) thin film, which has an ultralow refractive index, by silica particle surface modification using hexamethylisilazane (HMDS); this treatment decreases the refractive index of the silica thin film from 1.23 to 1.13. The formation mechanism of the ultralow refractive index thin film is proposed. The particle surface modification with HMDS significantly improves the hydrophobicity of the coated film; the water contact angle of the film increases from 23.4° to 160°. The bottom layer, which has a refractive index of 1.30, is prepared from acid‐catalyzed and base‐catalyzed mixed silica sol. A double‐layer silica AR coating is obtained with transmittances of 99.6% and 99.8% at 532 nm and 1064 nm, respectively. A double‐layer double wavelength antireflective (AR) coating that has 100% transmittance at both 1064 nm and 532 nm is designed with the assistance of a computer. This coating consists of top and bottom layers with refractive indices of 1.14 and 1.30. A template‐free sol‐gel method for the preparation of the superhydrophobic silica thin film with an ultralow refractive index is proposed.
  PubDate: 2013-03-25T03:20:17.277334-05:
  DOI: 10.1002/adfm.201203059
Temperature‐Insensitive (K,Na)NbO3‐Based Lead‐Free Piezoactuator Ceramics
- Authors: Ke Wang; Fang‐Zhou Yao, Wook Jo, Danka Gobeljic, Vladimir V. Shvartsman, Doru C. Lupascu, Jing‐Feng Li, Jürgen Rödel
  Pages: n/a - n/a
  Abstract: The development of lead‐free piezoceramics has attracted great interest because of growing environmental concerns. A polymorphic phase transition (PPT) has been utilized in the past to tailor piezoelectric properties in lead‐free (K,Na)NbO3 (KNN)‐based materials accepting the drawback of large temperature sensitivity. Here a material concept is reported, which yields an average piezoelectric coefficientd33 of about 300 pC/N and a high level of unipolar strain up to 0.16% at room temperature. Most intriguingly, field‐induced strain varies less than 10% from room temperature to 175 °C. The temperature insensitivity of field‐induced strain is rationalized using an electrostrictive coupling to polarization amplitude while the temperature‐dependent piezoelectric coefficient is discussed using localized piezoresponse probed by piezoforce microscopy. This discovery opens a new development window for temperature‐insensitive piezoelectric actuators despite the presence of a polymorphic phase transition around room temperature. The development of (K,Na)NbO3‐based lead‐free piezoceramics is attracting great interest because of growing environmental concerns. A material concept that yields an average piezoelectric coefficient, d33, of about 300 pC/N and a high level of unipolar strain up to 0.16% is reported. Most intriguingly, field‐induced strain varies less than 10% from room temperature to 175 °C.
  PubDate: 2013-03-19T02:20:25.243435-05:
  DOI: 10.1002/adfm.201203754
Micropatterning Alginate Substrates for In Vitro Cardiovascular Muscle on a Chip
- Authors: Ashutosh Agarwal; Yohan Farouz, Alexander Peyton Nesmith, Leila F. Deravi, Megan Laura McCain, Kevin Kit Parker
  Pages: n/a - n/a
  Abstract: Soft hydrogels such as alginate are ideal substrates for building muscle in vitro because they have structural and mechanical properties close to the in vivo extracellular matrix (ECM) network. However, hydrogels are generally not amenable to protein adhesion and patterning. Moreover, muscle structures and their underlying ECM are highly anisotropic, and it is imperative that in vitro models recapitulate the structural anisotropy in reconstructed tissues for in vivo relevance due to the tight coupling between sturcture and function in these systems. Two techniques to create chemical and structural heterogeneities within soft alginate substrates are presented and employed to engineer anisotropic muscle monolayers: i) microcontact printing lines of extracellular matrix proteins on flat alginate substrates to guide cellular processes with chemical cues and ii) micromolding of alginate surface into grooves and ridges to guide cellular processes with topographical cues. Neonatal rat ventricular myocytes as well as human umbilical artery vascular smooth muscle cells successfully attach to both these micropatterned substrates leading to subsequent formation of anisotropic striated and smooth muscle tissues. Muscular thin film cantilevers cut from these constructs are then employed for functional characterization of engineered muscular tissues. Thus, micropatterned alginate is an ideal substrate for in vitro models of muscle tissue because it facilitates recapitulation of the anisotropic architecture of muscle, mimics the mechanical properties of the ECM microenvironment, and is amenable to evaluation of functional contractile properties. Two new techniques to create chemical and structural heterogeneities within soft alginate substrates are presented and employed to engineer anisotropic cardiac and vascular smooth muscle monolayers. These micropatterned hydrogel substrates are ideally suited for building in vitro models of muscle contractility and tissue engineering applications as they recapitulate the mechanical properties of muscle microenvironment and their anisotropic structure.
  PubDate: 2013-03-19T02:20:18.121801-05:
  DOI: 10.1002/adfm.201203319
Nitrogen‐Doped Graphitic Nanoribbons: Synthesis, Characterization, and Transport
- Authors: Josue Ortiz‐Medina; M. Luisa García‐Betancourt, Xiaoting Jia, Rafael Martínez‐Gordillo, Miguel A. Pelagio‐Flores, David Swanson, Ana Laura Elías, Humberto R. Gutiérrez, Eduardo Gracia‐Espino, Vincent Meunier, Jonathan Owens, Bobby G. Sumpter, Eduardo Cruz‐Silva, Fernando J. Rodríguez‐Macías, Florentino López‐Urías, Emilio Muñoz‐Sandoval, Mildred S. Dresselhaus, Humberto Terrones, Mauricio Terrones
  Pages: n/a - n/a
  Abstract: Nitrogen‐doped graphitic nanoribbons (Nx‐GNRs), synthesized by chemical vapor deposition (CVD) using pyrazine as a nitrogen precursor, are reported for the first time. Scanning electron microscopy (SEM) and high‐resolution transmission electron microscopy (HRTEM) reveal that the synthesized materials are formed by multilayered corrugated GNRs, which in most cases exhibit the formation of curved graphene edges (loops). This suggests that during growth, nitrogen atoms promote loop formation; undoped GNRs do not form loops at their edges. Transport measurements on individual pure GNRs exhibit a linear I–V (current‐voltage) behavior, whereas Nx‐GNRs show reduced current responses following a semiconducting‐like behavior, which becomes more prominent for high nitrogen concentrations. To better understand the experimental findings, electron density of states (DOS), quantum conductance for nitrogen‐doped zigzag and armchair single‐layer GNRs are calculated for different N doping concentrations using density functional theory (DFT) and non‐equilibrium Green functions. These calculations confirm the crucial role of nitrogen atoms in the transport properties, confirming that the nonlinear I–V curves are due to the presence of nitrogen atoms within the Nx‐GNRs lattice that act as scattering sites. These characteristic Nx‐GNRs transport properties could be advantageous in the fabrication of electronic devices including sensors in which metal‐like undoped GNRs are unsuitable. Synthesis by chemical vapor deposition of nitrogen‐doped graphitic nanoribbons (Nx‐GNRs) is reported using pyrazine as a N precursor. Morphological, physico‐chemical, and electrical characterization of nitrogen‐doped graphitic nanoribbons reveal unique characteristics associated with doping sites, such as increased reactivity and changes in the electrical response towards semiconducting‐like features. These results are confirmed using first‐principle theoretical studies of N‐doped graphene nanoribbons.
  PubDate: 2013-03-19T02:20:11.848954-05:
  DOI: 10.1002/adfm.201202947
Realizing the Potential of ZnO with Alternative Non‐Metallic Co‐Dopants as Electrode Materials for Small Molecule Optoelectronic Devices
- Authors: Yong Hyun Kim; Jin Soo Kim, Won Mok Kim, Tae‐Yeon Seong, Jonghee Lee, Lars Müller‐Meskamp, Karl Leo
  Pages: n/a - n/a
  Abstract: High performance indium tin oxide (ITO)‐free small molecule organic solar cells and organic light‐emitting diodes (OLEDs) are demonstrated using optimized ZnO electrodes with alternative non‐metallic co‐dopants. The co‐doping of hydrogen and fluorine reduces the metal content of ZnO thin films, resulting in a low absorption coefficient, a high transmittance, and a low refractive index as well as the high conductivity, which are needed for the application in organic solar cells and OLEDs. While the established metal‐doped ZnO films have good electrical and optical properties, their application in organic devices is not as efficient as other alternative electrode approaches. The optimized ZnO electrodes presented here are employed in organic solar cells as well as OLEDs and allow not only the replacement of ITO, but also significantly improve the efficiency compared to lab‐standard ITO. The enhanced performance is attributed to outstanding optical properties and spontaneously nanostructured surfaces of the ZnO films with non‐metallic co‐dopants and their straightforward integration with molecular doping technology, which avoids several common drawbacks of ZnO electrodes. The observations show that optimized ZnO films with non‐metallic co‐dopants are a promising and competitive electrode for low‐cost and high performance organic solar cells and OLEDs. ZnO thin films are optimized by co‐doping of non‐metallic dopants for high optical and electrical performance. The ZnO‐based organic photovoltaic (OPV) cells and organic light‐emitting diodes (OLEDs) show highly improved efficiencies compared to indium tin oxide (ITO)‐based devices. The optimized ZnO films are very promising electrodes for highly efficient and cost‐effective OPV cells and OLEDs.
  PubDate: 2013-03-18T02:20:11.757865-05:
  DOI: 10.1002/adfm.201202799
Coordinatable and High Charge‐Carrier‐Mobility Water‐Soluble Conjugated Copolymers for Effective Aqueous‐Processed Polymer–Nanocrystal Hybrid Solar Cells and OFET Applications
- Authors: Haotong Wei; Hao Zhang, Gan Jin, Tianyi Na, Guoyan Zhang, Xue Zhang, Yan Wang, Haizhu Sun, Wenjing Tian, Bai Yang
  Pages: n/a - n/a
  Abstract: A water‐soluble conjugated polymer (WCP) poly[(3,4‐dibromo‐2,5‐thienylene vinylene)‐co‐(p‐phenylene‐vinylene)] (PBTPV), containing thiophene rings with high charge‐carrier mobility and benzene rings with excellent solubility is designed and prepared through Wessling polymerization. The PBTPV precursor can be easily processed by employing water or alcohols as the solvents, which are clean, environmentally friendly, and non‐toxic compared with the highly toxic organic solvents such as chloroform and chlorobenzene. As a novel photoelectric material, PBTPV presents excellent hole‐transport properties with a carrier mobility of 5 × 10−4 cm2 V−1 s−1 measured in an organic field‐effect transistor device. By integrating PBTPV with aqueous CdTe nanocrystals (NCs) to produce the active layer of water‐processed hybrid solar cells, the devices exhibit effective power conversion efficiency up to 3.3%. Moreover, the PBTPV can form strong coordination interactions with the CdTe NCs through the S atoms on the thiophene rings, and effective coordination with other nanoparticles can be reasonably expected. A new type of water‐soluble conjugated polymer composed of poly[(3,4‐dibromo‐2,5‐thienylene vinylene)‐co‐(p‐phenylene‐vinylene)] (PBTPV) is synthesized. Its excellent film‐forming properties, stability, and photoelectric response show good potential for organic field‐effect transistor and aqueous‐processable hybrid solar cell applications. The carrier mobility of the aqueous PBTPV is about 5 × 10−4 cm2 V−1 s−1 and the power conversion efficiency of the water‐processed PBTPV/CdTe nanocrystal hybrid photovoltaic devices reaches 3.3%.
  PubDate: 2013-03-15T04:20:27.14309-05:0
  DOI: 10.1002/adfm.201300333
Stretching‐Induced Growth of PEDOT‐Rich Cores: A New Mechanism for Strain‐Dependent Resistivity Change in PEDOT:PSS Films
- Authors: Yoo‐Yong Lee; Ji‐Hoon Lee, Ju‐Young Cho, Na‐Rae Kim, Dae‐Hyun Nam, In‐Suk Choi, Ki Tae Nam, Young‐Chang Joo
  Pages: n/a - n/a
  Abstract: It remains a fundamental challenge in the development of stretchable electronics to understand how mechanical strain changes the electrical properties of materials. Although the piezoresistive behavior of poly(3,4‐ethylene‐ dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been observed, its intrinsic origin is not yet fully understood because there are many extrinsic contributing factors and an experimental platform with which to assess such behavior has not been established. Here, systematic analysis shows that the matching Poisson's ratio and elastic modulus between PEDOT:PSS films and their underlying substrates is important in decoupling the factors that affect the material's piezoresistivity, allowing for tunable resistivity. Based on such a fundamental understanding, the conductivity of PEDOT:PSS can be controlled to be invariant and decrease as a function of applied tensile stress. Furthermore, a linear response of the resistivity with respect to mechanical strains of up to 60%, which has never before been realized, is shown. The irreversible conductivity enhancement is primarily caused by the coalescence‐induced growth of conductive PEDOT‐rich cores. Tensile strain‐resistivity response of poly(3,4‐ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) films is investigated according to doping with dimethylsulfoxide (DMSO) up to 60% strain without crack generation. The resistivity can be modulated under strain from invariant values to decreasing values up to 80%. The growth of conductive PEDOT‐rich cores is the primary mechanism for the decrease in resistivity induced by applied strain.
  PubDate: 2013-03-15T04:20:19.147909-05:
  DOI: 10.1002/adfm.201203670
Preparation of Iron Networks Hosted in Porous Alumina with Tunable Negative Permittivity and Permeability
- Authors: Zhi‐cheng Shi; Run‐hua Fan, Ke‐lan Yan, Kai Sun, Meng Zhang, Cheng‐guo Wang, Xiang‐fa Liu, Xi‐hua Zhang
  Pages: n/a - n/a
  Abstract: Random composites of iron particles hosted in porous alumina were prepared from a facile impregnation‐reduction process. Interestingly, when the iron content exceeds the percolation threshold, the interconnection of iron particles results in the formation of iron networks. The composites then change from capacitive to inductive and the conductive mechanism changes from hopping conduction to metal‐like conduction. The negative permittivity was attributed to the plasma oscillation of delocalized electrons in iron networks, while the negative permeability could be ascribed to the strong diamagnetic response of current loops in iron networks. The negative permittivity behavior of the iron/alumina composite was analyzed using Drude model. Additionally, the fitting results indicated that the effective plasma frequency of the iron/alumina composite is much lower than bulk iron. Further investigations show that, the iron content and reduction temperature can easily tune the amplitude and frequency ranges of the negative permittivity and permeability. Moreover, the negative permittivity region and the negative permeability region can be pushed to the same frequency region by adjusting the iron content and reduction temperature. The impregnation‐reduction process opens a new way for the realization of tunable negative permittivity and permeability in random composites, and has great potential for the preparation of new types of double negative materials. Iron particles are hosted in porous alumina via a facile impregnation‐reduction process. When the iron content exceeds the percolation threshold, iron networks are formed. The iron content and reduction temperature can easily tune the amplitude and frequency range of the negative permittivity and permeability. The impregnation‐reduction process has great potential for the preparation of random composites with tunable double negative properties.
  PubDate: 2013-03-15T04:20:09.326767-05:
  DOI: 10.1002/adfm.201202895
Visualizing MOF Mixed Matrix Membranes at the Nanoscale: Towards Structure‐Performance Relationships in CO2/CH4 Separation Over NH2‐MIL‐53(Al)@PI
- Authors: Tania Rodenas; Marion van Dalen, Elena García‐Pérez, Pablo Serra‐Crespo, Beatriz Zornoza, Freek Kapteijn, Jorge Gascon
  Pages: n/a - n/a
  Abstract: Mixed matrix membranes (MMMs) composed of metal organic framework (MOF) fillers embedded in a polymeric matrix represent a promising alternative for CO2 removal from natural gas and biogas. Here, MMMs based on NH2‐MIL‐53(Al) MOF and polyimide are successfully synthesized with MOF loadings up to 25 wt% and different thicknesses. At 308 K and ΔP = 3 bar, the incorporation of the MOF filler enhances CO2 permeability with respect to membranes based on the neat polymer, while preserving the relatively high separation factor. The rate of solvent evaporation after membrane casting proves key for the final configuration and dispersion of the MOF in the membrane. Fast solvent removal favours the contraction of the MOF structure to its narrow pore framework configuration, resulting in enhanced separation factor and, particularly, CO2 permeability. The study reveals an excellent filler‐polymer contact, with ca. 0.11% void volume fraction, for membranes based on the amino‐functionalized MOF, even at high filler loadings (25 wt%). By providing precise and quantitative insight into key structural features at the nanoscale range, the approach provides feedback to the membrane casting process and therefore it represents an important advancement towards the rational design of mixed matrix membranes with enhanced structural features and separation performance. Mixed‐matrix membranes composed of the flexible NH2‐MIL‐53(Al) metal‐organic framework (MOF) embedded in polyimide represent a promising alternative for CO2 removal from natural gas and biogas. Quantitative focused ion beam scanning electron microscopy (FIB‐SEM) tomography evidences an excellent filler‐polymer contact. The loading of MOF crystals and its framework configuration, which can be adjusted during membrane casting, are key parameters for the gas separation performance.
  PubDate: 2013-03-14T06:10:19.607063-05:
  DOI: 10.1002/adfm.201203462
Block‐Copolymer‐Assisted One‐Pot Synthesis of Ordered Mesoporous WO3−x/Carbon Nanocomposites as High‐Rate‐Performance Electrodes for Pseudocapacitors
- Authors: Changshin Jo; Jongkook Hwang, Hannah Song, Anh Ha Dao, Yong‐Tae Kim, Sang Hyup Lee, Seok Won Hong, Songhun Yoon, Jinwoo Lee
  Pages: n/a - n/a
  Abstract: An ordered mesoporous tungsten‐oxide/carbon (denoted as m‐WO3−x‐C‐s) nanocomposite is synthesized using a simple one‐pot method using polystyrene‐block‐poly(ethylene oxide) (PS‐b‐PEO) as a structure‐directing agent. The hydrophilic PEO block interacts with the carbon and tungsten precursors (resol polymer and WCl6), and the PS block is converted to pores after heating at 700 °C under a nitrogen flow. The m‐WO3−x‐C‐s nanocomposite has a high Brunauer–Emmett–Teller (BET) surface area and hexagonally ordered pores. Because of its mesoporous structure and high intrinsic density of tungsten oxide, this material exhibits a high average volumetric capacitance and gravimetric capacitance as a pseudocapacitor electrode. In comparison with reduced mesoporous tungsten oxide (denoted as m‐WO3−x‐h), which is synthesized by a tedious hard template approach and further reduction in a H2/N2 atmosphere, m‐WO3−x‐C‐s shows a high capacitance and enhanced rate performance, as confirmed by cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy. The good performance of m‐WO3−x‐C‐s is attributed to the high surface area arising from the mesoporous structure, the large interconnected mesopores, and the low internal resistance from the well‐dispersed reduced tungsten oxide and amorphous carbon composite structure. Here, the amorphous carbon acts as an electrical pathway for effective pseudocapacitor behavior of WO3‐x. An ordered mesoporous tungsten‐oxide/carbon (m‐WO3−x‐C‐s) nanocomposite is synthesized using a block‐copolymer‐assisted one‐pot self‐assembly method. As a pseudocapacitor electrode, m‐WO3−x‐C‐s exhibits a high average volumetric capacitance of 340 F cm−3 and a gravimetric capacitance of 103 F g−1. The amorphous carbon in the m‐WO3−x‐C‐s decreases the internal resistance of m‐WO3−x‐C‐s electrode by facilitating electric conduction.
  PubDate: 2013-03-13T03:20:39.027791-05:
  DOI: 10.1002/adfm.201202682
Molten‐Salt‐Assisted Self‐Assembly (MASA)‐Synthesis of Mesoporous Metal Titanate‐Titania, Metal Sulfide‐Titania, and Metal Selenide‐Titania Thin Films
- Authors: Cüneyt Karakaya; Yurdanur Türker, Ömer Dag
  Pages: n/a - n/a
  Abstract: New synthetic strategies are needed for the assembly of porous metal titanates and metal chalcogenite‐titania thin films for various energy applications. Here, a new synthetic approach is introduced in which two solvents and two surfactants are used. Both surfactants are necessary to accommodate the desired amount of salt species in the hydrophilic domains of the mesophase. The process is called a molten‐salt‐assisted self‐assembly (MASA) because the salt species are in the molten phase and act as a solvent to assemble the ingredients into a mesostructure and they react with titania to form mesoporous metal titanates during the annealing step. The mesoporous metal titanate (meso‐Zn2TiO4 and meso‐CdTiO3) thin films are reacted under H2S or H2Se gas at room temperature to yield high quality transparent mesoporous metal chalcogenides. The H2Se reaction produces rutile and brookite titania phases together with nanocrystalline metal selenides and H2S reaction of meso‐CdTiO3 yields nanocrystalline anatase and CdS in the spatially confined pore walls. Two different metal salts (zinc nitrate hexahydrate and cadmium nitrate tetrahydrate) are tested to demonstrate the generality of the new assembly process. The meso‐TiO2‐CdSe film shows photoactivity under sunlight. High quality mesoporous metal titanate thin films are synthesized using a molten‐phase‐assisted self‐assembly (MASA) method. The metal salts are used as a non‐volatile solvent in the new assembly process. The films are converted into mesoporous titania‐metal chalcogenides (TiO2‐CdS, TiO2‐CdSe, and TiO2‐ZnSe) under H2S or H2Se at room temperature and may find applications in solar cells, catalysis, photocatalysis, electronics, and optoelectronics.
  PubDate: 2013-03-13T03:20:33.664769-05:
  DOI: 10.1002/adfm.201202716
Magnetomechanical Four‐State Memory
- Authors: Chad S. Watson; Courtney Hollar, Kimball Anderson, William B. Knowlton, Peter Müllner
  Pages: n/a - n/a
  Abstract: With current non‐volatile memory technology approaching intrinsic storage density limits, new data storage technologies are under development. Probe‐based storage systems provide alternatives to conventional mass storage technologies. Ni‐Mn‐Ga, a ferromagnetic shape memory alloy (FSMA), is proposed as a medium for multi‐bit storage using scanning probe microscopy (SPM) techniques. Local modifications of the magnetic stray field were achieved using nanoindentation. Magnetic poles collect within the indentation, which is leveraged to control the magnetic stray field for the patterning of magnetic information. Four magnetic‐based memory states are possible due to magnetic field or stress‐induced twin rearrangement along two crystal orientations, each with two possible magnetic orientations. The magnetic shape memory alloy, Ni‐Mn‐Ga, is shown to exhibit multi‐bit non‐volatile memory behavior. Using nanoindentation, local modifications of the magnetic stray field enable the patterning of magnetic information. Four magnetic‐based memory states are demonstrated due to magnetic field or stress‐induced twin rearrangement along two crystal orientations, each with two possible magnetic orientations.
  PubDate: 2013-03-13T03:20:28.469655-05:
  DOI: 10.1002/adfm.201203015
Fused Thiophene Semiconductors: Crystal Structure–Film Microstructure Transistor Performance Correlations
- Authors: Jangdae Youn; Sumit Kewalramani, Jonathan D. Emery, Yanrong Shi, Shiming Zhang, Hsiu‐Chieh Chang, You‐jhih Liang, Chia‐Ming Yeh, Chieh‐Yuan Feng, Hui Huang, Charlotte Stern, Liang‐Hsiang Chen, Jia‐Chong Ho, Ming‐Chou Chen, Michael J. Bedzyk, Antonio Facchetti, Tobin J. Marks
  Pages: n/a - n/a
  Abstract: The molecular packing motifs within crystalline domains should be a key determinant of charge transport in thin‐film transistors (TFTs) based on small organic molecules. Despite this implied importance, detailed information about molecular organization in polycrystalline thin films is not available for the vast majority of molecular organic semiconductors. Considering the potential of fused thiophenes as environmentally stable, high‐performance semiconductors, it is therefore of interest to investigate their thin film microstructures in relation to the single crystal molecular packing and OTFT performance. Here, the molecular packing motifs of several new benzo[d,d′]thieno[3,2‐b;4,5‐b′]dithiophene (BTDT) derivatives are studied both in bulk 3D crystals and as thin films by single crystal diffraction and grazing incidence wide angle X‐ray scattering (GIWAXS), respectively. The results show that the BTDT derivative thin films can have significantly different molecular packing from their bulk crystals. For phenylbenzo[d,d′]thieno[3,2‐b;4,5‐b′]dithiophene (P‐BTDT), 2‐biphenylbenzo[d,d′]thieno‐[3,2‐b;4,5‐b′]dithiophene (Bp‐BTDT), 2‐naphthalenylbenzo[d,d′]thieno[3,2‐b;4,5‐b′]dithiophene (Np‐BTDT), and bisbenzo[d,d′]thieno[3,2‐b;4,5‐b′]dithiophene (BBTDT), two lattices co‐exist, and are significantly strained versus their single crystal forms. For P‐BTDT, the dominance of the more strained lattice relative to the bulk‐like lattice likely explains the high carrier mobility. In contrast, poor crystallinity and surface coverage at the dielectric/substrate interface explains the marginal OTFT performance of seemingly similar PF‐BTDT films. The thin film molecular packing motifs of several new benzo[d,d]thieno[3,2‐b;4,5‐b]dithiophene (BTDT) derivatives have different molecular packings from their bulk crystals. Co‐existence of strained lattices with their single crystal forms is speculated to have a significant effect on organic thin‐film transistor (OTFT) performance.
  PubDate: 2013-03-13T03:20:24.090266-05:
  DOI: 10.1002/adfm.201203439
Near‐Infrared SERS Nanoprobes with Plasmonic Au/Ag Hollow‐Shell Assemblies for In Vivo Multiplex Detection
- Authors: Homan Kang; Sinyoung Jeong, Younggeun Park, Joonhyuk Yim, Bong‐Hyun Jun, San Kyeong, Jin‐Kyoung Yang, Gunsung Kim, SoonGweon Hong, Luke P. Lee, Jong‐Ho Kim, Ho‐Young Lee, Dae Hong Jeong, Yoon‐Sik Lee
  Pages: n/a - n/a
  Abstract: For the effective application of surface‐enhanced Raman scattering (SERS) nanoprobes for in vivo targeting, the tissue transparency of the probe signals should be as high as it can be in order to increase detection sensitivity and signal reproducibility. Here, near‐infrared (NIR)‐sensitive SERS nanoprobes (NIR SERS dots) are demonstrated for in vivo multiplex detection. The NIR SERS dots consist of plasmonic Au/Ag hollow‐shell (HS) assemblies on the surface of silica nanospheres and simple aromatic Raman labels. The diameter of the HS interior is adjusted from 3 to 11 nm by varying the amount of Au3+ added, which results in a red‐shift of the plasmonic extinction of the Au/Ag nanoparticles toward the NIR (700–900 nm). The red‐shifted plasmonic extinction of NIR SERS dots causes enhanced SERS signals in the NIR optical window where endogenous tissue absorption coefficients are more than two orders of magnitude lower than those for ultraviolet and visible light. The signals from NIR SERS dots are detectable from 8‐mm deep in animal tissues. Three kinds of NIR SERS dots, which are injected into live animal tissues, produce strong SERS signals from deep tissues without spectral overlap, demonstrating their potential for in vivo multiplex detection of specific target molecules. Near‐infrared‐sensitive surface‐enhanced Raman scattering nanoprobes (NIR SERS dots) are fabricated by forming plasmonic Au/Ag hollow‐shells, which assemble on silica nanospheres. A single NIR SERS dot is capable of generating a strong SERS signal (average SERS enhancement factor value 2.8 × 105) with high reproducibility. In addition, the signals from NIR SERS dots are effectively detected from deep tissues of up to 8 mm depth and have exhibited a capability for in vivo multiplex detection in a live animal study.
  PubDate: 2013-03-13T03:20:19.853036-05:
  DOI: 10.1002/adfm.201203726
Nematic Liquid Crystals Embedded in Cubic Microlattices: Memory Effects and Bistable Pixels
- Authors: Francesca Serra; Shane Michael Eaton, Roberto Cerbino, Marco Buscaglia, Giulio Cerullo, Roberto Osellame, Tommaso Bellini
  Pages: n/a - n/a
  Abstract: The confinement of liquid crystals in geometries with frustrating boundary conditions gives rise to nontrivial effects such as bistability and memory. It is shown that large memory effects arise when nematic liquid crystals are embedded in cubic micrometer‐sized scaffolds made by two‐photon polymerization. The electric field alignment of the liquid crystals inside the porous medium is maintained when the applied field is above a threshold (approximately 2 V per micrometer of cell thickness). The onset of the memory is an on/off type process for each individual pore of the scaffold, and the memory typically starts emerging in one region of the structure and then propagates. The global memory effects in porous structures with controlled geometry are enhanced with respect to the case of random porous structures. This work is a proof of the “memory from topology” principle, which was previously suggested by computer simulations. These new materials can pave the way to new types of bistable displays. Nematic liquid crystals confined inside cubic scaffolds made by two‐photon polymerization exhibit bistability and large memory effects in response to electric fields, due to topological defects interacting with the solid structure. When viewed through crossed polarizers, the pixels, which are initially bright, remain dark after the application of strong electric fields.
  PubDate: 2013-03-13T03:20:14.074633-05:
  DOI: 10.1002/adfm.201203792
Creation of Ghost Illusions Using Wave Dynamics in Metamaterials
- Authors: Wei Xiang Jiang; Cheng‐Wei Qiu, Tiancheng Han, Shuang Zhang, Tie Jun Cui
  Pages: n/a - n/a
  Abstract: The creation of wave‐dynamic illusion functionality is of great interest to various scientific communities because it can potentially transform an actual perception into the pre‐controlled perception, thus empowering unprecedented applications in the advanced‐material science, camouflage, cloaking, optical and/or microwave cognition, and defense security. By using the space transformation theory and engineering capability of metamaterials, a functional “ghost” illusion device, which is capable of creating multiple virtual ghost images of the original object's position under the illumination of electromagnetic waves, is proposed and realized. The scattering signature of the object is thus ghosted and perceived as multiple ghost targets with different geometries and compositions. The ghost‐illusion material, which is being inhomogeneous and anisotropic, is realized using thousands of varying unit cells working at non‐resonance. The experimental demonstration of the ghost illusion validates the theory of scattering metamorphosis and opens a novel avenue to the wave‐dynamic illusion, cognitive deception, manipulate strange light (or matter) behaviors, and design novel optical and microwave devices. A functional “ghost” illusion device that uses inhomogeneous and anisotropic materials and is capable of creating multiple virtual images off the original object's position under the illumination of electromagnetic waves is presented. The scattering signature of the object is thus perceived as multiple targets with different geometries and compositions.
  PubDate: 2013-03-12T03:20:20.457462-05:
  DOI: 10.1002/adfm.201203806
Photochemical Generation of Light Responsive Surfaces
- Authors: Eva Blasco; Milagros Piñol, Luis Oriol, Bernhard V. K. J. Schmidt, Alexander Welle, Vanessa Trouillet, Michael Bruns, Christopher Barner‐Kowollik
  Pages: n/a - n/a
  Abstract: The preparation of patterned photoswitchable surfaces by employing the nitrile imine‐mediated tetrazole ene cycloaddition (NITEC) photoinduced reaction in the presence of dipolarophiles based on photoresponsive azobenzene moieties is reported. The dipolarophile used is a maleimide carrying either an azobenzene unit or a first generation dendron containing two azobenzene units. X‐ray photoelectron spectroscopy (XPS) is employed to analyze the functionalized silicon wafers, while time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) evidences the spatial control of the functionalization of the surface achieved by using a micropatterned shadow mask. Water contact angle measurements and optical inspection observing the behavior of a water droplet demonstrate the photoinduced change on wettability of the structured functionalized surfaces due to the reversible trans‐to‐cis isomerization of the azobenzene moities. Patterned photoswitchable surfaces are prepared by employing a nitrile imine‐mediated tetrazole ene cycloaddition (NITEC) photoinduced process in the presence of dipolarophiles based on photoresponsive azobenzene moieties. Trans‐to‐cis azobenzene isomerization of the surface allows for the spatially resolved tuning of the surface properties.
  PubDate: 2013-03-12T03:20:15.630339-05:
  DOI: 10.1002/adfm.201203602
Extreme Optical Properties Tuned Through Phase Substitution in a Structurally Optimized Biological Photonic Polycrystal
- Authors: Xia Wu; Andreas Erbe, Dierk Raabe, Helge‐Otto Fabritius
  Pages: n/a - n/a
  Abstract: Biological photonic structures evolved by insects provide inspiring examples for the design and fabrication of synthetic photonic crystals. The small scales covering the beetle Entimus imperialis are subdivided into irregularly shaped domains that mostly show striking colors, yet some appear colorless. The colors originate from photonic crystals consisting of cuticular material and air, which are geometrically separated by a triply periodic D‐surface (diamond). The structure and orientation of the photonic crystals are charactized and it is shown that in colorless domains SiO2 substitutes the air. The experimental results are incorporated into a precise D‐surface structure model used to simulate the photonic band structure. The study shows that the structural parameters in colored domains are optimized for maximum reflectivity by maximizing the stop gap width. The colorless domains provide a biological example of how the optical appearance changes through alteration of the refractive index contrast between the constituting phases. The cuticular photonic polycrystals formed by the beetle Entimus imperialis are a perfect example of a natural diamond‐type triply periodic bicontinuous cubic structure with structural parameters that are optimized to open up the largest possible photonic stop gaps. Depending on whether the cuticular network is complemented by air or SiO2, the optical properties of individual domains vary from bright coloration to no coloration and transparency.
  PubDate: 2013-03-12T03:20:07.25221-05:0
  DOI: 10.1002/adfm.201203597
Development of Degradable, pH‐Sensitive Star Vectors for Enhancing the Cytoplasmic Delivery of Nucleic Acids
- Authors: Yasemin Yuksel Durmaz; Yen‐Ling Lin, Mohamed E. H. ElSayed
  Pages: n/a - n/a
  Abstract: The report describes the synthesis of degradable, pH‐sensitive, membrane‐destabilizing, star‐shaped polymers where copolymers of hydrophobic hexyl methacrylate (HMA) and 2‐(dimethylamino)ethyl methacrylate (DMAEMA) monomers are grafted from the secondary face of a beta‐cyclodextrin (β‐CD) core via acid‐labile hydrazone linkages using atom transfer radical polymerization. The effect of the graft's molecular weight, HMA/DMAEMA molar ratio, and the fraction of DMAEMA converted to cationic N,N,N‐trimethylaminoethyl methacrylate (TMAEMA) monomers on polymer's transfection capacity is systematically investigated. Results show that all star‐shaped polymers condense anti‐GAPDH silencing RNA (siRNA) into nanosized particles at +/‐ ratio ≤ 4:1. Star polymers with shorter (25kDa) P(HMA‐co‐DMAEMA‐co‐TMAEMA) grafts are more efficient and less cytotoxic than carriers with longer (40kDa) grafts. The results show that increasing the ratio of hydrophobic HMA monomers in graft's composition higher than 50 mole% dramatically reduces polymer's aqueous solubility and abolishes their transfection capacity. Further, retention of DMAEMA monomers in graft's composition provide a buffering capacity that enhanced the endosomal escape and transfection capacity of the polymers. These systematic studies show that β‐CD‐P(HMA‐co‐DMAEMA‐co‐TMAEMA)4.8 polymer with a 25 kDa average graft's molecular weight and a 50/25/25 ratio of HMA/DMAEMA/TMAEMA monomers is the most efficient carrier in delivering the siRNA cargo into the cytoplasm of epithelial cancer cells. A series of degradable, pH‐sensitive, membrane‐destabilizing, star‐shaped polymers is synthesized. Star polymers are engineered to “sense” the drop in endosomal pH, which triggers the hydrolysis of acid‐labile hydrazone linkages and release of membrane‐active grafts that rupture the endosomal membrane and release the loaded siRNA cargo into the cytoplasm to produce the desired knockdown of targeted gene expression at both the mRNA and protein levels.
  PubDate: 2013-03-11T03:10:14.215447-05:
  DOI: 10.1002/adfm.201203762
Graphene/SiO2/p‐GaN Diodes: An Advanced Economical Alternative for Electrically Tunable Light Emitters
- Authors: Che‐Wei Chang; Wei‐Chun Tan, Meng‐Lin Lu, Tai‐Chun Pan, Ying‐Jay Yang, Yang‐Fang Chen
  Pages: n/a - n/a
  Abstract: Advanced materials that combine novel functionality and ease of applicability are central to the development of light‐emitting diodes (LEDs), which is of ever increasing commercial importance. Here a new metal‐insulator‐semiconductor (MIS) LED structure that combines economical fabrication with novel device properties is reported. The presented MIS‐LED consists of a graphene electrode on p‐GaN substrate separated by an insulating SiO2 layer. It is found that the MIS‐LED possesses a unique tunability of the electroluminescence spectra depending on the bias conditions. Tunnel injection from graphene into the p‐GaN can explain the difference in luminescence spectra under forward and reverse bias. The demonstrated MIS‐LED expands the use of graphene and also possibly allows the direct integration of light emitters with other circuit elements. Metal‐insulator‐semiconductor (MIS) light‐ emitting diodes (LEDs) consisting of a graphene electrode on p‐GaN substrate separated by an insulating SiO2 layer are reported. The novel MIS‐LEDs have a unique tunability of the electroluminescence (EL) spectra depending on the bias conditions. The underlying mechanism can be interpreted as the tunneling of electrons and holes through the insulating layer in both polarities, which is different from the standard p‐n junction model.
  PubDate: 2013-03-11T03:10:09.571746-05:
  DOI: 10.1002/adfm.201203035
Facile Fabrication of PbS Nanocrystal:C60 Fullerite Broadband Photodetectors with High Detectivity
- Authors: Rinku Saran; Muhammad N. Nordin, Richard J. Curry
  Pages: n/a - n/a
  Abstract: Hybrid PbS nanocrystal/C60 fullerite photodetectors are fabricated using a simple one‐step drop casting procedure onto pre‐patterned interdigitated electrodes. The devices exhibit a broad spectral response from the near UV through to the near infrared yielding a detectivity, D*, of above 1010 Jones from 400 nm to ≈1050 nm. The ability to further extend the spectral response to wavelengths ≈1350 nm in the near infrared via tuning of the PbS nanocrystal diameter is also demonstrated. The dynamic responses of the devices are presented, exhibiting a fast photocurrent rise time (
  PubDate: 2013-03-11T03:10:03.356432-05:
  DOI: 10.1002/adfm.201202818
Core‐Brominated Tetraazaperopyrenes as n‐Channel Semiconductors for Organic Complementary Circuits on Flexible Substrates
- Authors: Sonja Geib; Ute Zschieschang, Marcel Gsänger, Matthias Stolte, Frank Würthner, Hubert Wadepohl, Hagen Klauk, Lutz H. Gade
  Pages: n/a - n/a
  Abstract: Organic thin‐film transistors (TFTs) are prepared by vacuum deposition and solution shearing of 2,9‐bis(perfluoroalkyl)‐substituted tetraazaperopyrenes (TAPPs) with bromine substituents at the aromatic core. The TAPP derivatives are synthesized by reacting known unsubstituted TAPPs with bromine in fuming sulphuric acid, and their electrochemical properties are studied in detail by cyclic voltammetry and modelled with density functional theory (DFT) methods. Lowest unoccupied molecular orbital (LUMO) energies and electron affinities indicate that the core‐brominated TAPPs should exhibit n‐channel semiconducting properties. Current‐voltage characteristics of the TFTs established electron mobilities of up to μn = 0.032 cm2 V−1 s−1 for a derivative which was subsequently processed in the fabrication of a complementary ring oscillator on a flexible plastic substrate (PEN). The preparation of brominated tetraazaperopyrenes, their crystal structures, and their electrochemical properties are reported. These N‐heterocyclic peropyrene derivatives are well‐suited materials for the preparation of n‐channel conducting organic thin‐film transistors either by vacuum deposition or by solution processing. Additionally, the fabrication of a complementary ring oscillator on a flexible substrate is described.
  PubDate: 2013-03-08T02:10:20.929564-05:
  DOI: 10.1002/adfm.201203600
Biomimetic Structural Electrochemistry from Conducting Polymers: Processes, Charges, and Energies. Coulovoltammetric Results from Films on Metals Revisited
- Authors: Toribio F. Otero; Mercedes Alfaro, Venancio Martinez, Maria A. Perez, Jose G. Martinez
  Pages: n/a - n/a
  Abstract: Oxidation/reduction reactions in films of conducting polymers exchanging anions and solvent during reactions are revisited here and explored by voltammetric and coulovoltammetric results using Pt electrodes coated with films of different polymers, in different solvents and salts. The reactions induce molecular (conformational) and macroscopic (relaxation, swelling, shrinking, and compaction) structural changes. Coulovoltammetric loops constitute a direct graphical tool to identify, separate, and quantify the structural components of the reversible film reactions together the potential domains and the charges involved in irreversible reactions. Any abrupt slope variation is related to abrupt changes of the reaction rate associated with each of the structural induced processes. Charges, energies, and characteristic potentials for any of the structural or irreversible process are obtained. Reversible film reactions are present from –1.5 to 0.35 V versus Ag/AgCl, overlapping the irreversible hydrogen evolution at the metal/polymer interface below –1.1 V. Structural reduction energies are higher than structural oxidation energies and structural changes are energetically asymmetric. 3D reactive structural memories are envisaged stepping up storage density by orders of magnitude. Conducting polymers are 3D structural gel reactors at the polymer chain level: large anions in solution cannot penetrate inside the film. Composition changes and structural processes mimic intracellular matrix (ICM) biological reactions. Coulovoltammetric responses from films of conducting polymers coating metal electrodes allow a graphical separation, identification, and quantification of reversible and irreversible reactions; structural (shrinking, compaction, relaxation and swelling) components of the reversible film reaction; their potential domains; charges; and energies consumed by every structural process.
  PubDate: 2013-03-08T02:10:16.001801-05:
  DOI: 10.1002/adfm.201203502
Charge‐Tunable Autoclaved Silk‐Tropoelastin Protein Alloys That Control Neuron Cell Responses
- Authors: Xiao Hu; Min D. Tang‐Schomer, Wenwen Huang, Xiao‐Xia Xia, Anthony S. Weiss, David L. Kaplan
  Pages: n/a - n/a
  Abstract: Tunable protein composites are important for constructing extracellular matrix mimics of human nerve tissues with control of charge, structural, and mechanical properties. Molecular interaction mechanisms between silk fibroin protein and recombinant human tropoelastin, based on charge, are utilized to generate a new group of multifunctional protein alloys with different net charges. These new biomaterials are then utilized as a biomaterial platform to control neuron cell response. With a +38 net charge in water, tropoelastin molecules provide extraordinary elasticity and selective interactions with cell surface integrins. In contrast, negatively charged silk fibroin protein (net charge −36) provides remarkable toughness and stiffness with morphologic stability in material formats via autoclaving‐induced beta‐sheet crystal physical crosslinks. The combination of these properties in alloy format extends the versatility of both structural proteins, providing a new biocompatible, biodegradable, and charge‐tunable biomaterial platform for neural repair. The data point to these protein alloys as an alternative to commonly used charged synthetic polymers, particularly with regard to the versatility of material formats (e.g., gels, sponges, films, fibers). The results also provide a practical example of physically designed protein materials with control of net charge to direct biological outcomes, in this case for neuronal tissue engineering. Molecular interaction mechanisms between silk fibroin and recombinant human tropoelastin are utilized to generate multifunctional protein alloys with different net charges. The combination of their properties in alloy format extends the versatility of both structural proteins, providing a new biocompatible, biodegradable, and charge‐tunable biomaterial platform for neural repair.
  PubDate: 2013-03-08T02:10:11.364873-05:
  DOI: 10.1002/adfm.201202685
Random Lasing in Self‐Assembled Dye‐Doped Latex Nanoparticles: Packing Density Effects
- Authors: Luis Cerdán; Angel Costela, Eduardo Enciso, Inmaculada García‐Moreno
  Pages: n/a - n/a
  Abstract: Efficient random lasing (RL) from self‐assembled dye‐doped latex nanoparticles (d = 50–380 nm) presenting size polydispersity is reported. This new system exhibits a very good chemical compatibility between dye (Rhodamine 6G) and polymer as well as a high refractive index contrast between nanoparticle and surroundings (air), in such a way that its emission properties surpass the ones previously reported in similar systems. Furthermore, this system allows analyzing in detail the effects of the nanoparticle size polydispersity and the packing density on the RL emission properties. It is shown that size polydispersity gives rise to non‐uniformities in the filling fraction along the sample that lead to fluctuations on the scattering length across the sample and thus to a variation on the emission properties. Finally, it is observed that the increase of the filling fraction, enabled by the use of binary mixtures of nanoparticles with different sizes, results in remarkable improvements in the RL emission properties. Efficient random lasing (RL) from self‐assembled dye‐doped latex nanoparticles presenting size polydispersity is reported. Both the nanoparticle size and size polydispersity influence the RL emission properties. The use of binary mixtures of nanoparticles with different sizes improves the RL emission properties with respect to the mixture constituents separately due to an increase in the filling fraction.
  PubDate: 2013-03-06T03:10:35.280642-05:
  DOI: 10.1002/adfm.201202616
Direct Photopatterning of Electrochromic Polymers
- Authors: Jacob Jensen; Aubrey L. Dyer, D. Eric Shen, Frederik C. Krebs, John R. Reynolds
  Pages: n/a - n/a
  Abstract: Propylenedioxythiophene (ProDOT) polymers are synthesized using an oxidative polymerization route that results in methacrylate substituted poly(ProDOTs) having a Mn of 10–20 kDa wherein the methacrylate functionality constitutes from 6 to 60% of the total monomer units. Solutions of these polymers show excellent film forming abilities, with thin films prepared using both spray‐casting and spin‐coating. These polymers are demonstrated to crosslink upon UV irradiation at 350 nm, in the presence of an appropriate photoinitiator, to render the films insoluble to common organic solvents. Electrochemical, spectroelectrochemical, and colorimetric analyses of the crosslinked polymer films are performed to establish that they retain the same electrochromic qualities as the parent polymers with no detriment to the observed properties. To demonstrate applicability for multi‐film processing and patterning, photolithographic patterning is shown, as is desired for fully solution processed and patterned devices. Direct photopatterning of a conjugated electroactive dioxythiophene‐based polymer is presented. Thin films of the polymer are spray‐cast from organic solvents, followed by insolubilization via photocrosslinking. The crosslinking process does not cause any detriment to the electroactivity or optical properties of the polymer, which can be redox switched between a colored and bleached state, as desired for electrochromic displays and windows.
  PubDate: 2013-03-06T03:10:31.270204-05:
  DOI: 10.1002/adfm.201203005
Energetics of Donor‐Doping, Metal Vacancies, and Oxygen‐Loss in A‐Site Rare‐Earth‐Doped BaTiO3
- Authors: Colin L. Freeman; James A. Dawson, Hung‐Ru Chen, Liubin Ben, John H. Harding, Finlay D. Morrison, Derek C. Sinclair, Anthony R. West
  Pages: n/a - n/a
  Abstract: The energetics of La‐doping in BaTiO3 are reported for both (electronic) donor‐doping with the creation of Ti3+ cations and ionic doping with the creation of Ti vacancies. The experiments (for samples prepared in air) and simulations demonstrate that ionic doping is the preferred mechanism for all concentrations of La‐doping. The apparent disagreement with electrical conduction of these ionic doped samples is explained by subsequent oxygen‐loss, which leads to the creation of Ti3+ cations. Simulations show that oxygen‐loss is much more favorable in the ionic‐doped system than undoped BaTiO3 due to the unique local structure created around the defect site. These findings resolve the so‐called “donor‐doping” anomaly in BaTiO3 and explain the source of semiconductivity in positive temperature coefficient of resistance (PTCR) BaTiO3 thermistors. The defect structure of 4% La‐doped BaTiO3 with 4 LaBa and a Ti vacancy is shown. The loss of Ti encourages the creation of an oxygen vacancy and the subsequent electron compensation creates Ti3+, which generates the semiconducting behavior seen for the these samples.
  PubDate: 2013-03-06T03:10:26.207576-05:
  DOI: 10.1002/adfm.201203147
Soft Iron/Silicon Composite Tubes for Magnetic Peristaltic Pumping: Frequency‐Dependent Pressure and Volume Flow
- Authors: Roland Fuhrer; Christoph M. Schumacher, Martin Zeltner, Wendelin J. Stark
  Pages: n/a - n/a
  Abstract: The combination of force and flexibility enables controlled and soft movements. In sharp contrast, presently used machines are solid and mostly based on stiff driveshafts or cog wheels. Magnetic elastomers are realized through dispersion of small particles in polymer matrices and have attracted significant interest as soft actuators for controlled movement or conveying and are particularly attractive candidates for magnetic pump applications. At present, low magnetic particle loading and thus limited actuator strength have restricted the application of such materials. Here, the direct incorporation of metal microparticles into a very soft and flexible silicone and its application as an ultra‐flexible, yet strong magnetic tube, is described. Because metals have a far higher saturation magnetization and higher density than oxides, the resulting increased force/volume ratio afforded significantly stronger magnetic actuators with high mechanical stability, flexibility, and shape memory. Elliptical inner diameter shape of the tubing allowed a very efficient contraction of the tube by applying an external magnetic field. The combination of magnetic silicone tubes and a magnetic field generating device results in a magnetic peristaltic pump. Magnetic silicone tubes with 67 wt% magnetic particles and an inner diameter of elliptical shape allow very efficient contractions of the tube by applying an external magnetic field. The combination of magnetic silicone tubes and a magnetic field generating device results in a magnetic peristaltic pump.
  PubDate: 2013-03-06T03:10:23.051341-05:
  DOI: 10.1002/adfm.201203572
Buckling‐Based Strong Dry Adhesives Via Interlocking
- Authors: Chi‐Mon Chen; Chang‐Lung Chiang, Chien‐Lin Lai, Tao Xie, Shu Yang
  Pages: n/a - n/a
  Abstract: High‐aspect‐ratio shape‐memory polymer (SMP) pillar arrays are investigated as a new type of dry adhesive based on buckling and interlocking mechanism. When two identical SMP pillar arrays are engaged at 80 °C, above the glass transition temperature at a preload larger than the critical buckling threshold, the pillars are deformed and become interweaved and/or indented with each other. After cooling to room temperature, strong pull‐off forces are observed in the normal and shear directions, both of which are much larger than those from pillar‐to‐flat surface and flat‐to‐flat surface contact. From finite element anaylsis (FEA) and comparison of measured and calculated adhesion values using different contact mechanics models, it is shown that interweaved pillars are the main source that contributes to the pillar‐to‐pillar adhesion and the indented pillars set the lower limit, whereas the probability of interdigitation is very low. Further, it is found that interweaved pillars are primarily responsible for the decreased adhesion strength and increased anisotropy when the pillar spacing became larger. Finally, it is shown that the bonded pillars can be easily separated after reheating to 80 °C due to significant drop of modulus of SMPs. A strong dry adhesive based on interlocking of buckled shape‐memory polymer (SMP) pillars is designed and investigated. The strong adhesion originates from pillar interweaving and indenting with each other, as well as the elastic energy stored in the deformed SMP pillars at room temperature. The adhesion strength and anisotropy can be tuned using the detachment temperature and pillar spacing.
  PubDate: 2013-03-06T03:10:16.158184-05:
  DOI: 10.1002/adfm.201300052
Nanocontainer‐Based Anticorrosive Coatings: Effect of the Container Size on the Self‐Healing Performance
- Authors: Dimitriya Borisova; Dilek Akçakayıran, Matthias Schenderlein, Helmuth Möhwald, Dmitry G. Shchukin
  Pages: n/a - n/a
  Abstract: Organic coatings based on inhibitor loaded inorganic containers for smart corrosion inhibition are presented. The overall coating performance is strongly influenced by the containers as well as their inhibitor capacity, compatibility with the coating matrix, and size. The important effect of container size is described for the first time in this work by investigating two types of mesoporous silica containers of different diameters: 80 and 700 nm. The coating physical properties (thickness and adhesion) are comparable for both container types. In contrast, the coating barrier properties are strongly influenced by the container size as assessed with electrochemical impedance spectroscopy (EIS). The incorporation of bigger containers reduces the coating resistance by a factor of two. Surprisingly, despite the similar amounts (20 wt%) of loaded inhibitor (2‐mercaptobenzothiazole), different active inhibition ability is detected with the scanning vibrating electrode technique (SVET). Therefore, it is found that coatings with smaller containers exhibit better self‐healing performance. Nanocontainer‐based anticorrosive coatings are formed by dispersing inhibitor‐loaded mesoporous silica particles throughout an epoxy primer. They offer self‐healing functionality and outperform the unmodified primer. Such coatings demonstrate worse passive and active protection. Coatings containing smaller nanocontainers offer enhanced self‐healing performance due to homogeneous distribution and preservation of the coating integrity.
  PubDate: 2013-03-05T03:10:26.168034-05:
  DOI: 10.1002/adfm.201203715
Cathodoluminescence Modulation of ZnS Nanostructures by Morphology, Doping, and Temperature
- Authors: Hui Liu; Linfeng Hu, Kentaro Watanabe, Xinhua Hu, Benjamin Dierre, Bongsoo Kim, Takashi Sekiguchi, Xiaosheng Fang
  Pages: n/a - n/a
  Abstract: Spatially and spectrally resolved cathodoluminescence (CL) is one of the most effective methods to explore the optical properties of a nanomaterials and reveals the spatial distribution as well as the correlation between the luminescence and the sample morphology and microstructure. Here, CL modulation of ZnS nanostructures by controlled morphologies, Fe/Mn doping, and measurement temperature is demonstrated. High quality ZnS nanobelts and nanorods are synthesized on an Au‐coated Si substrate and an Au‐coated GaAs substrate via a facile thermal evaporation route. A room‐temperature sharp ultraviolet (UV) lasing‐like peak in various ZnS is achieved. The main UV luminescence peaks appear at wavelengths between 330 and 338 nm. The low temperature (32 K) CL spectrum consists of a narrow and strong UV peak centered at 330 nm and two broad, low‐intensity peaks in the visible region (514 and 610 nm). Temperature‐dependent CL from such single‐crystalline ZnS nanobelts in the temperature range of 32 to 296 K reveals two UV peaks at 3.757 and 3.646 eV. The effects of Fe doping and Fe/Mn co‐doping on the CL property of ZnS nanobelts are further investigated. These results imply that ZnS nanostructures can be used for potential luminescent materials as well as short‐wavelength nanolaser light sources. Cathodoluminescence (CL) modulation of ZnS nanostructures by controlled morphologies, Fe/Mn doping, and measurement temperature is demonstrated. A comprehensive investigation of CL of ZnS nanostructures reveals a sharp UV band‐gap emission at room temperature. The ZnS nanostructures show potential applications in luminescent materials as well as short‐wavelength nanolaser light sources.
  PubDate: 2013-03-05T03:10:22.648105-05:
  DOI: 10.1002/adfm.201203711
Printed, Flexible, Organic Nano‐Floating‐Gate Memory: Effects of Metal Nanoparticles and Blocking Dielectrics on Memory Characteristics
- Authors: Minji Kang; Kang‐Jun Baeg, Dongyoon Khim, Yong‐Young Noh, Dong‐Yu Kim
  Pages: n/a - n/a
  Abstract: The effects of using a blocking dielectric layer and metal nanoparticles (NPs) as charge‐trapping sites on the characteristics of organic nano‐floating‐gate memory (NFGM) devices are investigated. High‐performance NFGM devices are fabricated using the n‐type polymer semiconductor, poly{[N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} (P(NDI2OD‐T2)), and various metal NPs. These NPs are embedded within bilayers of various polymer dielectrics (polystyrene (PS)/poly(4‐vinyl phenol) (PVP) and PS/poly(methyl methacrylate) (PMMA)). The P(NDI2OD‐T2) organic field‐effect transistor (OFET)‐based NFGM devices exhibit high electron mobilities (0.4–0.5 cm2 V−1 s−1) and reliable non‐volatile memory characteristics, which include a wide memory window (≈52 V), a high on/off‐current ratio (Ion/Ioff ≈ 105), and a long extrapolated retention time (>107 s), depending on the choice of the blocking dielectric (PVP or PMMA) and the metal (Au, Ag, Cu, or Al) NPs. The best memory characteristics are achieved in the ones fabricated using PMMA and Au or Ag NPs. The NFGM devices with PMMA and spatially well‐distributed Cu NPs show quasi‐permanent retention characteristics. An inkjet‐printed flexible P(NDI2OD‐T2) 256‐bit transistor memory array (16 × 16 transistors) with Au‐NPs on a polyethylene naphthalate substrate is also fabricated. These memory devices in array exhibit a high Ion/Ioff (≈104 ± 0.85), wide memory window (≈43.5 V ± 8.3 V), and a high degree of reliability. Highly stable top‐gated organic nano‐floating‐gate memory (NFGM) devices with quasi‐permanent retention characteristics are fabricated using an n‐type conjugated polymer. The best polymer NFGM devices show excellent memory‐related characteristics along with a wide memory window, a high on‐off current ratio, and a long retention time.
  PubDate: 2013-03-05T03:10:17.293479-05:
  DOI: 10.1002/adfm.201203417
Polymer‐Based Magnetoelectric Materials
- Authors: Pedro Martins; Senentxu Lanceros‐Méndez
  Pages: n/a - n/a
  Abstract: Polymer‐based magnetoelectric (ME) materials are an interesting, challenging and innovative research field, that will bridge the gap between fundamental research and applications in the near future. Here, the current state of the art on the different materials, the used configurations for the development of sensors and actuators, as well as the main values of the ME coupling obtained for the different polymer‐based systems are summarized. Further, some of the specific applications that are being developed for those polymer‐based ME materials are addressed as well as the main advantages and remaining challenges in this research field. Polymer‐based magnetoelectric materials have great potential for advanced applications such as four‐state memories, energy harvesting materials and magnetic sensors. New flexible, low cost, and easily processeble magnetoelectric materials are being developed by combining electroactive polymers and magnetostrictive materials, allowing the achievement of larger areas or non‐planar structures and meeting the current technological challenges.
  PubDate: 2013-03-05T03:10:11.55438-05:0
  DOI: 10.1002/adfm.201202780
Multi Variant Surface Mounted Metal–Organic Frameworks
- Authors: Bo Liu; Min Tu, Denise Zacher, Roland A. Fischer
  Pages: n/a - n/a
  Abstract: Hybrid surface mounted metal–organic frameworks (h‐SURMOFs) of multi variant core‐shell (cs) and core–shell–shell (css) structures (SURMOF A‐B and A‐B‐C, A: [Cu2(bdc)2(dabco)]; B: [Cu2(NH2‐bdc)2(dabco)]; C: [Cu2(ndc)2(dabco)], bdc = 1,4‐benzenedicarboxylate; NH2‐bdc = 2‐amino‐1,4‐benzenedicarboxylate; ndc = 1,4‐naphtalenedicarboxylate; dabco = 1,4‐diazabicyclo[2.2.2]octane) with specific crystallographic [001] orientation and incorporated amino groups at a controllable depth within the bulk are deposited via liquid phase epitaxial (LPE) approach on pyridyl‐terminated self‐assembled monolayers (SAM). The location of the (amino) functionality can be precisely controlled through tuning the thickness (number of deposition cycles) of each sub‐multilayer block according to the LPE deposition protocol. The chemo‐selective and location‐specific post deposition (chemical) modification of the amino groups in the cs and css‐type h‐SURMOF samples is achieved. The h‐SURMOFs allow one to probe functional groups at certain location in the volume of hybrid MOF crystallites attached to surfaces as thin film coatings. Multiplex adsorption kinetics of FPI (FPI = 4‐fluorophenyl isothiocyanate) is observed in h‐SURMOFs due to their multi‐variant pore structures in samples of A‐B and A‐B‐C. Conceptually, the stepwise LPE growth method enables fabrication of hybrid SURMOFs and incorporation of multi‐variant functionalities into one homogeneous thin film material, providing precisely tunable pore environment for selective adsorption, separation, etc. Multi variant surface mounted metal–organic frameworks (SURMOFs) are fabricated by sequential and stepwise growth of different MOF sub‐layers on a functionalized substrate. Varied pore structures and environments in the hybrid SURMOF result in multiplex adsorption kinetics of guest molecules and thus reveal the excellent potential for advanced separation tasks.
  PubDate: 2013-03-01T05:10:09.593943-05:
  DOI: 10.1002/adfm.201202996
A Near‐Infrared cis‐Configured Squaraine Co‐Sensitizer for High‐Efficiency Dye‐Sensitized Solar Cells
- Authors: Chuanjiang Qin; Youhei Numata, Shufang Zhang, Ashraful Islam, Xudong Yang, Keitaro Sodeyama, Yoshitaka Tateyama, Liyuan Han
  Pages: n/a - n/a
  Abstract: A cis‐configured squaraine dye (HSQ1), synthesized by incorporation of a strongly electron‐withdrawing dicyanovinyl group into the central squaric acid moiety, is employed in dye‐sensitized solar cells (DSCs). In solution, HSQ1 displays an intense absorption in the near‐infrared region with a maximum at 686 nm and when the dye is adsorbed on a TiO2 surface, the absorption spectrum broadens in both the blue and the near‐infrared regions, which is favorable for efficient light harvesting over a broad wavelength range. A solar cell sensitized with HSQ1 shows a broader incident photon‐to‐current conversion efficiency (IPCE) spectrum (from 400 to 800 nm) and a higher IPCE in the long‐wavelength region (71% at 700 nm) than a cell sensitized with squaraine dye SQ1. Furthermore, a solar cell co‐sensitized with HSQ1 and N3 dye shows remarkably improved short‐circuit current density and open‐circuit voltage compared to those of a DSC based on N3 alone and fabricated under the same conditions. The energy‐conversion efficiency of the co‐sensitized DSC is 8.14%, which is the highest reported efficiency for a squaraine dye–based co‐sensitized DSC without using Al2O3 layer. The absorption maximum of a cis‐configured squaraine dye (HSQ1) is red‐shifted to 686 nm using a simple molecular design strategy. Dye‐sensitized solar cells (DSCs) co‐sensitized with HSQ1 and N3 (a Ru bipyridyl complex) show an energy‐conversion efficiency of 8.14%. HSQ1 is the first squaraine dye to possess such a broad incident photon‐to‐current conversion efficiency (IPCE) response spectrum and high conversion efficiency at long wavelengths.
  PubDate: 2013-02-28T03:10:15.606073-05:
  DOI: 10.1002/adfm.201203384
Quantum Dot–Carbon Nanotube Hybrid Phototransistor with an Enhanced Optical Stark Effect
- Authors: Chandan Biswas; Hyun Jeong, Mun Seok Jeong, Woo Jong Yu, Didier Pribat, Young Hee Lee
  Pages: n/a - n/a
  Abstract: Enhanced carrier–carrier interactions in hybrid nanostructures exhibit exceptional electronic and optoelectronic properties. Carbon nanotubes demonstrate excellent switching behavior with high on/off ratio and high mobility but do not show photoresponse in the visible range, whereas quantum dots (QDs) shows excellent optical response in various optical ranges which can be tuned with diameter. Here, a simple and effective way to develop hybrid phototransistors with extraordinary optoelectronic properties is presented by decorating semiconducting QDs on the surface of a single‐walled carbon nanotube (SWCNT). This hybrid structure demonstrates clear negative photoresponse and optical switching behavior, which could be further tuned by applying external gate bias in the future. A clear type conversion of SWCNT transistor from p‐type to n‐type caused by a charge transfer from attached QDs to CNT is demonstrated. Moreover, this hybrid structure also demonstrates an enhancement in ‘optical Stark effect’ without applying any external electric field. Charged SWCNT surface plays a key role behind the enhancement of optical Stark effect in QDs. The carrier dynamics of the QD and CNT heterostructures system highlights the potential application opportunity of the quantum dot systems, which can be adaptable to the current technologies. A simple and effective way to develop hybrid phototransistor with extraordinary optoelectronic properties is achieved by decorating semiconducting quantum dots (QDs) on a single‐walled carbon nanotube (SWCNT) surface. This hybrid structure demonstrates clear negative photoresponse and optical switching behavior, which could be further tuned by applying external gate bias in the future. Moreover, this hybrid structure also demonstrates an enhancement in the optical Stark effect without applying any external electric field.
  PubDate: 2013-02-27T07:10:17.142022-05:
  DOI: 10.1002/adfm.201203469
Mechanisms for High‐Performance and Non‐Local Photoisomerization Gratings in a Sol–Gel Material
- Authors: Francisco Gallego‐Gómez; Francisco del Monte, Klaus Meerholz
  Pages: n/a - n/a
  Abstract: Novel azo‐containing sol–gel films exhibiting outstanding properties for optical applications via nonlocal photoisomerization gratings were reported recently, although the underlying mechanisms were not well understood, especially regarding the unexpected non‐local effect. Here, this photoisomerizable sol–gel material is characterized in‐depth, analyzing the design and fabrication strategy, and discussing the aspects that enable the efficient photoresponse, with focus on the holographic recording. The material consists of an azochromophore‐rich silica matrix containing glycidoxypropyl groups, which provide increased flexibility and internal free volume for improved dye photoresponse. The matrix characteristics allow a novel procedure for fabrication of thick optical films, in which chromophore aggregation is ruptured by thermal annealing while keeping the material centrosymmetry (beneficial for high hologram contrast). The molecular photo‐orientation promotes alignment of microscopic domains in a cooperative motion, not reported previously in sol–gel materials. This collective mechanism enhances the material response and explains some intriguing features of photoisomerization gratings. In particular, there is evidence that spatially shifted domains are related to the grating nonlocal nature. Different recording (write–erase–write) procedures that distinctly affect the photoalignment at both molecular and microscopic level are studied. The holographic performance drastically changes, which can be selectively exploited for either long‐term or dynamic holography. The fabrication strategy and underlying mechanisms of a highly photoisomerizable sol–gel material is investigated. Glycidoxypropyl groups and thermal annealing decisively improve the material properties for better dye photoresponse. Molecular photo‐orientation promotes cooperative alignment of microscopic domains, enhancing the material response and explaining intriguing features of photoisomerization gratings, such as their potential nonlocal nature. Holographic performance is selectively exploited for either long‐term or dynamic holography.
  PubDate: 2013-02-27T06:30:25.414911-05:
  DOI: 10.1002/adfm.201201281
High‐Concentration Aqueous Dispersions of MoS2
- Authors: Yagang Yao; Lorenzo Tolentino, Zhongzheng Yang, Xiaojuan Song, Wen Zhang, Yongsheng Chen, Ching‐ping Wong
  Pages: n/a - n/a
  Abstract: Molybdenum disulfide (MoS2) nanosheets have been attracting increasing research interests due to their unique material properties. However, the lack of a reliable large‐scale production method impedes their practical applications. Here a facile, efficient, and scalable method for the fabrication of high‐concentration aqueous dispersion of MoS2 nanosheets using combined grinding and sonication is reported. The 26.7 ± 0.7 mg/mL concentration achieved is the highest concentration in an aqueous solution reported up to now. Grinding generates pure shear forces to detach the MoS2 layers from the bulk materials. Subsequent sonication further breaks larger crystallites into smaller crystallites, which promotes the dispersion of MoS2 nanosheets in ethanol/water solutions. The exfoliation process establishes a new paradigm in the top‐down fabrication of 2D nanosheets in aqueous solution. In the meantime, MoS2‐based sensing film produced using this approach has successfully demonstrated the feasibility of a low‐cost and efficient NH3 gas sensor using inkjet printing as a viable method. The lack of a reliable large‐scale production method inhibits practical applications of MoS2 nanosheets. To address this, a facile, efficient, and scalable method for the fabrication of high‐concentration aqueous dispersion of MoS2 nanosheets using combined grinding and sonication is developed. The exfoliation process establishes a new paradigm in the top‐down fabrication of 2D nanosheets in aqueous solution.
  PubDate: 2013-02-27T06:30:18.673672-05:
  DOI: 10.1002/adfm.201201843
Immobilization of Quantum Dots in Nanostructured Porous Silicon Films: Characterizations and Signal Amplification for Dual‐Mode Optical Biosensing
- Authors: Girija Gaur; Dmitry S. Koktysh, Sharon M. Weiss
  Pages: n/a - n/a
  Abstract: Highly sensitive dual‐mode labeled detection of biotin in well‐characterized porous silicon (PSi) films using colloidal quantum dots (QDs) as signal amplifiers are demonstrated. Optimization of the PSi platform for targeted QD infiltration and immobilization is carried out by characterizing and tuning the porosity, film depth, and pore size. Binding events of target QD‐biotin conjugates with streptavidin probes immobilized on the pore walls are monitored by reflective interferometric spectroscopy and fluorescence measurements. QD labeling of the target biotin molecules enables detection based on a distinct fluorescent signal as well as a greater than 5‐fold enhancement in the measured spectral reflectance fringe shift and a nearly three order of magnitude improvement in the detection limit for only 6% surface area coverage of QDs inside the porous matrix. Utilizing the QD signal amplifiers, an exceptional biotin detection limit of ≈6 fg mm−2 is demonstrated with sub‐fg mm−2 detection limits achievable. White light reflective interferometric spectroscopy and fluorescence measurements are used to implement a novel dual‐mode optical porous silicon biosensor. Quantum dots act as signal amplifiers resulting in over an order of magnitude increase in sensor response and providing a secondary means of biomolecule‐specific recognition through their distinct fluorescence spectra.
  PubDate: 2013-02-27T06:30:12.480972-05:
  DOI: 10.1002/adfm.201202697
Short Synthetic β‐Sheet Forming Peptide Amphiphiles as Broad Spectrum Antimicrobials with Antibiofilm and Endotoxin Neutralizing Capabilities
- Authors: Zhan Yuin Ong; Shu Jun Gao, Yi Yan Yang
  Pages: n/a - n/a
  Abstract: Although naturally occurring membrane lytic antimicrobial peptides (AMPs) and their analogs hold enormous promise for antibiotics‐resistant infectious disease therapies, significant challenges such as systemic toxicities, long peptide sequences, poor understanding of structure‐activity relationships, and the potential for compromising innate host defense immunity have greatly limited their clinical applicability. To improve the clinical potential of AMPs, a facile approach is adopted to design a series of short synthetic β‐sheet folding peptide amphiphiles comprised of short recurring (X1Y1X2Y2)n‐NH2 sequences, where X1 and X2: hydrophobic residues (Val, Ile, Phe or Trp), Y1 and Y2: cationic residues (Arg or Lys), and n: number of repeat units; with systematic variations to the cationic and hydrophobic residues to obtain optimized AMP sequences bearing minimal resemblance to naturally occurring sequences. The designed β‐sheet forming peptides exhibit broad spectrum antimicrobial activities against various clinically relevant microorganisms, including Gram‐positive Staphylococcus epidermidis and Staphylococcus aureus, Gram‐negative Escherichia coli and Pseudomonas aeruginosa, and yeast Candida albicans, with excellent selectivities for microbial membranes. Optimal synthetic peptides with n = 2 and n = 3 repeat units, i.e., (IRIK)2‐NH2 and (IRVK)3‐NH2, efficiently inhibit sessile biofilm bacteria growth leading to biomass reduction. Additionally, sequences with n = 3 repeat units effectively neutralize endotoxins while causing minimal cytotoxicities. Taken together, these findings clearly demonstrate that the rationally designed synthetic β‐sheet folding peptides are highly selective, non‐cytotoxic at antimicrobial levels and have tremendous potential for use as broad spectrum antimicrobial agents to overcome multidrug resistance in a wide range of localized, systemic, or external therapeutic applications. Synthetic β‐sheet folding antimicrobial peptides (AMPs) based upon simple recurring amphiphilic sequences of (X1Y1X2Y2)n are designed to enhance their clinical applicability. The peptide length, types of cationic, and hydrophobic residues are systematically varied to identify broad spectrum AMPs with excellent selectivities for microbial membranes. Additionally, the strong antibiofilm and endotoxin neutralization activities of optimized β‐sheet forming peptide sequences are demonstrated.
  PubDate: 2013-02-26T04:10:44.051203-05:
  DOI: 10.1002/adfm.201202850
Ultrathin Graphene Nanofiltration Membrane for Water Purification
- Authors: Yi Han; Zhen Xu, Chao Gao
  Pages: n/a - n/a
  Abstract: A method of fabricating ultrathin (≈22–53 nm thick) graphene nanofiltration membranes (uGNMs) on microporous substrates is presented for efficient water purification using chemically converted graphene (CCG). The prepared uGNMs show well packed layer structure formed by CCG sheets, as characterized by scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. The performance of the uGNMs for water treatment was evaluated on a dead end filtration device and the pure water flux of uGNMs was high (21.8 L m−2 h−1 bar−1). The uGNMs show high retention (>99%) for organic dyes and moderate retention (≈20–60%) for ion salts. The rejection mechanism of this kind of negatively charged membranes is intensively studied, and the results reveal that physical sieving and electrostatic interaction dominate the rejection process. Because of the ultrathin nature of uGNMs, 34 mg of CCG is sufficient for making a square meter of nanofiltration membrane, indicating that this new generation graphene‐based nanofiltration technology would be resource saving and cost‐effective. The integration of high performance, low cost, and simple solution‐based fabrication process promises uGNMs great potential application in practical water purification. Ultrathin graphene nanofiltration membranes (uGNMs) are fabricated on microporous substrates. These graphene membranes (no more than 53‐nm thick) are thin enough to have excellent flexibility and can be bent without any breakage. The uGNMs show high pure water flux and high retention for organic dyes. The dark red Direct Red 81 solution turns into colorless after filtration.
  PubDate: 2013-02-26T04:10:37.603072-05:
  DOI: 10.1002/adfm.201202601
Improving Graphene Diffusion Barriers via Stacking Multiple Layers and Grain Size Engineering
- Authors: Susmit Singha Roy; Michael S. Arnold
  Pages: n/a - n/a
  Abstract: It is shown that the performance of graphene diffusion barriers can be enhanced by stacking multiple layers of graphene and increasing grain size. The focus is on large‐area barriers of graphene grown by chemical vapor deposition (CVD) in the context of passivating an underlying Cu substrate from oxidation in air at 200 °C and use imaging Raman spectroscopy as a tool to temporally and spatially map the barrier performance and to guide barrier design. At 200 °C in air, Cu oxidation proceeds in multiple regimes: first slowly via transport through atomic‐scale grain boundary defects inherent to CVD‐graphene and then more rapidly as the graphene itself degrades and new defects are formed. In the initial regime, the graphene passivates better than previously reported. Whereas oxidation through single sheets primarily occurs through grain boundaries, oxidation through multiple sheets is spatially confined to their intersection. Performance further increases with grain‐size. The degradation of the graphene itself at 200 °C ultimately limits high temperature but suggests superior low temperature barrier performance. This study is expected to improve the understanding of mass transport through CVD‐graphene materials and lead to improved large area graphene materials for barrier applications. It is demonstrated that atomistic defects/grain boundaries in monolayer‐graphene, grown via chemical vapor deposition, can act as diffusion pathways. Transport through these pathways can be substantially reduced by either independently growing separate membranes of graphene and then stacking them together to decrease the line‐of‐sight pathways or increasing the internucleation distance/grain‐size of the graphene monolayer to reduce the grain boundary density.
  PubDate: 2013-02-26T04:10:35.069531-05:
  DOI: 10.1002/adfm.201203179
High CO2/CH4 and C2 Hydrocarbons/CH4 Selectivity in a Chemically Robust Porous Coordination Polymer
- Authors: Jingui Duan; Masakazu Higuchi, Satoshi Horike, Maw Lin Foo, Koya Prabhakara Rao, Yasutaka Inubushi, Tomohiro Fukushima, Susumu Kitagawa
  Pages: n/a - n/a
  Abstract: A new porous coordination polymer, ([La(BTB)H2O]·solvent (1⊃guest)), is synthesized. Gas adsorption, ideal adsorbed solution theory (IAST) and breakthrough experiments of it exhibits high CH4 separation capability toward CO2 and C2 hydrocarbons at 273 K. In addition, this also shows good water and chemical stability, in particular, it is stable at pH = 14 at 100 °C, which is unprecedented for carboxylate‐based porous coordination polymers. Furthermore, the effective adsorption site for separation is revealed by using an in situ diffuse reflectance IR fourier transform ( DRIFT) spectra study. A new porous coordination polymer, ([La(BTB)H2O]·solvent (1⊃guest)), exhibits high CH4 separation capability toward CO2 and C2 hydrocarbons at room temperature. In particular, it shows good water and chemical stability and is stable at pH = 14 at 100 °C.
  PubDate: 2013-02-26T04:10:29.598744-05:
  DOI: 10.1002/adfm.201203288
Achieving High Efficiency and Improved Stability in ITO‐Free Transparent Organic Light‐Emitting Diodes with Conductive Polymer Electrodes
- Authors: Yong Hyun Kim; Jonghee Lee, Simone Hofmann, Malte C. Gather, Lars Müller‐Meskamp, Karl Leo
  Pages: n/a - n/a
  Abstract: Efficient transparent organic light‐emitting diodes (OLEDs) with improved stability based on conductive, transparent poly(3,4‐ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) electrodes are reported. Based on optical simulations, the device structures are carefully optimized by tuning the thickness of doped transport layers and electrodes. As a result, the performance of PEDOT:PSS‐based OLEDs reaches that of indium tin oxide (ITO)‐based reference devices. The efficiency and the long‐term stability of PEDOT:PSS‐based OLEDs are significantly improved. The structure engineering demonstrated in this study greatly enhances the overall performances of ITO‐free transparent OLEDs in terms of efficiency, lifetime, and transmittance. These results indicate that PEDOT:PSS‐based OLEDs have a promising future for practical applications in low‐cost and flexible device manufacturing. Transparent organic light‐emitting diodes (OLEDs) with conductive, transparent poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electrodes are carefully optimized by tuning the device structure. The efficiency of PEDOT:PSS‐based OLEDs is comparable to that of conventional indium tin oxide‐based OLEDs. Long‐term stability of OLEDs with PEDOT:PSS is significantly improved, showing a promising future for practical applications.
  PubDate: 2013-02-26T04:10:25.195129-05:
  DOI: 10.1002/adfm.201203449
From Melamine‐Cyanuric Acid Supramolecular Aggregates to Carbon Nitride Hollow Spheres
- Authors: Young‐Si Jun; Eun Zoo Lee, Xinchen Wang, Won Hi Hong, Galen D. Stucky, Arne Thomas
  Pages: n/a - n/a
  Abstract: Graphitic carbon nitride (g‐CN) is a promising heterogeneous metal‐free catalyst for organic photosynthesis, solar energy conversion, and photodegradation of pollutants. Its catalytic performance is easily adjustable by modifying texture, optical, and electronic properties via nanocasting, doping, and copolymerization. However, simultaneous optimization has yet to be achieved. Here, a facile synthesis of mesoporous g‐CN using molecular cooperative assembly between triazine molecules is reported. Flower‐like, layered spherical aggregates of melamine cyanuric acid complex (MCA) are formed by precipitation from equimolecular mixtures in dimethyl sulfoxide (DMSO). Thermal polycondensation of MCA under nitrogen at 550 °C produces mesoporous hollow spheres comprised of tri‐s‐triazine based g‐CN nanosheets (MCA‐CN) with the composition of C3N4.14H1.98. The layered structure succeeded from MCA induces stronger optical absorption, widens the bandgap by 0.16 eV, and increases the lifetime of photoexcited charge carriers by twice compared to that of the bulk g‐CN, while the chemical structure remains similar to that of the bulk g‐CN. As a result of these simultaneous modifications, the photodegradation kinetics of rhodamine B on the catalyst surface can be improved by 10 times. Simple molecular engineering of triazine precursors enables simultaneous optimization of the texture and photoelectric properties of graphitic carbon nitride (g‐CN). Thermolysis of flower‐like supramolecular aggregates of melamine and cyanuric acid yields the formation of mesoporous g‐CN hollow spheres with the typical nanosheet‐type structure preserved in the microspheres. Such structures are highly active in the photocatalytic degradation of organic pollutants.
  PubDate: 2013-02-26T04:10:18.191978-05:
  DOI: 10.1002/adfm.201203732
Dynamic Doping in Planar Ionic Transition Metal Complex‐Based Light‐Emitting Electrochemical Cells
- Authors: Sebastian B. Meier; Stephan van Reenen, Bastien Lefevre, David Hartmann, Henk J. Bolink, Albrecht Winnacker, Wiebke Sarfert, Martijn Kemerink
  Pages: n/a - n/a
  Abstract: Using a planar electrode geometry, the operational mechanism of iridium(III) ionic transition metal complex (iTMC)‐based light‐emitting electrochemical cells (LECs) is studied by a combination of fluorescence miscroscopy and scanning Kelvin probe microscopy (SKPM). Applying a bias to the LECs leads to the quenching of the photoluminescence (PL) in between the electrodes and to a sharp drop of the electrostatic potential in the middle of the device, far away from the contacts. The results shed light on the operational mechanism of iTMC‐LECs and demonstrate that these devices work essentially the same as LECs based on conjugated polymers do, i.e., according to an electrochemical doping mechanism. Moreover, with proceeding operation time the potential drop shifts towards the cathode coincident with the onset of light emission. During prolonged operation the emission zone and the potential drop both migrate towards the anode. This event is accompanied by a continuous quenching of the PL in two distinct regions separated by the emission line. A combination of fluorescence and scanning Kelvin probe microscopy is used to gain insight into the operational mechanism of planar ionic transition metal complex‐based light‐emitting electrochemical cells. Quenching of the photoluminescence in the interelectrode gap in accord with a sharp potential drop far away from the electrodes confirm an electrochemical doping mechanism, which results in the formation of a light‐emitting p‐i‐n junction.
  PubDate: 2013-02-26T04:10:10.282077-05:
  DOI: 10.1002/adfm.201202689
Single Pulse Laser‐Induced Phase Transitions of PLD‐Deposited Ge2Sb2Te5 Films
- Authors: Hongbing Lu; Erik Thelander, Jürgen W. Gerlach, Ulrich Decker, Benpeng Zhu, Bernd Rauschenbach
  Pages: n/a - n/a
  Abstract: Phase transformations between amorphous and crystallized states are induced by irradiation with a single nanosecond laser pulse in Ge2Sb2Te5 films grown by pulsed laser deposition. By adjusting the laser fluence, the two different phases are obtained and can be distinguished by their different optical reflectivity. The effect of laser fluence on the crystalline nature of the films is studied in detail. Large structural differences between the laser‐irradiated and thermally annealed films are revealed, due to the high heating rate and short duration of the laser pulse. X‐ray reflectivity measurements show a density increase of 3.58% upon laser‐induced crystallization. The effect of laser fluence on the crystalline nature of Ge2Sb2Te5 films is studied in detail. Large structural differences between the laser‐irradiated and thermally annealed films are revealed to be caused by the high heating rate and short duration of the laser pulse.
  PubDate: 2013-02-25T07:10:30.070462-05:
  DOI: 10.1002/adfm.201202665
Nongeminate Recombination and Charge Transport Limitations in Diketopyrrolopyrrole‐Based Solution‐Processed Small Molecule Solar Cells
- Authors: Christopher M. Proctor; Chunki Kim, Dieter Neher, Thuc‐Quyen Nguyen
  Pages: n/a - n/a
  Abstract: Charge transport and nongeminate recombination are investigated in two solution‐processed small molecule bulk heterojunction solar cells consisting of diketopyrrolopyrrole (DPP)‐based donor molecules, mono‐DPP and bis‐DPP, blended with [6,6]‐phenyl‐C71‐butyric acid methyl ester (PCBM). While the bis‐DPP system exhibits a high fill factor (62%) the mono‐DPP system suffers from pronounced voltage dependent losses, which limit both the fill factor (46%) and short circuit current. A method to determine the average charge carrier density, recombination current, and effective carrier lifetime in operating solar cells as a function of applied bias is demonstrated. These results and light intensity measurements of the current‐voltage characteristics indicate that the mono‐DPP system is severely limited by nongeminate recombination losses. Further analysis reveals that the most significant factor leading to the difference in fill factor is the comparatively poor hole transport properties in the mono‐DPP system (2 × 10−5 cm2 V−1 s−1 versus 34 × 10−5 cm2 V−1 s−1). These results suggest that future design of donor molecules for organic photovoltaics should aim to increase charge carrier mobility thereby enabling faster sweep out of charge carriers before they are lost to nongeminate recombination. Charge transport and voltage‐dependent recombination losses are studied in two diketopyrrolopyrrole (DPP)‐based solution‐processed small molecule bulk heterojunction solar cells. Light intensity and impedance spectroscopy measurements probe the influence of nongeminate recombination losses in both systems. Further analysis suggests the increase in fill factor observed in the bis‐DPP system is a direct result of the higher hole mobility.
  PubDate: 2013-02-25T07:10:22.250449-05:
  DOI: 10.1002/adfm.201202643
Polymer Grafting by Inkjet Printing: A Direct Chemical Writing Toolset
- Authors: Alexandre Garcia; Nassim Hanifi, Bruno Jousselme, Pascale Jégou, Serge Palacin, Pascal Viel, Thomas Berthelot
  Pages: n/a - n/a
  Abstract: Among all the patterning techniques, inkjet printing has lately become a reliable technique at micrometer scale to produce localized modifications on material surfaces. Printing of polymer on material surface however leads to adsorbed patterns with poor adhesion. To overcome this drawback, a new process combining for the first time inkjet printing and an efficient covalent polymer grafting method was developed. This latter method is based on a photo‐assisted reduction of aryldiazonium salt/acrylate monomer ink, derived from the already published GraftFast process. In order to demonstrate its versatility, this new localized polymer grafting process is here combined as an example with the ligand induced electroless plating (LIEP) process to obtain metal interconnects onto flexible and transparent substrates with excellent mechanical and electrical properties for applications in flexible electronics devices. A photoassisted and inkjet‐printed covalent polymer grafting method based on the photoreduction of aryldiazonium is presented. This powerful and versatile method to obtain local surface functionalization is combined with the ligand induced electroless plating process to obtain metal patterns onto flexible and transparent substrates with excellent mechanical and electrical properties, which may find applications in flexible electronics devices.
  PubDate: 2013-02-19T15:10:16.450363-05:
  DOI: 10.1002/adfm.201203544
A Multifunction Heterojunction Formed Between Pentacene and a Single‐Crystal Silicon Nanomembrane
- Authors: Jung‐Hun Seo; Tae‐Yeon Oh, Jungho Park, Weidong Zhou, Byeong‐Kwon Ju, Zhenqiang Ma
  Pages: n/a - n/a
  Abstract: A mesh patterned n‐type single‐crystalline silicon nanomembrane (SiNM) created from a silicon‐on‐insulator (SOI) wafer is complementally combined with a p‐type pentacene layer to form a heterogeneous p‐n junction on a flexible plastic substrate. Excellent rectifying characteristics are obtained from the heterogeneous p‐n diode. The diode also exhibits photosensitivity at visible wavelengths with a photo‐to‐dark current ratio exceeding four orders, a responsivity of 0.7 A/W, and an external quantum efficiency of 21.9% at 633 nm. Over 60% average transmittance in the visible spectrum is measured from the heterogeneous multilayer junction on a plastic substrate. Outstanding mechanical bending characteristics are observed with up to 1.08% of strain applied to the diode. These results suggest that organic‐inorganic heterogeneous integration may be a viable strategy to build flexible organic‐inorganic heterojunction devices and thus enable a number of novel multifunctional applications. A mesh patterned n‐type single‐crystalline silicon nanomembrane is complementally combined with a p‐type pentacene layer to form a heterogeneous p‐n junction on a flexible plastic substrate. The flexible heterogeneous photodetector also exhibits good photosensitivity and external quantum efficiency at visible wavelengths. Over 60% average transmittance in the visible spectrum is measured and outstanding mechanical bending characteristics are observed.
  PubDate: 2013-02-19T15:10:12.264295-05:
  DOI: 10.1002/adfm.201203309
A Mechanistic Study of Wetting Superhydrophobic Porous 3D Meshes
- Authors: Stefan T. Yohe; Jonathan D. Freedman, Eric J. Falde, Yolonda L. Colson, Mark W. Grinstaff
  Pages: n/a - n/a
  Abstract: Superhydrophobic, porous, 3D materials composed of poly(ϵ‐caprolactone) (PCL) and the hydrophobic polymer dopant poly(glycerol monostearate‐co‐ϵ‐caprolactone) (PGC‐C18) are fabricated using the electrospinning technique. These 3D materials are distinct from 2D superhydrophobic surfaces, with maintenance of air at the surface as well as within the bulk of the material. These superhydrophobic materials float in water, and when held underwater and pressed, an air bubble is released and will rise to the surface. By changing the PGC‐C18 doping concentration in the meshes and/or the fiber size from the micro‐ to nanoscale, the long‐term stability of the entrapped air layer is controlled. The rate of water infiltration into the meshes, and the resulting displacement of the entrapped air, is quantitatively measured using X‐ray computed tomography. The properties of the meshes are further probed using surfactants and solvents of different surface tensions. Finally, the application of hydraulic pressure is used to quantify the breakthrough pressure to wet the meshes. The tools for fabrication and analysis of these superhydrophobic materials as well as the ability to control the robustness of the entrapped air layer are highly desirable for a number of existing and emerging applications. Superhydrophobic, porous, 3D materials composed of poly(ϵ‐caprolactone) (PCL) and poly(glycerol monostearate‐co‐ϵ‐caprolactone) (PGC‐C18) as a hydrophobic dopant are fabricated using the electrospinning technique. These materials are distinct from 2D superhydrophobic surfaces with maintenance of air at the surface and within the bulk of the material, and, thus, these functional materials can be considered for new applications where time and rate of wetting dictate performance.
  PubDate: 2013-02-19T09:10:20.956888-05:
  DOI: 10.1002/adfm.201203111
A One‐Step and Binder‐Free Method to Fabricate Hierarchical Nickel‐Based Supercapacitor Electrodes with Excellent Performance
- Authors: Guoge Zhang; Wenfang Li, Keyu Xie, Fei Yu, Haitao Huang
  Pages: n/a - n/a
  Abstract: Research is currently being carried out in the search for alternative electrode materials to replace the expensive and toxic RuO2‐based electrode. As a typical example, nickel oxide or hydroxide has been widely studied but the results are far from satisfactory. Here, using a facile one‐step anodization method, a hierarchical nickel compound (HNC) film with an interconnecting 3D nanoflake structure is obtained, providing large electrochemically active surface area and interconnecting nanoscale pore channels for ion transport. The HNC electrode demonstrates significantly improved capacitance, 70 times higher than the reported NiO‐TiO2 nanotube array electrode with similar thickness. The charge/discharge kinetics are also superior, showing only a 24% capacitance reduction when the scan rate is increased by 50 times, as compared with the typical 70% capacitance reduction for pseudocapacitor electrodes under the same conditions. HNC exhibits an extraordinary excellent cycle life; capacitance increases to 115% after 4500 test cycles. Furthermore, because HNC is in intimate contact with the current collector, it is not necessary to use conducting agents or binders, which reduces the electrode weight and facilitates the electrode preparation process. The method is low cost, facile, scalable, additive free, and is promising for fabricating supercapacitor electrode with excellent performance. A hierarchical nickel compound (HNC) film is prepared using a facile, one‐step, and binder‐free method. The HNC demonstrates both large capacitance and superior rate capability. The capacitance reduction is only 24%, even when the scan rate is increased by 50 times. The HNC also exhibits an excellent cycle life.
  PubDate: 2013-02-19T09:10:14.369437-05:
  DOI: 10.1002/adfm.201203418
Self‐Supporting Graphene Hydrogel Film as an Experimental Platform to Evaluate the Potential of Graphene for Bone Regeneration
- Authors: Jiayu Lu; Yu‐Shi He, Chi Cheng, Yi Wang, Ling Qiu, Dan Li, Derong Zou
  Pages: n/a - n/a
  Abstract: Graphene, a two dimensional carbonaceous material possessing a range of extraordinary properties, is considered promising for biomedical applications. Here, a simple form of graphene‐based bulk material–self‐supporting graphene hydrogel (SGH) film is used as a suitable platform to study the intrinsic properties of graphene both in vitro and in vivo. The free‐standing film show good cell adhesion, spreading, and proliferation. Films are implanted into subcutaneous sites of rats, and produce minimal fibrous capsule formation, and mild host tissue response in vivo. New blood vessel formation is also seen. The films swell and cracked in vivo, indicating the beginning of degradation. Of particular interest is that the film alone is found to be able to stimulate osteogenic differentiation of stem cells, without additional inducer, both in vitro and in vivo. Thus, this SGH film appears to be highly biocompatible and osteoinductive, demonstrating graphene's potential for bone regenerative medicine. A self‐supporting graphene hydrogel film prepared by a simple vacuum filtration technology is used as a unique experimental platform to study how graphene interacts with biological tissues both in vitro and in vivo. This graphene‐based bulk material appears to be highly biocompatible, biodegradable, and osteoinductive, demonstrating graphene's potential for bone regenerative medicine.
  PubDate: 2013-02-18T03:29:13.513686-05:
  DOI: 10.1002/adfm.201203637
A Fabricated siRNA Nanoparticle for Ultralong Gene Silencing In Vivo
- Authors: Seung Koo Lee; Ching‐Hsuan Tung
  Pages: n/a - n/a
  Abstract: Persistent gene silencing is crucially required for the successful therapeutics of short interfering RNA (siRNA). Here, a nanoparticle‐based delivery system is presented which assembles by layering siRNAs between protease degradable polypeptides to extend the therapeutic window. These tightly packed nanoparticles are efficiently taken up by cells by endocytosis, and the fabricated siRNAs are gradually released following intracellular degradation of the polypeptide layers. During cell division, the particles are distributed to the daughter cells. Due to the slow degradation through the multiple layers, the particles continuously release siRNA in all cells. Using this controlled release construct, the in vivo gene silencing effect of siRNA is consistent for an ultralong period of time (>3 weeks) with only a single treatment. An ultralong siRNA gene silencing effect is achieved in cells and in animal using a functionalized siRNA nanoparticle that consists of multiple siRNA and degradable polypeptide layers. Due to the slow degradation through the layers, the particles continuously release siRNA and the in vivo gene silencing effect is consistent for more than 3 weeks with only a single treatment.
  PubDate: 2013-02-18T03:29:08.987417-05:
  DOI: 10.1002/adfm.201202777
Improving the Photovoltage of Dithienopyrrole Dye‐Sensitized Solar Cells via Attaching the Bulky Bis(octyloxy)biphenyl Moiety to the Conjugated π‐Linker
- Authors: Ning Cai; Jing Zhang, Mingfei Xu, Min Zhang, Peng Wang
  Pages: n/a - n/a
  Abstract: The judicious design of 3D giant organic dye molecules to enable the formation of a porous photoactive layer on the surface of titania is one of the viable tactics to abate the adverse interfacial charge recombination in dye‐sensitized solar cells (DSCs) employing outer‐sphere redox couples. Here 2′,6′‐bis(octyloxy)‐biphenyl substituted dithieno[3,2‐b:2′,3′‐d]pyrrole segment is constructed and employed as the π‐linker of a high molar absorption coefficient organic push‐pull dye. With respect to its congener possessing the hexyl substituted dithieno[3,2‐b:2′,3′‐d]pyrrole linker, the new dye can self‐assemble on the surface of titania to afford a porous organic coating, which effectively slow down the kinetics of charge recombination of titania electrons with both outer‐sphere tris(1,10‐phenanthroline)cobalt(III) ions and photooxidized dye molecules, improving the cell photovoltage. In addition, the diminishments of charge recombination via modulating the microstructure of interfacial functional zone can also overcompensate the disadvantageous impact of reduced light‐harvesting and evoke an enhanced photocurrent output, bringing forth an efficiency improvement from 7.5% to 9.3% at the 100 mW cm−2, simulated AM1.5 conditions. A high molar absorption coefficient dithienopyrrole dye featuring the 3D giant bis(octyloxy)biphenyl segment is synthesized and employed to fabricate a 9.3% dye‐sensitized solar cell at the AM1.5G conditions. The solar cell exhibits a reduced interfacial charge recombination of photoinjected electrons with both tris(1,10‐phenanthroline)cobalt(III) ions and dye cations, in comparison with its congener possessing the hexyl substitutent.
  PubDate: 2013-02-18T03:16:59.357778-05:
  DOI: 10.1002/adfm.201203348
Monolayer Doping via Phosphonic Acid Grafting on Silicon: Microscopic Insight from Infrared Spectroscopy and Density Functional Theory Calculations
- Authors: Roberto C. Longo; Kyeongjae Cho, Wolf Gero Schmidt, Yves J. Chabal, Peter Thissen
  Pages: n/a - n/a
  Abstract: Monolayer doping (MLD) is a promising technique for creating ultra shallow junctions (USJs). Here, a novel self assembled monolayer (SAM) grafting technique is proposed through a single oxygen atom capable of MLD. Consequently, this approach can use simple forms of alkylphosphonic acids and avoid carbon contamination altogether during the doping process. In this paper, density functional theory (DFT) is used to explore the way how alkylphosphonic acid molecules can in just one chemical step be grafted on H‐terminated Si(111). A maximum coverage of alkylphosphonic acids is found at 2/3 due to steric constrain forces. It is further demonstrated by means of in situ infrared spectroscopy and DFT calculations that the weak link of an alkylphosphonic acid, such as octadecylphosphonic acid (ODPA), is the P‐C bond, with typical release of the carbon ligand around 500 °C. Finally, after release of the carbon ligand, an unsaturated electron configuration is driving force for the phosphorous to start the MLD process. Density functional theory (DFT) is used to explore the way how alkylphosphonic acid molecules can in just one chemical step be grafted on H‐terminated Si(111). It is further demonstrated by means of in situ infrared spectroscopy and DFT calculations that the weak link of an alkylphosphonic acid is the P‐C bond, with typical release of the carbon ligand around 500 °C. Finally, after release of the carbon ligand, an unsaturated electron configuration is driving force for the phosphorous to start the monolayer doping process.
  PubDate: 2013-02-18T03:16:56.226516-05:
  DOI: 10.1002/adfm.201202808
Hierarchical Layered Double Hydroxide Microspheres with Largely Enhanced Performance for Ethanol Electrooxidation
- Authors: Mingfei Shao; Fanyu Ning, Jingwen Zhao, Min Wei, David G. Evans, Xue Duan
  Pages: n/a - n/a
  Abstract: Hierarchical MgFe‐layered double hydroxide (LDH) microspheres with tunable interior structure are synthesized by a facile and cost‐effective surfactant‐templated method. Scanning and transmission electron microscopy images reveal that the obtained microspheres display a three‐dimensional architecture with hollow, yolk−shell and solid interior structure, respectively, with continuous changes in specific surface area and pore‐size distribution. Moreover, the hollow MgFe‐LDH microspheres exhibit excellent electrocatalytic oxidation of ethanol in alkaline fuel cell, including high activity, long‐term durability and cycling stability, owing to the significantly improved faradaic redox reaction and mass transport. Therefore, this work provides a promising approach for the design and synthesis of structure tunable materials with largely enhanced ethanol electrooxidation behavior, which can be potentially used in noble metal‐free alkaline fuel cells. Hierarchical MgFe‐layered double hydroxide (LDH) microspheres with tunable interior structure (hollow, yolk−shell, and solid) are synthesized by a facile and cost‐effective surfactant‐templated method. The resulting hollow LDH microspheres yields largely enhanced activity as well as robust durability towards ethanol electro‐oxidation, owing to the significantly improved mass transport and faradaic redox reaction.
  PubDate: 2013-02-18T03:16:54.512698-05:
  DOI: 10.1002/adfm.201202825
High‐Mobility n‐Type Organic Transistors Based on a Crystallized Diketopyrrolopyrrole Derivative
- Authors: Won Sik Yoon; Sang Kyu Park, Illhun Cho, Jeong‐A Oh, Jong H. Kim, Soo Young Park
  Pages: n/a - n/a
  Abstract: A new high‐performing small molecule n‐channel semiconductor based on diketopyrrolopyrrole (DPP), 2,2′‐(5,5′‐(2,5‐bis(2‐ethylhexyl)‐3,6‐dioxo‐2,3,5,6‐tetrahydropyrrolo[3,4‐c]pyrrole‐1,4‐diyl)bis(thiophene‐5,2‐diyl))bis(methan‐1‐yl‐1‐ylidene)dimalononitrile (DPP‐T‐DCV), is successfully synthesized. The frontier molecular orbitals in this designed structure are elaborately tuned by introducing a strong electron‐accepting functionality (dicyanovinyl). The well‐defined lamellar structures of the crystals display a uniform terrace step height corresponding to a molecular monolayer in the solid‐state. As a result of this tuning and the remarkable crystallinity derived from the conformational planarity, organic field‐effect transistors (OFETs) based on dense‐packed solution‐processed single‐crystals of DPP‐T‐DCV exhibit an electron mobility (μe) up to 0.96 cm2 V−1 s−1, one of the highest values yet obtained for DPP derivative‐based n‐channel OFETs. Polycrystalline OFETs show promise (with an μe up to 0.64 cm2 V−1 s−1) for practical utility in organic device applications. A new dicyanovinyl‐substituted diketopyrrolopyrrole (DPP)‐based small molecule, DPP‐T‐DCV, shows outstanding electron mobility (μe) in solution‐processed single‐crystal OFETs (SC‐OFETs). The molecular structure of DPP‐T‐DCV, SC‐OFETs device structure, and atomic force microscopy images of the film surface of the DPP‐T‐DCV crystal device are shown. SC‐OFETs exhibit μe as high as 0.96 cm2 V−1 s−1 with on/off current ratio of ≈105.
  PubDate: 2013-02-18T03:16:53.285097-05:
  DOI: 10.1002/adfm.201203065
Graphene‐Based Mesoporous SnO2 with Enhanced Electrochemical Performance for Lithium‐Ion Batteries
- Authors: Sheng Yang; Wenbo Yue, Jia Zhu, Yu Ren, Xiaojing Yang
  Pages: n/a - n/a
  Abstract: Graphene‐based metal oxides generally show outstanding electrochemical performance due to the superior properties of graphene. However, the aggregation of active metal oxide nanoparticles on the graphene surface may result in a capacity fading and poor cycle performance. Here, a mesostructured graphene‐based SnO2 composite is prepared through in situ growth of SnO2 particles on the graphene surface using cetyltrimethylammonium bromide as the structure‐directing agent. This novel mesoporous composite inherits the advantages of graphene nanosheets and mesoporous materials and exhibits higher reversible capacity, better cycle performance, and better rate capability compared to pure mesoporous SnO2 and graphene‐based nonporous SnO2. It is concluded that the synergetic effect between graphene and mesostructure benefits the improvement of the electrochemical properties of the hybrid composites. This facile method may offer an attractive alternative approach for preparation of the graphene‐based mesoporous composites as high‐ performance electrodes for lithium‐ion batteries. A graphene‐based mesoporous SnO2 composite is prepared via in situ growth of mesoporous SnO2 on the graphene surface using cetyltrimethylammonium bromide (CTAB) as a template. Because of the high specific surface area and stable mesostructure of SnO2 nanocrystalline on graphene, it displays higher reversible capacity, better cycle performance, and better rate capability compared to mesoporous SnO2 and graphene‐based non‐porous SnO2.
  PubDate: 2013-02-18T03:16:49.526409-05:
  DOI: 10.1002/adfm.201203286
On‐Substrate Preparation of an Electroactive Conjugated Polyazomethine from Solution‐Processable Monomers and its Application in Electrochromic Devices
- Authors: Lambert Sicard; Daminda Navarathne, Thomas Skalski, W. G. Skene
  Pages: n/a - n/a
  Abstract: An electroactive polyazomethine is prepared from a solution processable 2,5‐diaminothiophene derivative and 4,4′‐triphenylamine dialdehyde by spray‐coating the monomers on substrates, including indium tin oxide (ITO) coated glass and native glass slides. The conjugated polymer was rapidly formed in situ by heating the substrates at 120 °C for 30 min in an acid saturated atmosphere. The resulting immobilized polymer is easily purified by rinsing the substrate with dichloromethane. The on‐substrate polymerization is tolerant towards large stoichiometry imbalances of the comonomers, unlike solution step‐growth polymerization. The resulting polyazomethine is electroactive and it can be switched reversibly between its neutral and oxidized states both electrochemically and chemically without degradation. A transmissive electrochromic device is fabricated from the immobilized polyazomethine on an ITO electrode. The resulting device is successfully cycled between its oxidized (dark blue) and neutral (cyan/light green) states with applied biases of +3.2 and ‐1.5 V under ambient conditions without significant color fatigue or polymer degradation. The coloration efficiency of the oxidized state at 690 nm is 102 cm2 C−1. Thermal polymerization of a conjugated polymer is performed directly on the device electrode using solution‐processable monomers. The resulting immobilized polymer is electroactive and it is reversibly switched electrotrochemically between its colored states. The polymer is successfully used as the electrochromic layer in a working device with repeated color switching.
  PubDate: 2013-02-18T03:16:44.460083-05:
  DOI: 10.1002/adfm.201203657
Nanoparticle Direct Doping: Novel Method for Manufacturing Three‐Dimensional Bulk Plasmonic Nanocomposites
- Authors: Marcin Gajc; Hancza B. Surma, Andrzej Klos, Katarzyna Sadecka, Krzysztof Orlinski, Andrey E. Nikolaenko, Krzysztof Zdunek, Dorota A. Pawlak
  Pages: n/a - n/a
  Abstract: Metallodielectric materials with plasmonic resonances at optical and infrared wavelengths are attracting increasing interest, due to their potential novel applications in the fields of photonics, plasmonics and photovoltaics. However, simple and fast fabrication methods for three‐dimensional bulk plasmonic nanocomposites that offer control over the size, shape and chemical composition of the plasmonic elements have been missing. Here, such a manufacturing method and examples of experimental realizations of volumetric isotropic nanocomposites doped with plasmonic nanoparticles that exhibit resonances at visible and infrared wavelengths are presented. This method is based on doping a low‐melting dielectric material with plasmonic nanoparticles, using a directional glass‐solidification process. Transmission‐spectroscopy experiments confirm a homogenous distribution of the nanoparticles, isotropy of the material and resonant behavior. The phenomenon of localized surface plasmon resonance is also observed visually. This approach may enable rapid and cost‐efficient manufacturing of bulk nanoplasmonic composites with single or multiple resonances at various wavelength ranges. These composites could be isotropic or anisotropic, and potentially co‐doped with other chemical agents, in order to enhance different optical processes. Metallodielectric materials with plasmonic resonances at optical and infrared wavelengths are attracting increasing interest, due to their potential novel applications in photonics, plasmonics and photovoltaics. Here, a manufacturing method is presented with experimental realizations of volumetric nanocomposites doped with plasmonic nanoparticles that exhibit resonances at visible/infrared wavelengths.
  PubDate: 2013-02-15T10:07:27.805258-05:
  DOI: 10.1002/adfm.201203116
Fluorinated Polymer Yields High Organic Solar Cell Performance for a Wide Range of Morphologies
- Authors: John R. Tumbleston; Andrew C. Stuart, Eliot Gann, Wei You, Harald Ade
  Pages: n/a - n/a
  Abstract: Device performance is recognized to be generally sensitive to morphology in bulk heterojunction solar cells. Through the use of quantitative morphological measurements, it is demonstrated that devices based on benzodithiophene and fluorinated benzotriazole moieties constitute an exception to this design rule and exhibit a range of morphologies that yield similar high performance. In particular, the fill factor (FF) remains above 65% even with factor of two changes in domain size and factor of two changes in relative domain purity. Devices with active layer thicknesses of 250 nm are employed, which are capable of increasing optical absorption to produce high photocurrent. The general insensitivity to both morphology and thickness is likely related to the measured low equilibrium miscibility of fullerene in the polymer of 3‐4%. The materials and processes investigated therefore provide insights into functional material design that yield increased processing latitude and may be more amenable to roll‐to‐roll processing. Narrow ranges of morphological properties are typically required to achieve high performance in bulk heterojunction organic solar cells. This is not the case for the particular fluorinated polymer‐based blend investigated, for which excellent performance is achieved for a range of domain sizes and domain purities. Broad processing latitudes and unconventionally thick active layers afforded by this material offer the potential to rapidly fabricate high performing devices without strict morphological control.
  PubDate: 2013-02-15T10:06:16.721378-05:
  DOI: 10.1002/adfm.201300093
Microfabricated Porous Silk Scaffolds for Vascularizing Engineered Tissues
- Authors: Lindsay S. Wray; Konstantinos Tsioris, Eun Seok Gil, Fiorenzo G. Omenetto, David L. Kaplan
  Pages: n/a - n/a
  Abstract: There is critical clinical demand for tissue‐engineered (TE), 3D constructs for tissue repair and organ replacements. Current efforts toward this goal are prone to necrosis at the core of larger constructs because of limited oxygen and nutrient diffusion. Therefore, critically sized 3D TE constructs demand an immediate vascular system for sustained tissue function upon implantation. To address this challenge the goal of this project was to develop a strategy to incorporate microchannels into a porous silk TE scaffold that could be fabricated reproducibly using microfabrication and soft lithography. Silk is a suitable biopolymer material for this application because it is mechanically robust, biocompatible, slowly degrades in vivo, and has been used in a variety of TE constructs. Here, the fabrication of a silk‐based TE scaffold that contains an embedded network of porous microchannels is reported. Enclosed porous microchannels support endothelial lumen formation, a critical step toward development of the vascular niche, while the porous scaffold surrounding the microchannels supports tissue formation, demonstrated using human mesenchymal stem cells. This approach for fabricating vascularized TE constructs is advantageous compared to previous systems, which lack porosity and biodegradability or degrade too rapidly to sustain tissue structure and function. The broader impact of this research will enable the systemic study and development of complex, critically‐sized engineered tissues, from regenerative medicine to in vitro tissue models of disease states. A silk scaffold is patterned with porous microchannels for engineering vascularized tissues. Microchannel dimensions range from 25 to 300 μm and endothelial cells proliferate to confluence within the channels. The microchannels are enclosed with biocompatible tissue adhesive. The scaffold supports endothelial lumen formation in the enclosed micochannels and supports stem cell co‐culture in the bulk space.
  PubDate: 2013-02-13T02:30:18.434653-05:
  DOI: 10.1002/adfm.201202926
Self‐Healing and Antifouling Multifunctional Coatings Based on pH and Sulfide Ion Sensitive Nanocontainers
- Authors: Zhaoliang Zheng; Xing Huang, Matthias Schenderlein, Dimitriya Borisova, Rong Cao, Helmuth Möhwald, Dmitry Shchukin
  Pages: n/a - n/a
  Abstract: Application of mesoporous silica nanoparticles (MSNs) as delivery tools for self‐healing coatings is limited by spontaneous leakage and specifically responsive release of small molecular inhibitors. In this work, a pH/sulfide ion responsive release system based on MSNs using a Cu‐BTA complex forms at the openings of the mesopores into which BTA (corrosion inhibitor) and benzalkonium chloride (biocide) are loaded. The spontaneous leakage of active species is completely avoided and premature release of the loaded composition was lowered to 0.02. The responsive release begins when the pH is lower than 5 or [S2−] is higher than 0.02 mM (about 0.6 ppm). The hybrid coating containing the responsive release system exhibits feedback self‐healing property sensitive to lowering of pH and sulfide ion concentration and, at the same time, provides a high barrier level for a long time. Due to incorporation of biocide in the release system, the coating is also provided with antifouling properties. A system for controlled release of small molecular corrosion inhibitors and antifouling agents is designed to entrap the active compounds and open the nanovalves only in the presence of pH lowering and sulfide ions, obtaining a multifunctional coating with self‐healing anticorrosion and antifouling properties.
  PubDate: 2013-02-13T02:30:04.977021-05:
  DOI: 10.1002/adfm.201203180
Synthesis, Characterization, and Application of Antibody Functionalized Fluorescent Silica Nanoparticles
- Authors: Matthew T. Hurley; Zifan Wang, Amanda Mahle, Daniel Rabin, Qing Liu, Douglas S. English, Michael R. Zachariah, Daniel Stein, Philip DeShong
  Pages: n/a - n/a
  Abstract: Fluorescent silica nanoparticles (FSNs) are prepared by incorporating dye into a mesoporous silica nanoparticle (MSN) synthesis procedure. FSNs containing sulforhodamine B, hydrophobically modified sulforhodamine B, and Casdade Blue hydrazide are made. The MSN‐based FSNs do not leach dye under simulated physiological conditions and have strong, stable fluorescence. FSNs prepared with sulforhodamine B are compared to FSNs prepared with hydrophobically modified sulforhodamine B. The data indicate that FSNs prepared with sulforhodamine B are equally as stable but twice as fluorescent as particles made with hydrophobically modified sulforhodamine B. The fluorescence of a FSN prepared with sulforhodamine B is 10 times more intense than the fluorescence of a 4.5 nm core–shell CdSe/ZnS quantum dot. For diagnostic applications, a method to selectively and covalently bind antibodies to the surface of the FSNs is devised. FSNs that are functionalized with antibodies specific for Neisseria gonorrhoeae specifically bind to Neisseria gonorrhoeae in flow cytometry experiments, thus demonstrating the functionality of the attached antibodies and the potential of MSN‐based FSNs to be used in diagnostic applications. Fluorescent silica nanoparticles (FSNs) are prepared by incorporating dye into a mesoporous silica nanoparticle synthesis procedure. The particles do not leach dye and have strong, stable fluorescence. FSNs functionalized with antibody specific for Neisseria gonorrhoeae selectively bind Neisseria gonorrhoeae in flow cytometry experiments.
  PubDate: 2013-02-11T03:40:12.947656-05:
  DOI: 10.1002/adfm.201202699
Rigidity‐Patterned Polyelectrolyte Films to Control Myoblast Cell Adhesion and Spatial Organization
- Authors: Claire Monge; Naresh Saha, Thomas Boudou, Cuauhtemoc Pózos‐Vásquez, Virginie Dulong, Karine Glinel, Catherine Picart
  Pages: n/a - n/a
  Abstract: In vivo, cells are sensitive to the stiffness of their microenvironment and to the spatial organization of the stiffness. In vitro studies of this phenomenon can help to better understand the mechanisms of the cell response to spatial variations of the matrix stiffness. Here, polelyelectrolyte multilayer films made of poly(L‐lysine) and a photoreactive hyaluronan derivative are designed. These films can be photo‐crosslinked through a photomask to create spatial patterns of rigidity. Quartz substrates incorporating a chromium mask are prepared to expose selectively the film to UV light (in a physiological buffer), without any direct contact between the photomask and the soft film. It is shown that these micropatterns are chemically homogeneous and flat, without any preferential adsorption of adhesive proteins. Three groups of pattern geometries differing by their shape (circles or lines), size (from 2 to 100 μm), or interspacing distance between the motifs are used to study the adhesion and spatial organization of myoblast cells. The results pave the way for the study of the different steps of myoblast fusion in response to matrix rigidity in well‐defined geometrical conditions. Rigidity photopatterns are created by the photo‐crosslinking of a polyelectrolyte multilayer film made of a photoreactive hyaluronan derivative. Orientation of C2C12 myoblasts and nuclear elongation is observed on linear micropatterns.
  PubDate: 2013-02-07T13:30:04.6532-05:00
  DOI: 10.1002/adfm.201203580
Chameleon Nonwovens by Green Electrospinning
- Authors: Elisabeth Giebel; Claudia Mattheis, Seema Agarwal, Andreas Greiner
  Pages: n/a - n/a
  Abstract: Electrospun ionic nonwovens are obtained by green electrospinning of aqueous dispersions. The resulting nonwovens are termed as chameleon nonwovens since their surface properties can be tailored in a large variety by coating of different functionalities following the protocol of the layer‐by‐layer process (LBL). The dimensional stability of the electrospun fibers in the chameleon nonwovens is achieved by photo‐cross‐linking after electrospinning and thereby overcoming the repulsive forces of the ionic moieties in the fibers. Depending on the nature of the ionic moieties different materials are coated by LBL including dyes, antibacterial materials, silver, and gold nanoparticles. Enhanced coating efficiency for coating of metal nanoparticles is observed when the chameleon nonwovens were precoated by a polyelectrolyte. Electrospun nonwovens with chameleon‐type surface properties are obtained by green electrospinning of acrylate dispersions with ionic moieties followed by photo cross‐linking. The resulting chameleon nonwovens can be coated by layer‐by‐layer deposition with a large variety of different functional materials including dyes, antibacterial compounds, silver, and gold nanoparticles.
  PubDate: 2013-02-07T08:10:27.640106-05:
  DOI: 10.1002/adfm.201201873
Substantial Recoverable Energy Storage in Percolative Metallic Aluminum‐Polypropylene Nanocomposites
- Authors: Lisa A. Fredin; Zhong Li, Michael T. Lanagan, Mark A. Ratner, Tobin J. Marks
  Pages: n/a - n/a
  Abstract: Chemisorption of the activated metallocene polymerization catalyst derived from [rac‐ethylenebisindenyl]zirconium dichlororide (EBIZrCl2) on the native Al2O3 surfaces of metallic aluminum nanoparticles, followed by exposure to propylene, affords 0–3 metal‐isotactic polypropylene nanocomposites. The microstructures of these nanocomposites are characterized by X‐ray diffraction, transmission electron microscopy, scanning electron microscopy, and atomic force microscopy. Electrical measurements show that increasing the concentration of the filler nanoparticles increases the effective permittivity of the nanocomposites to ϵr values as high as 15.4. Because of the high contrast in the complex permittivities and conductivities between the metallic aluminum nanoparticles and the polymeric polypropylene matrix, these composites obey the percolation law for two‐phase composites, reaching maximum permittivities just before the percolation threshold volume fraction, vf ≈ 0.16. This unique method of in situ polymerization from the surface of metallic Al particles produces a new class of materials that perform as superior pulse‐power capacitors, with low leakage current densities of ≈10−7–10−9 A/cm2 at an applied field of 105 V/cm, low dielectric loss in the 100 Hz–1 MHz frequency range, and recoverable energy storage as high as 14.4 J/cm3. Al‐polypropylene nanocomposites are promising pulse‐power capacitor materials, with resistivities of ≈ 1012–1015 Ω·cm, low dielectric loss in the 100 Hz–1 MHz frequency range, and recoverable energy storage as high as 14.4 J/cm3. These conductive‐insulator composites obey the percolation law for two‐phase composites, reaching maximum permittivity values, ϵr, as high as 15.4 before the percolation threshold volume fraction, vf = 0.16.
  PubDate: 2013-02-07T08:10:23.933846-05:
  DOI: 10.1002/adfm.201202469
Rheological and Drying Considerations for Uniformly Gravure‐Printed Layers: Towards Large‐Area Flexible Organic Light‐Emitting Diodes
- Authors: Gerardo Hernandez‐Sosa; Nils Bornemann, Ingo Ringle, Michaela Agari, Edgar Dörsam, Norman Mechau, Uli Lemmer
  Pages: n/a - n/a
  Abstract: Printing organic semiconductor inks by means of roll‐to‐roll compatible techniques will allow a continuous, high‐volume fabrication of large‐area flexible optoelectronic devices. The gravure printing technique is set to become a widespread process for the high throughput fabrication of functional layers. The gravure printing process of a poly‐phenylvinylene derivative light‐emitting polymer dissolved in a two solvent mixture on poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is studied. The surface tensions, contact angles, viscosities, and drying times of the formulations are investigated as a function of the solvent volume fraction and polymer concentration. The properties of the ink grant a homogeneous printed layer, suitable for device fabrication, when the calculated film leveling time is shorter than a critical time, at which the film has been frozen due to loss of solvent via evaporation. The knowledge obtained from the printing process is applied to fabricate organic light‐emitting diodes (OLEDs) on flexible substrates, yielding a luminance of ≈5000 cd m−2. The interplay between surface tension, viscosity, and drying time of organic semiconducting inks is taken into account to fabricate homogeneous active layers for organic light‐emitting diodes (OLEDs) by gravure printing. The optimal formulation of the ink is identified when its properties allow a film leveling time shorter that the critical “freezing” time.
  PubDate: 2013-02-07T08:10:20.807549-05:
  DOI: 10.1002/adfm.201202862
Rapid, One‐step, Digital Selective Growth of ZnO Nanowires on 3D Structures Using Laser Induced Hydrothermal Growth
- Authors: Junyeob Yeo; Sukjoon Hong, Manorotkul Wanit, Hyun Wook Kang, Daeho Lee, Costas P. Grigoropoulos, Hyung Jin Sung, Seung Hwan Ko
  Pages: n/a - n/a
  Abstract: For functional nanowire based electronics fabrication, conventionally, combination of complex multiple steps, such as (1) chemical vapor deposition (CVD) growth of nanowire, (2) harvesting of nanowire, (3) manipulation and placement of individual nanowires, and (4) integration of nanowire to circuit are necessary. Each step is very time consuming, expensive, and environmentally unfriendly, and only a very low yield is achieved through the multiple steps. As an alternative to conventional complex multistep approach, original findings are presented on the first demonstration of rapid, one step, digital selective growth of nanowires directly on 3D micro/nanostructures by developing a novel approach; laser induced hydrothermal growth (LIHG) without any complex integration of series of multiple process steps such as using any conventional photolithography process or CVD. The LIHG process can grow nanowires by scanning a focused laser beam as a local heat source in a fully digital manner to grow nanowires on arbitrary patterns and even on the non‐flat, 3D micro/nano structures in a safer liquid environment, as opposed to a gas environment. The LIHG process can greatly reduce the processing lead time and simplify the nanowire‐based nanofabrication process by removing multiple steps for growth, harvest, manipulation/placement, and integration of the nanowires. LIHG process can grow nanowire directly on 3D micro/nano structures, which will be extremely challenging even for the conventional nanowire integration processes. LIHG does not need a vacuum environment to grow nanowires but can be performed in a solution environment which is safer and cheaper. LIHG can also be used for flexible substrates such as temperature‐sensitive polymers due to the low processing temperature. Most of all, the LIHG process is a digital process that does not require conventional vacuum deposition or a photolithography mask. Laser induced hydrothermal growth (LIHG) is developed for rapid, one step, digital selective growth of nanowire directly on 3D micro/nanostructures without using conventional photolithography or chemical vapor deposition. The LIHG process greatly reduces the process lead time and simplifies nanowire‐based nanofabrication by removing multiple steps for growth, harvest, manipulation/placement, and integration of the nanowires. Furthermore, the LIHG process can grow nanowires directly on 3D micro/nano structures.
  PubDate: 2013-02-07T08:10:13.592491-05:
  DOI: 10.1002/adfm.201203863
Controllable Particle Adhesion with a Magnetically Actuated Synthetic Gecko Adhesive
- Authors: Andrew G Gillies; Jonghun Kwak, Ronald S Fearing
  Pages: n/a - n/a
  Abstract: Particle capture and release using controllable adhesion is of growing interest in many fields including reusable adhesives, solar panel cleaning, micro‐manipulation and robot locomotion where current methods of particle management are not sufficient. Here controllable adhesion to glass spheres with a new magnetically actuated synthetic gecko adhesive made from a magnetoelastomer composite is demonstrated. Adhesion is controlled by changing the effective elastic modulus of the surface through actuation of micro surface features with an external magnetic field. A compliant mechanics and magnetic torque analysis explains this general principle and generalizes the results for various geometries. Results show sphere pull‐off forces can be increased 10‐fold by changing the ridge orientation via the external magnetic field, and that the effective elastic modulus can be changed from 65 kPa to 1.5 MPa. Particle transport and release of 500‐ and 1000‐micrometer glass spheres is also demonstrated. Controllable adhesion to glass spheres with a new magnetically actuated synthetic gecko adhesive made from a magnetoelastomer is demonstrated. Adhesion is controlled by orienting the microfabricated surface features with an external magnetic field. Results show sphere pull‐off forces can be increased 10‐fold by changing the ridge orientation. Applications include solar panel cleaning, reusable adhesives, and microfabrication.
  PubDate: 2013-02-07T04:10:06.484625-05:
  DOI: 10.1002/adfm.201203122
Charge‐Compensated Compound Defects in Ga‐containing Thermoelectric Skutterudites
- Authors: Yuting Qiu; Lili Xi, Xun Shi, Pengfei Qiu, Wenqing Zhang, Lidong Chen, James R. Salvador, Jung Y. Cho, Jihui Yang, Yuan‐chun Chien, Sinn‐wen Chen, Yinglu Tang, G. Jeffrey Snyder
  Pages: n/a - n/a
  Abstract: Heavy doping changes an intrinsic semiconductor into a metallic conductor by the introduction of impurity states. However, Ga impurities in thermoelectric skutterudite CoSb3 with lattice voids provides an example to the contrary. Because of dual‐site occupancy of the single Ga impurity charge‐compensated compound defects are formed. By combining first‐principle calculations and experiments, we show that Ga atoms occupy both the void and Sb sites in CoSb3 and couple with each other. The donated electrons from the void‐filling Ga (GaVF) saturate the dangling bonds from the Sb‐substitutional Ga (GaSb). The stabilization of Ga impurity as a compound defect extends the region of skutterudite phase stability toward Ga0.15Co4Sb11.95 whereas the solid–solution region in other directions of the ternary phase diagram is much smaller. A proposed ternary phase diagram for Ga‐Co‐Sb is given. This compensated defect complex leads to a nearly intrinsic semiconductor with heavy Ga doping in CoSb3 and a much reduced lattice thermal conductivity (κL) which can also be attributed to the effective scattering of both the low‐ and high‐frequency lattice phonons by the dual‐site occupant Ga impurities. Such a system maintains a low carrier concentration and therefore high thermopower, and the thermoelectric figure of merit quickly increases to 0.7 at a Ga doping content as low as 0.1 per Co4Sb12 and low carrier concentrations on the order of 1019 cm−3. Ga occupies both the void and Sb sites in CoSb3 which is proven by combining first‐principles calculations and experiments. The stabilization of the Ga impurity as a compound defect extends the region of skutterudite phase stability toward Ga0.15Co4Sb11.95 whereas the solid–solution region becomes much smaller in other directions of the phase diagram. This compensated defect complex leads to a nearly intrinsic semiconductor with low carrier concentration, and therefore high thermopower, which possesses a much reduced lattice thermal conductivity.
  PubDate: 2013-02-06T09:23:12.042409-05:
  DOI: 10.1002/adfm.201202571
Matrix Metalloproteinase Responsive, Proximity‐Activated Polymeric Nanoparticles for siRNA Delivery
- Authors: Hongmei Li; Shann S. Yu, Martina Miteva, Christopher E. Nelson, Thomas Werfel, Todd D. Giorgio, Craig L. Duvall
  Pages: n/a - n/a
  Abstract: Small interfering RNA (siRNA) has significant potential to evolve into a new class of pharmaceutical inhibitors, but technologies that enable robust, tissue‐specific intracellular delivery must be developed before effective clinical translation can be achieved. A pH‐responsive, smart polymeric nanoparticle (SPN) with matrix metalloproteinase (MMP)‐7‐dependent proximity‐activated targeting (PAT) is described here. The PAT‐SPN is designed to trigger cellular uptake and cytosolic delivery of siRNA once activated by MMP‐7, an enzyme whose overexpression is a hallmark of cancer initiation and progression. The PAT‐SPN is composed of a corona‐forming polyethylene glycol (PEG) block, an MMP‐7‐cleavable peptide, a cationic siRNA‐condensing block, and a pH‐responsive, endosomolytic terpolymer block that drives self‐assembly and forms the PAT‐SPN core. With this novel design, the PEG corona shields cellular interactions until it is cleaved in MMP‐7‐rich environments, shifting the SPN ζ‐potential from +5.8 to +14.4 mV and triggering a 2.5 fold increase in carrier internalization. The PAT‐SPN exhibits pH‐dependent membrane disruptive behavior that enables siRNA escape from endo‐lysosomal pathways. Intracellular siRNA delivery and knockdown of the model enzyme luciferase in R221A‐Luc mammary tumor cells is significantly increased by MMP‐7 pre‐activation (p < 0.05). These combined data indicate that the PAT‐SPN provides a promising new platform for tissue‐specific, proximity‐activated siRNA delivery to MMP‐rich pathological environments. The pH‐responsive, smart polymeric nanocarrier (SPN) with matrix metalloproteinase (MMP)‐7‐dependent proximity‐activated targeting (PAT) incorporates polyethylene glycol (PEG) shielding that is removable in MMP‐7‐rich environments (i.e., breast cancer metastases). The up‐regulated MMP‐7 activity in pathological tissue exposes the cationic component of the SPN polymer, triggering cell uptake. Following internalization of polymers into the endosomal pathway, pH‐dependent endosomal escape facilitates cytosolic siRNA delivery.
  PubDate: 2013-02-06T08:23:17.033983-05:
  DOI: 10.1002/adfm.201202215
Tuning the Poisson's Ratio of Biomaterials for Investigating Cellular Response
- Authors: Wande Zhang; Pranav Soman, Kyle Meggs, Xin Qu, Shaochen Chen
  Pages: n/a - n/a
  Abstract: Cells sense and respond to mechanical forces, regardless of whether the source is from a normal tissue matrix, an adjacent cell or a synthetic substrate. In recent years, cell response to surface rigidity has been extensively studied by modulating the elastic modulus of poly(ethylene glycol) (PEG)‐based hydrogels. In the context of biomaterials, Poisson's ratio, another fundamental material property parameter has not been explored, primarily because of challenges involved in tuning the Poisson's ratio in biological scaffolds. Two‐photon polymerization is used to fabricate suspended web structures that exhibit positive and negative Poisson's ratio (NPR), based on analytical models. NPR webs demonstrate biaxial expansion/compression behavior, as one or multiple cells apply local forces and move the structures. Unusual cell division on NPR structures is also demonstrated. This methodology can be used to tune the Poisson's ratio of several photocurable biomaterials and could have potential implications in the field of mechanobiology. A methodology to develop suspended structures with tunable Poisson's ratios is reported. Two‐photon polymerization is used to fabricate suspended web structures with a negative Poisson's ratio (NPR), based on analytical models. This technique could be used to investigate effects of altering the Poisson's ratio of several photocurable biomaterials on a variety of cellular aspects including morphology, gene expression, and migration using different cell types.
  PubDate: 2013-02-06T08:23:10.005517-05:
  DOI: 10.1002/adfm.201202666
Cationic Polyfluorenes with Phosphorescent Iridium(III) Complexes for Time‐Resolved Luminescent Biosensing and Fluorescence Lifetime Imaging
- Authors: Huifang Shi; Huibin Sun, Huiran Yang, Shujuan Liu, Gareth Jenkins, Wei Feng, Fuyou Li, Qiang Zhao, Bin Liu, Wei Huang
  Pages: n/a - n/a
  Abstract: The application of a time‐resolved photoluminescence technique and fluorescence lifetime imaging microscopy for biosensing and bioimaging based on phosphorescent conjugated polyelectrolytes (PCPEs) containing Ir(III) complexes and polyfluorene units is reported. The specially designed PCPEs form 50 nm nanoparticles with blue fluorescence in aqueous solutions. Electrostatic interaction between the nanoparticles and heparin improves the energy transfer between the polyfluorene units to Ir(III) complex, which lights up the red signal for naked‐eye sensing. Good selectivity has been demonstrated for heparin sensing in aqueous solution and serum with quantification ranges of 0–70 μM and 0–5 μM, respectively. The signal‐to‐noise ratio can be further improved through time‐resolved emission spectra, especially when the detection is conducted in complicated environment, e.g., in the presence of fluorescent dyes. In addition to heparin sensing, the PCPEs have also been used for specific labeling of live KB cell membrane with high contrast using both confocal fluorescent cellular imaging and fluorescence lifetime imaging microscopies. This study provides a new perspective for designing promising CPEs for biosensing and bioimaging applications. The application of time‐resolved photoluminescence and fluorescence lifetime imaging for heparin sensing and bioimaging based on phosphorescent conjugated polyelectrolytes (PCPEs) containing iridium(III) complexes to eliminate background fluorescence with enhanced signal‐to‐noise ratio is reported.
  PubDate: 2013-02-06T08:10:16.993923-05:
  DOI: 10.1002/adfm.201202385
A Large Magnetoresistance Effect in p–n Junction Devices by the Space‐Charge Effect
- Authors: Dezheng Yang; Fangcong Wang, Yang Ren, Yalu Zuo, Yong Peng, Shiming Zhou, Desheng Xue
  Pages: n/a - n/a
  Abstract: The finding of an extremely large magnetoresistance effect on silicon based p–n junction with vertical geometry over a wide range of temperatures and magnetic fields is reported. A 2500% magnetoresistance ratio of the Si p–n junction is observed at room temperature with a magnetic field of 5 T and the applied bias voltage of only 6 V, while a magnetoresistance ratio of 25 000% is achieved at 100 K. The current‐voltage (I–V) behaviors under various external magnetic fields obey an exponential relationship, and the magnetoresistance effect is significantly enhanced by both contributions of the electric field inhomogeneity and carrier concentrations variation. Theoretical analysis using classical p–n junction transport equation is adapted to describe the I–V curves of the p–n junction at different magnetic fields and reveals that the large magnetoresistance effect origins from a change of space‐charge region in the p–n junction induced by external magnetic field. The results indicate that the conventional p–n junction is proposed to be used as a multifunctional material based on the interplay between electronic and magnetic response, which is significant for future magneto‐electronics in the semiconductor industry. A large magnetoresistance effect in conventional silicon p–n junctions is reported. By utilizing the magnetic field to manipulate the space‐charge region of the p–n junction, a 2500% magnetoresistance ratio is observed at room temperature with H = 5 T. The p–n junction controlled by both electric field and magnetic field will open a new avenue for future magneto‐electronics.
  PubDate: 2013-02-06T07:23:13.688846-05:
  DOI: 10.1002/adfm.201202695
Efficient Persistent Room Temperature Phosphorescence in Organic Amorphous Materials under Ambient Conditions
- Authors: Shuzo Hirata; Kenro Totani, Junxiang Zhang, Takashi Yamashita, Hironori Kaji, Seth R. Marder, Toshiyuki Watanabe, Chihaya Adachi
  Pages: n/a - n/a
  Abstract: Persistent emission with a long lifetime (>1 s) from organic materials can only be observed at a low temperature, because of the significant nonradiative deactivation pathway that occurs at room‐temperature (RT). If organic materials with persistent RT emission in air could be developed, they could potentially be utilized for a variety of applications. Here, organic host‐guest materials with efficient persistent RT phosphorescence (RTP) are developed by minimizing the nonradiative deactivation pathway of triplet excitons. The nonradiative deactivation pathway is dependent on both nonradiative deactivation of the guest and quenching by diffusional motion of the host. The rigidity and oxygen barrier properties of the steroidal compound used as the host suppressed the quenching, and the aromatic hydrocarbon used as the guest is highly deuterated to minimize nonradiative deactivation of the guest. Red‐green‐blue persistent RTP with a lifetime >1 s and a quantum yield >10% in air is realized for a pure organic material. Efficient persistent room temperature phosphorescence with a quantum efficiency of greater than 10% and a lifetime longer than 1 s from pure organic amorphous host‐guest materials is demonstrated in air. Physical rigidity and oxygen blocking characteristics of amorphous steroidal compounds as the host greatly minimize quenching of long‐lifetime triplet exitons of the guest by interaction with the host and oxygen.
  PubDate: 2013-02-06T06:40:21.129322-05:
  DOI: 10.1002/adfm.201203706
A High‐Performance Graphene Oxide‐Doped Ion Gel as Gel Polymer Electrolyte for All‐Solid‐State Supercapacitor Applications
- Authors: Xi Yang; Fan Zhang, Long Zhang, Tengfei Zhang, Yi Huang, Yongsheng Chen
  Pages: n/a - n/a
  Abstract: A high‐performance graphene oxide (GO)‐doped ion gel (P(VDF‐HFP)‐EMIMBF4‐GO gel) is prepared by exploiting copolymer (poly(vinylidene fluoride‐hexafluoro propylene), P(VDF‐HFP)) as the polymer matrix, ionic liquid (1‐ethyl‐3‐methylimidazolium tetrafluoroborate, EMIMBF4) as the supporting electrolyte, and GO as the ionic conducting promoter. This GO‐doped ion gel demonstrates significantly improved ionic conductivity compared with that of pure ion gel without the addition of GO, due to the homogeneously distributed GO as a 3D network throughout the GO‐doped ion gel by acting like a ion “highway” to facilitate the ion transport. With the incorporation of only a small amount of GO (1 wt%) in ion gel, there has been a dramatic improvement in ionic conductivity of about 260% compared with that of pure ion gel. In addition, the all‐solid‐state supercapacitor is fabricated and measured at room temperature using the GO‐doped ion gel as gel polymer electrolyte, which demonstrates more superior electrochemical performance than the all‐solid‐state supercapacitor with pure ion gel and the conventional supercapacitor with neat EMIMBF4, in the aspect of smaller internal resistance, higher capacitance performance, and better cycle stability. These excellent performances are due to the high ionic conductivity, excellent compatibility with carbon electrodes, and long‐term stability of the GO‐doped ion gel. A high‐performance graphene oxide (GO)‐doped ion gel is developed, which may have great potential for applications in wearable energy storage devices. This GO‐doped ion gel demonstrates significantly improved ionic conductivity compared with that of pure ion gel, due to the homogeneously distributed GO as a 3D network throughout the ion gel by acting like an ion “highway”.
  PubDate: 2013-02-06T06:40:17.236065-05:
  DOI: 10.1002/adfm.201203556
Deformable, Programmable, and Shape‐Memorizing Micro‐Optics
- Authors: Hangxun Xu; Cunjiang Yu, Shuodao Wang, Viktor Malyarchuk, Tao Xie, John A. Rogers
  Pages: n/a - n/a
  Abstract: The use of shape memory polymers is demonstrated for deformable, programmable, and shape‐memorizing micro‐optical devices. A semi‐crystalline shape memory elastomer, crosslinked poly(ethylene‐co‐vinyl acetate), is used to prepare various micro‐optic components, ranging from microlens and microprism arrays to diffraction gratings and holograms. The precise replication of surface features at the micro‐ and nanoscale and the formation of crosslinked shape memory polymer networks can be achieved in a single step via compression molding. Further deformation via hot pressing or stretching of micro‐optics formed in this manner allows manipulation of the microscopic surface features, and thus the corresponding optical properties. Due to the shape memory effect, the original surface structures and the optical properties can be recovered and the devices be reprogrammed, with excellent reversibility in the optical properties. Furthermore, arrays of transparent resistive microheaters can be integrated with deformed micro‐optical devices to selectively trigger the recovery of surface features in a spatially programmable manner, thereby providing additional capabilities in user‐definable optics. Micro‐optical components, ranging from microprism and microlens arrays, to diffraction gratings, and holograms are fabricated from a semi‐crystalline shape memory elastomer. These resulting components represent a class of micro‐optical devices with programmable optical properties. Spatio‐selective manipulation of these optical devices can be further achieved by the integration of individually addressable arrays of microheaters.
  PubDate: 2013-02-06T06:40:11.077699-05:
  DOI: 10.1002/adfm.201203396
Structure and Energetics of Dislocations at Micro‐Structured Complementary Interfaces Govern Adhesion
- Authors: Congrui Jin; Anand Jagota, Chung‐Yuen Hui
  Pages: n/a - n/a
  Abstract: Highly enhanced adhesion can be achieved between surfaces patterned with complementary micro‐channel structures. An elastic material, poly(dimethylsiloxane) (PDMS), is used to fabricate such surfaces by molding into a silicon master with micro‐channel profiles patterned by photolithography. For each pair of complementary surfaces, dislocation defects are observed in the form of visible striations, where ridges fail to fully insert into the channels, and the rotational misalignment angle was found to be the key factor controlling the dislocation distribution and adhesion strength. Adhesion between complementary interfaces, as measured by energy release rate required to propagate an interfacial crack, can be enhanced by up to 30 times compared to a flat control depending on the misalignment angle. The ability to control the orientation and periodicity of dislocation patterns by changing misalignment angle makes this system eminently controllable. This system could be a useful experimental tool in assisting research on geometry‐controlled adhesion, while providing a test‐bed for stability theories of interacting dislocations and crack fronts. Poly (dimethylsiloxane) (PDMS) is used to fabricate micro‐structured complementary surfaces by molding into a silicon master with micro‐channel profiles patterned by photolithography. For each pair of complementary surfaces, dislocation defects are observed in the form of visible striations, and misalignment angle is found to be the key factor controlling dislocation distribution and adhesion strength. The ability to control the orientation and periodicity of dislocation patterns by changing misalignment angle makes this system eminently controllable.
  PubDate: 2013-02-06T06:30:30.965865-05:
  DOI: 10.1002/adfm.201203337
Amorphous Zinc Stannate (Zn2SnO4) Nanofibers Networks as Photoelectrodes for Organic Dye‐Sensitized Solar Cells
- Authors: Seung‐Hoon Choi; Daesub Hwang, Dong‐Young Kim, Yann Kervella, Pascale Maldivi, Sung‐Yeon Jang, Renaud Demadrille, Il‐Doo Kim
  Pages: n/a - n/a
  Abstract: A new strategy for developing dye‐sensitised solar cells (DSSCs) by combining thin porous zinc tin oxide (Zn2SnO4) fiber‐based photoelectrodes with purely organic sensitizers is presented. The preparation of highly porous Zn2SnO4 electrodes, which show high specific surface area up to 124 m2/g using electrospinning techniques, is reported. The synthesis of a new organic donor‐conjugate‐acceptor (D‐π‐A) structured orange organic dye with molar extinction coefficient of 44 600 M−1 cm−1 is also presented. This dye and two other reference dyes, one organic and a ruthenium complex, are employed for the fabrication of Zn2SnO4 fiber‐based DSSCs. Remarkably, organic dye‐sensitized DSSCs displayed significantly improved performance compared to the ruthenium complex sensitized DSSCs. The devices based on a 3 μm‐thick Zn2SnO4 electrode using the new sensitizer in conjunction with a liquid electrolyte show promising photovoltaic conversion up to 3.7% under standard AM 1.5G sunlight (100 mW cm−2). This result ranks among the highest reported for devices using ternary metal oxide electrodes. Dye‐sensitized solar cells (DSSCs) combining Zn2SnO4 fiber‐based photoelectrodes and purely organic sensitizers are fabricated. The highly porous amorphous Zn2SnO4 electrodes are prepared using electrospinning techniques and combined with a new organic dye. Using this strategy, devices that show photovoltaic conversion up to 3.7% are produced. The results rank among the highest reported for devices using ternary metal oxide sensitized electrodes.
  PubDate: 2013-02-06T06:30:22.170816-05:
  DOI: 10.1002/adfm.201203278
Effect of Exchange Interactions on the Coercivity of SmCo5 Nanoparticles Made by Cluster Beam Deposition
- Authors: O. Akdogan; W. Li, B. Balasubramanian, D. J. Sellmyer, G. C. Hadjipanayis
  Pages: n/a - n/a
  Abstract: Single crystal SmCo5 nanoparticles with an average size of 3.5 nm are produced by cluster‐beam deposition. When deposited without matrix, the nanoparticles showed a super‐paramagnetic behavior with a blocking temperature of 145 K. Dispersion of the SmCo5 nanoparticles in a carbon matrix results in an increase in both the coercivity and the blocking temperature. Room temperature coercivities as high as 12 kOe are obtained for the first time in mono‐layers of SmCo5 nanoparticles dispersed in C matrix. δM plots show that the interactions in the samples are of exchange type, which can decrease the overall effective anisotropy and coercivity according to the random‐anisotropy model. Coercivity is found to be inversely proportional to the packing density of the particles. SmCo5 nanoparticles with high coercivity are potential candidates for the next generation ultra high‐density magnetic recording media. The effect of SmCo5 nanoparticle dispersion in a carbon matrix on the coercivity is investigated. Poor dispersion of these nanoparticles results in a moderate room temperature coercivity of ≈1 kOe. By increasing the inter‐particle distance substantially, coercivity increases up to 12 kOe due to the overall anisotropy increase with the decrease in exchange interactions according to the random anisotropy model.
  PubDate: 2013-02-06T06:30:05.129585-05:
  DOI: 10.1002/adfm.201201353
Molecular or Nanoscale Structures' The Deciding Factor of Surface Properties on Functionalized Poly(3,4‐ethylenedioxythiophene) Nanorod Arrays
- Authors: Hsing‐An Lin; Shyh‐Chyang Luo, Bo Zhu, Chi Chen, Yoshiro Yamashita, Hsiao‐hua Yu
  Pages: n/a - n/a
  Abstract: Nanostructures of poly(3,4‐ethylenedioxythiophene) (PEDOT) are assembled by using an anodic alumium oxide template directly fabricated on gold‐coated silicon wafers. Inside these templates, PEDOT and hydroxy functionalized PEDOT form tubes. On the other hand, alkyl‐ and perfluoro‐functionalized PEDOTs assembled as nanorods. This approach allows a platform to understand the molecular and nanostructural effect on the surface wettability of these materials. In the water/air interface, the contact angle of water droplet (CAwater) for the smooth alkyl‐functionalized PEDOT films increases when alkyl chain gets longer. In contrast, the contact angle reachs saturation at 130° with alkyl chain longer than ethyl in assembled nanorod arrays. It remains the same even in the case of perfluoro‐functionalized PEDOT. Moreover, ethyl‐functionalized PEDOT (PEDOT‐C2) nanorods displays superoleophilicity and the oil deoplet cannot stay on the film in water. Based on the wettability studies, it is concluded that the nanostructures contribute predominantly for the surface wettability of these nanomaterials when the length of alkyl chain crosses certain threshold. Nanorod arrays of functionalized poly(3,4‐ethylenedioxythiophene) (PEDOT) are assembled by using an anodic alumium oxide template directly fabricated on gold‐coated silicon wafers. These nanorod arrays are promising for organic electronic and biomedical applications. This approach allows a platform to understand the molecular and nanostructural effect on the surface wettability of these materials.
  PubDate: 2013-02-06T03:23:16.219581-05:
  DOI: 10.1002/adfm.201203006
Layered Gradient Nonwovens of In Situ Crosslinked Electrospun Collagenous Nanofibers Used as Modular Scaffold Systems for Soft Tissue Regeneration
- Authors: Marco Angarano; Simon Schulz, Martin Fabritius, Robert Vogt, Thorsten Steinberg, Pascal Tomakidi, Christian Friedrich, Rolf Mülhaupt
  Pages: n/a - n/a
  Abstract: In a versatile modular scaffold system, gradient nonwovens of in situ crosslinked gelatin nanofibers (CGN), fabricated by reactive electrospinning, are laminated with perforated layers and nonwovens of thermoplastic non‐crosslinked biodegradable polyesters. The addition of glyoxal to a gelatin solution in a non‐toxic solvent mixture consisting of acetic acid, ethyl acetate, and water (5:3:2 w/w/w) enables the in situ crosslinking of gelatin nanofibers during electrospinning. The use of this fluorine‐free crosslinking system eliminates the need of post‐treatment crosslinking and purification steps typical for conventional CGN scaffolds. The slowly progressing crosslinking of the dissolved gelatin in the presence of glyoxal increases the viscosity of the gelatin solution during electrospinning so that the average diameter of the crosslinked gelatin nanofibers gradually increases from 90 to 680 nm. During the subsequent lamination process, alternating layers of CGN and polycaprolactone (PCL) nonwovens, produced by 3D microextrusion of micrometer‐sized PCL fibers, are bonded together upon heating above the PCL melting temperature. In contrast to the water‐soluble gelatin nanofibers and the comparatively weak CGN, the CGN/PCL/CGN layered biocomposites are water‐resistant and very robust. In such modular scaffold systems, strength, biodegradation rate, and biological functions can be controlled by varying the type, composition, fiber diameter, porosity, number, and sequence of the individual layers. The CGN/PCL multilayer biocomposites can be cut into any desired scaffold shape and attached to tissue by surgical sutures in order to suit the needs of individual patients. Water resistant, in situ crosslinked gelatin nanofibers are produced in a versatile one‐step electrospinning process, using glyoxal as a crosslinking agent. The slowly progressing crosslinking reaction during electrospinning increases the solution viscosity and the average fiber diameter, thus forming gradient nonwovens. To improve the mechanical stability, we laminate the crosslinked gelatin nonwovens with perforated polycaprolactone layers.
  PubDate: 2013-02-06T03:10:47.45268-05:0
  DOI: 10.1002/adfm.201202816
Charge Photogeneration for a Series of Thiazolo‐Thiazole Donor Polymers Blended with the Fullerene Electron Acceptors PCBM and ICBA
- Authors: Safa Shoaee; Selvam Subramaniyan, Hao Xin, Chaz Keiderling, Pabitra Shakya Tuladhar, Fiona Jamieson, Samson A. Jenekhe, James R. Durrant
  Pages: n/a - n/a
  Abstract: Photoinduced charge separation in bulk heterojunction solar cells is studied using a series of thiazolo‐thiazole donor polymers that differ in their side groups (and bridging atoms) blended with two acceptor fullerenes, phenyl‐C71‐butyric acid methyl ester (PC71BM) and a fullerene indene‐C60 bisadduct (ICBA). Transient absorption spectroscopy is used to determine the yields and lifetimes of photogenerated charge carriers, complimented by cyclic voltammetry studies of materials energetics, wide angle X‐ray diffraction and transmission electron microscopy studies of neat and blend film crystallinity and photoluminescence quenching studies of polymer/fullerene phase segregation, and the correlation of these measurements with device photocurrents. Good correlation between the initial polaron yield and the energetic driving force driving charge separation, ΔECS is observed. All blend films exhibit a power law transient absorption decay phase assigned to non‐geminate recombination of dissociated charges; the amplitude of this power law decay phase shows excellent correlation with photocurrent density in the devices. Furthermore, for films of one (relatively amorphous) donor polymer blended with ICBA, we observe an additional 100 ns geminate recombination phase. The implications of the observations reported are discussed in terms of the role of materials' crystallinity in influencing charge dissociation in such devices, and thus materials design requirements for efficient solar cell function. Model of charge separation from polymer singlet excitons, including both interfacial charge‐transfer (CT) states, loosely bound polaron pairs and dissociated polarons. For blend films where at least one material is relatively crystalline, efficient dissociation of photogenerated polarons is observed. For amorphous blend films, significant recombination of loosely bound polaron pairs is observed on the 100 ns timescale.
  PubDate: 2013-02-06T03:10:44.196971-05:
  DOI: 10.1002/adfm.201203148
Novel Atmospheric Growth Technique to Improve Both Light Absorption and Charge Collection in ZnO/Cu2O Thin Film Solar Cells
- Authors: Andrew T. Marin; David Muñoz‐Rojas, Diana C. Iza, Talia Gershon, Kevin P. Musselman, Judith L. MacManus‐Driscoll
  Pages: n/a - n/a
  Abstract: In low temperature grown ZnO/Cu2O solar cells, there is a discrepancy between collection length and depletion width in the Cu2O which makes the simultaneous achievement of efficient charge collection and high open‐circuit voltage problematic. This is addressed in this study by fabricating ZnO/Cu2O/Cu2O+ back surface field devices using an atmospheric atomic layer deposition (AALD) printing method to grow a sub‐200‐nm Cu2O+ film on top of electrodeposited ZnO and Cu2O layers. The AALD Cu2O+ has a carrier concentration around 2 orders of magnitude higher than the electrodeposited Cu2O, allowing the electrodeposited Cu2O layer thickness in a back surface field cell to be reduced from 3 μm to the approximate charge collection length, 1 μm, while still allowing a high potential to be built into the cell. The dense conformal nature of the AALD layer also blocks shunt pathways allowing the voltage enhancement to be maintained. The thinner cell design reduces recombination losses and increases charge collection from both incident light and light reflected off the back electrode. Using this design, a short circuit current density of 6.32 mA cm−2 is achieved–the highest reported JSC for an atmospherically deposited ZnO/Cu2O device to date. A Cu2O‐based solar cell synthesized using atmospheric atomic layer deposition (AALD) and employing a back surface field architecture is demonstrated for the first time. Enhanced charge collection is observed when a layer of AALD Cu2O+ is overlaid on electrodeposited ZnO/Cu2O layers. These cells produce a record short circuit current density of >6.3 mA cm−2 for a fully‐atmospherically deposited ZnO/Cu2O device.
  PubDate: 2013-02-06T03:10:37.690891-05:
  DOI: 10.1002/adfm.201203243
Aqueous Two‐Phase System Patterning of Microbubbles: Localized Induction of Apoptosis in Sonoporated Cells
- Authors: John P. Frampton; Zhenzhen Fan, Arlyne Simon, Di Chen, Cheri X. Deng, Shuichi Takayama
  Pages: n/a - n/a
  Abstract: Ultrasound‐driven microbubbles produce mechanical forces that can disrupt cell membranes (sonoporation). However, it is difficult to control microbubble location with respect to cells. This lack of control leads to low sonoporation efficiencies and variable outcomes. In this study, aqueous two‐phase system (ATPS) droplets are used to localize microbubbles in select micro‐regions at the surface of living cells. This is achieved by stably partitioning microbubbles in dextran (DEX) droplets, deposited on living adherent cells in medium containing polyethylene glycol (PEG). The interfacial energy at the PEG‐DEX interface overcomes microbubble buoyancy and prevents microbubbles from floating away from the cells. Spreading of the small DEX droplets retains microbubbles at the cell surface in defined lateral positions without the need for antibody or cell‐binding ligand conjugation. The patterned microbubbles are activated on a cell monolayer exposed to a broadly applied ultrasound field (center frequency 1.25 MHz, active element diameter 0.6 cm, pulse duration 8 μs or 30 s). This system enables efficient testing of different ultrasound conditions for their effects on sonoporation‐mediated membrane disruption and cell viability. Regions of cells without patterned microbubbles show no injury or membrane disruption. In microbubble patterned regions, 8 μs ultrasound pulses (0.2‐0.6 MPa) produce cell death that is primarily apoptotic. Ultrasound‐induced apoptosis increases with higher extracellular calcium concentrations, with cells displaying all of the hallmarks of apoptosis including annexinV labeling, loss of mitochondrial membrane potential, caspase activation and changes in nuclear morphology. A new method is described for patterning microbubbles on cell monolayers to target ultrasound treatment to cells. This novel platform provides a controlled system for high throughput testing of the effects of ultrasound‐mediated cell membrane disruption on cell physiology. Using this patterning method, it is possible to induce apoptosis in select populations of cells.
  PubDate: 2013-02-06T03:10:31.185978-05:
  DOI: 10.1002/adfm.201203321
Efficient Dye‐Sensitized Solar Cells with Potential‐Tunable Organic Sulfide Mediators and Graphene‐Modified Carbon Counter Electrodes
- Authors: Xiong Li; Linfeng Liu, Guanghui Liu, Yaoguang Rong, Ying Yang, Heng Wang, Zhiliang Ku, Mi Xu, Cheng Zhong, Hongwei Han
  Pages: n/a - n/a
  Abstract: A new class of organic sulfide mediators with programmable redox properties is designed via density functional theory calculations and synthesized for efficient dye‐sensitized solar cells (DSCs). Photophysical and electrochemical properties of these mediators derived from systematical functionalization of the framework with electron donating and withdrawing groups (MeO, Me, H, Cl, CF3, and NO2) are investigated. With this new class of organic mediators, the redox potential can be fine‐tuned over a 170 mV range, overlapping the conventional I−/I3−couple. Due to the suitable interplay of physical properties and electrochemical characteristics of the mediator involving electron‐donating MeO group, the DSCs based on this mediator behave excellently in various kinetic processes such as dye regeneration, electron recombination, and mass transport. Thus, the MeO derivative of the mediator is identified as having the best performance of this series of redox shuttles. As inferred from electrochemical impedance spectroscopy and cyclic voltammetry measurements, the addition of graphene into the normal carbon counter electrode material dramatically improves the apparent catalytic activity of the counter electrode towards the MeO derivative of mediator, resulting in N719 based DSCs showing a promising conversion efficiency of 6.53% under 100 mW·cm−2 simulated sunlight illumination. A new series of organic sulfide mediators with programmable redox properties is synthesized for efficient dye‐sensitized solar cells (DSCs) through simple structural modification. Furthermore, addition of graphene components into the normal carbon counter electrode material can dramatically improve the catalytic activity of the counter electrode towards these sulfide mediators, giving the DSCs formed using these organic mediators good conversion efficiency.
  PubDate: 2013-02-06T03:10:15.116354-05:
  DOI: 10.1002/adfm.201203374
Top‐Emission Organic Light‐Emitting Diode with a Novel Copper/Graphene Composite Anode
- Authors: Hu Meng; Jianxing Luo, Wei Wang, Zujin Shi, Qiaoli Niu, Lun Dai, Guogang Qin
  Pages: n/a - n/a
  Abstract: The relatively high sheet resistance of graphene compared with indium tin oxide (ITO) blocks the applications of graphene as transparent electrodes in organic light‐emitting diodes. A novel copper (Cu)/graphene composite electrode is presented and employed as the anode of a top‐emission organic light‐emitting diode with the structure of Cu/graphene/V2O5/NPB/Alq3/Alq3: C545T/Bphen: Cs2CO3/Sm/Au. The Cu/graphene composite electrodes are fabricated by growing graphene directly on Cu substrates via the chemical vapor deposition method without any transfer process. The maxima of current efficiency and power efficiency of a typical Cu/graphene composite anode device reach 6.1 cd/A and 7.6 lm/W, respectively, which are markedly higher than those of the control devices with a graphene anode, a Cu anode or an ITO anode. The low sheet resistance of the composite electrode, the high quality of graphene without any transfer process and the avoidance of wave guiding loss in glass or polyethylene terephthalate substrates result in the improvements of light emission efficiencies. A novel copper/graphene composite anode is proposed and applied as the anode of a top‐emission organic light‐emitting diode without any graphene transfer process. The maxima of current and power efficiency of a typical copper/graphene composite anode device reach 6.1 cd/A and 7.6 lm/W, respectively, which are markedly higher than those of the control devices with a graphene anode or an ITO anode.
  PubDate: 2013-01-31T02:20:12.84973-05:0
  DOI: 10.1002/adfm.201203283
One‐Pot Synthesis of Luminescent Polymer‐Nanoparticle Composites from Task‐Specific Ionic Liquids
- Authors: Paul S. Campbell; Chantal Lorbeer, Joanna Cybinska, Anja‐Verena Mudring
  Pages: n/a - n/a
  Abstract: A multifunctional polymerizable ionic liquid, diallyldimethylammonium tetrafluoroborate (DADMA BF4), is used in a one‐pot synthesis of novel luminescent polymer‐nanoparticle composites. First, small monodisperse lanthanide fluoride nanoparticles are formed by microwave irradiation in the presence of Ln(OAc)3·xH2O (Ln = Gd, Eu, Tb; OAc = acetate) in the ionic liquid. The nanoparticles can be precipitated for structural characterization or kept in the solution, which yields after irradition by high intensity UV light colorless, processable polymer materials with good photophysical properties. Both green‐emitting Tb‐containing and red‐emitting Eu‐containing IL‐ polymers are described. A multifunction polymerizable ionic liquid, diallyldimethylammonium tetrafluoroborate, is used in a one‐pot synthesis of novel luminescent polymer‐nanoparticle composites. Small monodisperse lanthanide fluoride nanoparticles are formed by microwave irradiation in the presence of Ln(OAc)3·xH2O (Ln = Gd, Eu, Tb, OAc = acetate) in the ionic liquid. Irradiation under high intensity UV gives colorless, processable polymer materials with good photophysical properties.
  PubDate: 2013-01-31T02:10:17.131319-05:
  DOI: 10.1002/adfm.201202472
Imaging‐Guided pH‐Sensitive Photodynamic Therapy Using Charge Reversible Upconversion Nanoparticles under Near‐Infrared Light
- Authors: Chao Wang; Liang Cheng, Yumeng Liu, Xiaojing Wang, Xinxing Ma, Zhaoyi Deng, Yonggang Li, Zhuang Liu
  Pages: n/a - n/a
  Abstract: Photodynamic therapy (PDT) based on upconversion nanoparticles (UCNPs) can effectively destroy cancer cells under tissue‐penetrating near‐infrared light (NIR) light. Herein, we synthesize manganese (Mn2+)‐doped UCNPs with strong red light emission at ca. 660 nm under 980 nm NIR excitation to activate Chlorin e6 (Ce6), producing singlet oxygen (1O2) to kill cancer cells. A layer‐by‐layer (LbL) self‐assembly strategy is employed to load multiple layers of Ce6 conjugated polymers onto UCNPs via electrostatic interactions. UCNPs with two layers of Ce6 loading (UCNP@2xCe6) are found to be optimal in terms of Ce6 loading and 1O2 generation. By further coating UCNP@2xCe6 with an outer layer of charge‐reversible polymer containing dimethylmaleic acid (DMMA) groups and polyethylene glycol (PEG) chains, we obtain a UCNP@2xCe6‐DMMA‐PEG nanocomplex, the surface of which is negatively charged and PEG coated under pH 7.4; this could be converted to have a positively charged naked surface at pH 6.8, significantly enhancing cell internalization of nanoparticles and increasing in vitro NIR‐induced PDT efficacy. We then utilize the intrinsic optical and paramagnetic properties of Mn2+‐doped UCNPs for in vivo dual modal imaging, and uncover an enhanced retention of UCNP@2xCe6‐DMMA‐PEG inside the tumor after intratumoral injection, owing to the slightly acidic tumor microenvironment. Consequently, a significantly improved in vivo PDT therapeutic effect is achieved using our charge‐reversible UCNP@2xCe6‐DMMA‐PEG nanoparticles. Finally, we further demonstrate the remarkably enhanced tumor‐homing of these pH‐responsive charge‐switchable nanoparticles in comparison to a control counterpart without pH sensitivity after systemic intravenous injection. Our results suggest that UCNPs with finely designed surface coatings could serve as smart pH‐responsive PDT agents promising in cancer theranostics. Smart charge‐reversible upconversion nanoparticles are developed for pH‐responsive in vivo near‐infrared light ‐excited photodynamic therapy. While stable under normal physiological pH, highly sensitive acid‐induced charge conversion of those nanoparticles is observed in slightly acidic tumor microenvironments, resulting in significantly enhanced cellular internalization of nanoparticles and remarkably improved photodynamic cancer‐cell killing efficacy.
  PubDate: 2013-01-31T02:10:13.123022-05:
  DOI: 10.1002/adfm.201202992
Highly Efficient Greenish‐Yellow Phosphorescent Organic Light‐Emitting Diodes Based on Interzone Exciton Transfer
- Authors: Yi‐Lu Chang; Brett A. Kamino, Zhibin Wang, Michael G. Helander, Yingli Rao, Lily Chai, Suning Wang, Timothy P. Bender, Zheng‐Hong Lu
  Pages: n/a - n/a
  Abstract: Phosphorescent organic light emitting diodes (PHOLEDs) have undergone tremendous growth over the past two decades. Indeed, they are already prevalent in the form of mobile displays, and are expected to be used in large‐area flat panels recently. To become a viable technology for next generation solid‐state light source however, PHOLEDs face the challenge of achieving concurrently a high color rendering index (CRI) and a high efficiency at high luminance. To improve the CRI of a standard three color white PHOLED, one can use a greenish‐yellow emitter to replace the green emitter such that the gap in emission wavelength between standard green and red emitters is eliminated. However, there are relatively few studies on greenish‐yellow emitters for PHOLEDs, and as a result, the performance of greenish‐yellow PHOLEDs is significantly inferior to those emitting in the three primary colors, which are driven strongly by the display industry. Herein, a newly synthesized greenish‐yellow emitter is synthesized and a novel device concept is introduced featuring interzone exciton transfer to considerably enhance the device efficiency. In particular, high external quantum efficiencies (current efficiencies) of 21.5% (77.4 cd/A) and 20.2% (72.8 cd/A) at a luminance of 1000 cd/m2 and 5000 cd/m2, respectively, have been achieved. These efficiencies are the highest reported to date for greenish‐yellow emitting PHOLEDs. A model for this unique design is also proposed. This design could potentially be applied to enhance the efficiency of even longer wavelength yellow and red emitters, thereby paving the way for a new avenue of tandem white PHOLEDs for solid‐state lighting. A new greenish‐yellow emitter is synthesized and a novel device concept is introduced featuring interzone exciton transfer to achieve unprecedented performance: external quantum efficiencies (current efficiencies) of 21.5% (77.4 cd/A) and 20.2% (72.8 cd/A) at 1000 cd/m2 and 5000 cd/m2, respectively, for greenish‐yellow emitting phosphorescent organic light‐emitting diodes, excellent for displays and solid‐state lighting.
  PubDate: 2013-01-30T03:20:20.165639-05:
  DOI: 10.1002/adfm.201202944
Property Control of Graphene by Employing “Semi‐Ionic” Liquid Fluorination
- Authors: Jong Hak Lee; Gavin Kok Wai Koon, Dong Wook Shin, V. E. Fedorov, Jae‐Young Choi, Ji‐Beom Yoo, Barbaros Özyilmaz
  Pages: n/a - n/a
  Abstract: Semi‐ionically fluorinated graphene (s‐FG) is synthesized with a one step liquid fluorination treatment. The s‐FG consists of two different types of bonds, namely a covalent C‐F bond and an ionic C‐F bond. Control is achieved over the properties of s‐FG by selectively eliminating ionic C‐F bonds from the as prepared s‐FG film which is highly insulating (current < 10−13 A at 1 V). After selective elimination of ionic C‐F bonds by acetone treatment, s‐FG recovers the highly conductive property of graphene. A 109 times increase in current from 10−13 to 10−4A at 1 V is achieved, which indicates that s‐FG recovers its conducting property. The properties of reduced s‐FG vary according to the number of layers and the single layer reduced s‐FG has mobility of more than 6000 cm2 V−1 s−1. The mobility drastically decreases with increasing number of layers. The bi‐layered s‐FG has a mobility of 141cm2 V−1 s−1 and multi‐layered s‐FG film showed highly p‐type doped electrical property without Dirac point. The reduction via acetone proceeds as 2C2F(semi‐ionic) + CH3C(O)CH3(l) → HF + 2C(s) + C2F(covalent) + CH3C(O)CH2(l). The fluorination and reduction processes permit the safe and facile non‐destructive property control of the s‐FG film. Semi‐ionically fluorinated graphene has good insulating properties. After selective elimination of ionic C–F bonds, s‐FG recovers in current by a factor of 109, from 10−13 to 10−4 A. The fluorination and reduction processes permit the safe and facile non‐destructive property control of the s‐FG film.
  PubDate: 2013-01-30T03:20:14.050597-05:
  DOI: 10.1002/adfm.201202822
Functionalized Fe‐Filled Multiwalled Carbon Nanotubes as Multifunctional Scaffolds for Magnetization of Cancer Cells
- Authors: Riccardo Marega; Federica De Leo, Florent Pineux, Jacopo Sgrignani, Alessandra Magistrato, Anil Damodar Naik, Yann Garcia, Lionel Flamant, Carine Michiels, Davide Bonifazi
  Pages: n/a - n/a
  Abstract: With the aim to design addressable magnetically‐active carbon nanotubes (CNTs) for cancer treatment, the use of Fe‐filled CNTs (Fe@MWCNTs) as multifunctional scaffolds is reported for exohedrally anchoring a monoclonal antibody (mAb) known to bind a plasma membrane receptor over‐expressed in several cancer cells (EGFR). Comprehensive microscopic (transmission electron microscopy, atomic force microscopy, and scanning electron microscopy) and spectroscopic (Raman, 57Fe Mossbauer, energy dispersive spectroscopy, X‐ray photoelectron spectroscopy (XPS), X‐ray diffraction) characterizations reveal the efficient confinement of magnetically‐active Fe phases (α‐Fe and Fe3C), while compositional evaluations through XPS, thermogravimetric analysis and gel electrophoresis confirm that mAb immobilization onto Fe@MWCNTs occurs. Enzyme‐linked immunosorbent assay (ELISA), confocal microscopy imaging and western blotting confirm the targeting action toward EGFR‐overexpressing cell lines (EGFR+). In vitro magnetic filtration experiments demonstrate that a selective removal of EGFR+ cells from a mixed population of healthy cell lines could be obtained in very short times (≈10 min). Cytotoxicity evaluations by classic cell staining procedures after application of an electromagnetic radiation inducing magnetic fluid hyperthermia (MFH), show a selective suppression of the EGFR+ cell line. Molecular dynamics and docking simulations of the hybrid mAb/Fe@MWCNTs conjugates nicely show how the presence of the CNT framework does not sterically affect the conformational properties of the two antigen binding regions, further supporting the biochemical findings. Encapsulation of Fe phases inside multiwalled carbon nanotubes (MWCNTs) allows isolating reactive magnetic phases exerting magnetic fluid hyperthermia responses. In particular, bioconjugation of Fe@MWCNTs with monoclonal antibody Cetuximab enables the selective in vitro cancer cell sorting and stimuli‐induced cytotoxicity under application of external magnetic inputs. Molecular dynamics calculations shed further light on binding modes and conformational properties of the Ab moieties linked on to the tubular carbon framework.
  PubDate: 2013-01-29T03:10:14.068662-05:
  DOI: 10.1002/adfm.201202898
Polymer Brush Electrets
- Authors: Xinlei Ma; Zhuang Xie, Zhilu Liu, Xuqing Liu, Tingbing Cao, Zijian Zheng
  Pages: n/a - n/a
  Abstract: The charge storage properties of polymer brushes are reported for the first time. Poly(methyl methacrylate) (PMMA) brushes are explored as electrets to store electrostatic charges. Micrometer‐ and nanometer‐scale patterns of electrostatic charges are successfully fabricated on planar and non‐planar PMMA brush films by means of conductive microcontact printing and atomic force microscope lithography, where the charge storage density and stability are studied in detail with Kelvin force microscopy. Importantly, because PMMA brushes are chemically tethered on the substrate, their charge storage properties can be studied in various organic solvents, in which their bulk counterparts will be dissolved. It is found that patterned charges on PMMA brushes are stable enough in organic media, such as hexane and toluene, for guiding the assembly of Au nanoparticles in organic media and the dewetting of polymer thin films with solvent annealing. The electrets properties shall add a new dimension of functionality, apart from the conventional chemical and physical properties, to polymer brushes for a wide range of applications in materials science, nanotechnology, and electronic devices. Poly(methyl methacrylate) (PMMA) brushes are demonstrated for the first time as electrets. Micro/nano patterns of electrostatic charges are fabricated on PMMA brushes by conductive microcontact printing and AFM lithography. The electrostatic charges on PMMA brushes are stable enough in organic solvents for guiding the assembly of nanoparticles and directing the dewetting of bulk polymer thin films.
  PubDate: 2013-01-28T06:10:16.348563-05:
  DOI: 10.1002/adfm.201203181
Discovery and Evaluation of a Functional Ternary Polymer Blend for Bone Repair: Translation from a Microarray to a Clinical Model
- Authors: Ferdous Khan; James O. Smith, Janos M. Kanczler, Rahul. S. Tare, Richard O.C. Oreffo, Mark Bradley
  Pages: n/a - n/a
  Abstract: Skeletal tissue regeneration is often required following trauma, where substantial bone or cartilage loss may be encountered and is a significant driver for the development of biomaterials with a defined 3D structural network. Solvent blending is a process that avoids complications associated with conventional thermal or mechanical polymer blending or synthesis, opening up large areas of chemical and physical space, while potentially simplifying regulatory pathways towards in vivo application. Here ternary mixtures of natural and synthetic polymers were solvent blended and evaluated as potential bone tissue engineering matrices for osteoregeneration by the assessment of growth and differentiation of STRO‐1+ skeletal stem cells. Several of the blend materials were found to be excellent supports for human bone marrow‐derived STRO‐1+ skeletal cells and fetal skeletal cells, with the optimized blend exhibiting in vivo osteogenic potential, suggesting that these polymer blends could act as suitable matrices for bioengineering of hard tissues. Functional biomaterials with a 3D porous architecture are a requirement for biomimetic scaffolds for skeletal tissue remodeling. Microarrays of ternary polymer blends are fabricated and screened, resulting in the generation of structural and functional 3D biomimetic scaffolds capable of directing skeletal tissue formation.
  PubDate: 2013-01-25T04:40:13.064261-05:
  DOI: 10.1002/adfm.201202710
Silk as a Multifunctional Biomaterial Substrate for Reduced Glial Scarring around Brain‐Penetrating Electrodes
- Authors: Lee W. Tien; Fan Wu, Min D. Tang‐Schomer, Euisik Yoon, Fiorenzo G. Omenetto, David L. Kaplan
  Pages: n/a - n/a
  Abstract: The reliability of chronic, brain‐penetrating electrodes must be improved for these ‐neural recording technologies to be viable in widespread clinical applications. One approach to improving electrode reliability is to reduce the foreign body response at the probe‐tissue interface. In this work, silk fibroin is investigated as a candidate material for fabricating mechanically dynamic neural probes with enhanced biocompatibility compared to traditional electrode materials. Silk coatings are applied to flexible cortical electrodes to produce devices that transition from stiff to flexible upon hydration. Theoretical modeling and in vitro testing show that the silk coatings impart mechanical properties sufficient for the electrodes to penetrate brain tissue. Further, it is demonstrated that silk coatings may reduce some markers of gliosis in an in vitro model and that silk can encapsulate and release the gliosis‐modifying enzyme chondroitinase ABC. This work establishes a basis for future in vivo studies of silk‐based brain‐penetrating electrodes, as well as the use of silk materials for other applications in the central nervous system where gliosis must be controlled. Silk fibroin is investigated as a novel material for fabricating brain‐penetrating electrodes with dynamic mechanical properties and the capacity to deliver sensitive therapeutics. Silk coatings are shown to natively reduce some markers of gliosis in vitro, and a further reduction is demonstrated by encapsulation and release of the enzyme chondroitinase ABC.
  PubDate: 2013-01-24T05:10:22.907696-05:
  DOI: 10.1002/adfm.201203716
Microstructured Nematic Liquid Crystalline Elastomer Surfaces with Switchable Wetting Properties
- Authors: Zi Liang Wu; Axel Buguin, Hong Yang, Jean‐Marie Taulemesse, Nicolas Le Moigne, Anne Bergeret, Xiaogong Wang, Patrick Keller
  Pages: n/a - n/a
  Abstract: Inspired by the lotus leaf, scientists have developed many superhydrophobic surfaces, some of which show remarkable switching between hydrophobic and hydrophilic state under external stimuli. However, the switch usually relies on the change of chemical properties rather than on the modification of the topographic structure of the surface. In this paper, the roughness‐change‐related switchable wetting properties of microstructured responsive surfaces made of nematic liquid crystalline elastomers (LCEs) is reported. First, various carbonate LC monomers and side‐on LCEs are synthesized with low nematic‐to‐isotropic transition temperature, TNI. Then, LCEs prepared from 3″‐vinylcarbonyloxypropyl 2,5‐di(4′‐octyloxybenzoyloxy)benzoate monomer, with TNI of 76 °C and contraction of 34% are used to construct a surface covered with micropillar arrays by using a replica molding technique. The contraction of the micropillars induces a reversible roughness change of the microstructured surface. Water contact angle of this microstructured surface changed with temperature, indicating a successful approach at building a surface with switchable wetting properties. Microstructured liquid crystalline elastomer surfaces covered with micropillar arrays are developed using a replica molding technique. When modulating the temperature, the contraction of the pillars along their long axis induces a roughness change and therefore a change of the wetting properties of the microstructured surfaces.
  PubDate: 2013-01-24T05:10:14.610751-05:
  DOI: 10.1002/adfm.201203291
Photochemical Transformation of Fullerenes
- Authors: Jia Wang; Jenny Enevold, Ludvig Edman
  Pages: n/a - n/a
  Abstract: Experimental findings and associated theoretical insights regarding the photochemical transformation of fullerenes are reported, which challenge the conventional wisdom in the field and point out a viable path towards improved fullerene‐based electronic devices. It is shown that the efficiency of the photochemical monomer‐to‐dimer transformation of the fullerene [6,6′]‐phenyl‐C61‐butyric acid methyl ester (PCBM) is strongly dependent on the light intensity, and this is utilized to demonstrate that direct patterning of an electroactive PCBM film can be effectuated by sub‐second UV‐light exposure followed by development in a tuned developer solution. By straightforward analytical reasoning, it is demonstrated that the observed intensity‐dependent monomer‐to‐dimer transformation dictates that a significant back‐reaction to the ground state must be in effect, which presumably originates from the excited‐triplet state. By a combination of numerical modeling and analytical argumentation, it is further shown that the final dimer formation must constitute a bi‐excited reaction between two neighboring monomers photo‐excited to the triplet state. The photochemical monomer‐to‐dimer transformation of the fullerene [6,6′]‐phenyl‐C61‐butyric acid methyl ester (PCBM) is strongly dependent on the light intensity. This is utilized to realize sub‐second UV‐light induced patterning of electronically active PCBM films. Dimer formation takes place between two monomers photo‐excited to the triplet state. These findings challenge the conventional wisdom in the field.
  PubDate: 2013-01-23T06:10:14.466092-05:
  DOI: 10.1002/adfm.201203386
Efficient Ag@AgCl Cubic Cage Photocatalysts Profit from Ultrafast Plasmon‐Induced Electron Transfer Processes
- Authors: Yuxin Tang; Zhelong Jiang, Guichuan Xing, Anran Li, Pushkar D. Kanhere, Yanyan Zhang, Tze Chien Sum, Shuzhou Li, Xiaodong Chen, Zhili Dong, Zhong Chen
  Pages: n/a - n/a
  Abstract: Photon‐coupling and electron dynamics are the key processes leading to the photocatalytic activity of plasmonic metal‐semiconductor nanohybrids. To better utilize and explore these effects, a facile large‐scale synthesis route to form Ag@AgCl cubic cages with well‐defined hollow interiors is carried out using a water‐soluble sacrificial salt‐crystal‐template process. Theoretical calculations and experimental probes of the electron transfer process are used in an effort to gain insight into the underlying plasmonic properties of the Ag@AgCl materials. Efficient utilization of solar energy to create electron‐hole pairs is attributed to the significant light confinement and enhancement around the Ag/AgCl interfacial plasmon hot spots and multilight‐reflection inside the cage structure. More importantly, an ultrafast electron transfer process (≤150 fs) from Ag nanoparticles to the AgCl surface is detected, which facilitates the charge separation efficiency in this system, contributing to high photocatalytic activity and stability of Ag@AgCl photocatalyst towards organic dye degradation. A novel and economic water‐soluble sacrificial salt‐crystal‐template process is developed for the large‐scale production of hollow Ag@AgCl cage materials. The hollow Ag@AgCl cages show superior photocatalytic performance (28 times larger) compared with the solid form, which profits from the highly efficient electron‐hole pair separation that results from ultrafast plasmon‐induced electron transfer from Ag nanoparticles to the AgCl surface.
  PubDate: 2013-01-23T03:20:26.29627-05:0
  DOI: 10.1002/adfm.201203379
Impact of Materials versus Geometric Parameters on the Contact Resistance in Organic Thin‐Film Transistors
- Authors: Manfred Gruber; Egbert Zojer, Ferdinand Schürrer, Karin Zojer
  Pages: n/a - n/a
  Abstract: The contact resistance is known to severely hamper the performance of organic thin‐film transistors, especially when dealing with large injection barriers, high mobility organic semiconductors, or short channel lengths. Here, the relative significance of how it is affected by materials‐parameters (mobility and interfacial level‐offsets) and geometric factors (bottom‐contact vs top‐contact geometries) is assessed. This is done using drift‐diffusion‐based simulations on idealized device structures aiming at a characterization of the “intrinsic” situation in the absence of traps, differences in the film morphology, or metal‐atoms diffusing into the organic semiconductor. It is found that, in contrast to common wisdom, in such a situation the top‐contact devices do not always outperform the bottom‐contact ones. In fact, the observed ratio between the contact resistances of the two device structures changes by up to two orders of magnitude depending on the assumed materials parameters. The contact resistance is also shown to be strongly dependent on the hole mobility in the organic semiconductor and influenced by the chosen point of operation of the device. The contact resistance in organic thin‐film transistors is affected by many more parameters than just the injection barrier at the metal/organic interface. The arrangement of the electrodes, the operation conditions, as well as the carrier mobility can change the contact resistance by many orders of magnitude. This can be understood from the details of the potential distribution in the devices.
  PubDate: 2013-01-23T03:20:21.84558-05:0
  DOI: 10.1002/adfm.201203250
The Impacts of Cation Stoichiometry and Substrate Surface Quality on Nucleation, Structure, Defect Formation, and Intermixing in Complex Oxide Heteroepitaxy–LaCrO3 on SrTiO3(001)
- Authors: L. Qiao; K. H. L. Zhang, M. E. Bowden, T. Varga, V. Shutthanandan, R. Colby, Y. Du, B. Kabius, P. V. Sushko, M. D. Biegalski, S. A. Chambers
  Pages: n/a - n/a
  Abstract: The ability to design and fabricate electronic devices with reproducible properties using complex oxides is critically dependent on our ability to controllably synthesize these materials in thin‐film form. Structure‐property relationships are intimately tied to film and interface composition. Here the effect of cation stoichiometry on structural quality and defect formation in LaCrO3 heteroepitaxial films prepared using molecular beam epitaxy is reported. From first principles the regions of stability of various candidate defects, along with the predicted effects of these defects on structural parameters, are calculated as a function of Cr and O chemical potential. Epitaxial LaCrO3 films readily nucleate and remain coherently strained on SrTiO3(001) over a wide range of La‐to‐Cr atom ratios, but La‐rich films are of considerably lower structural quality than stoichiometric and Cr‐rich films. Cation imbalances are accompanied by anti‐site defect formation. Cation mixing occurs at the interface for all La‐to‐Cr ratios investigated and is not quenched by deposition on SrTiO3(001) at ambient temperature. Indiffused La atoms occupy Sr sites. Intermixing is effectively quenched by using molecular beam epitaxy to deposit LaCrO3 at ambient temperature on defect free Si(001). However, analogous pulsed laser deposition on Si is accompanied by cation mixing. The effect of cation stoichiometry and substrate defect density on nucleation, structure, defect formation, and interfacial mixing during III‐III perovskite complex oxide heteroepitaxy is explored. Cation imbalance is shown to result in anti‐site defect formation within the film, and subsurface cation vacancies promote cation mixing.
  PubDate: 2013-01-23T03:20:14.229882-05:
  DOI: 10.1002/adfm.201202655
Fluorene‐Based Asymmetric Bipolar Universal Hosts for White Organic Light Emitting Devices
- Authors: Ejabul Mondal; Wen‐Yi Hung, Hung‐Chi Dai, Ken‐Tsung Wong
  Pages: n/a - n/a
  Abstract: Two new bipolar host molecules composed of hole‐transporting carbazole and electron‐transporting cyano (CzFCN) or oxadiazole (CzFOxa)‐substituted fluorenes are synthesized and characterized. The non‐conjugated connections, via an sp3‐hybridized carbon, effectively block the electronic interactions between electron‐donating and ‐accepting moieties, giving CzFCN and CzFOxa bipolar charge transport features with balanced mobilities (10−5 to 10−6 cm2 V−1 s−1). The meta–meta configuration of the fluorene‐based acceptors allows the bipolar hosts to retain relatively high triplet energies [ET = 2.70 eV (CzFOxa) and 2. 86 eV (CzFCN)], which are sufficiently high for hosting blue phosphor. Using a common device structure – ITO/PEDOT:PSS/DTAF/TCTA/host:10% dopants (from blue to red)/DPPS/LiF/Al – highly efficient electrophosphorescent devices are successfully achieved. CzFCN‐based devices demonstrate better performance characteristics, with maximum ηext of 15.1%, 17.9%, 17.4%, 18%, and 20% for blue (FIrpic), green [(PPy)2Ir(acac)], yellowish‐green [m‐(Tpm)2Ir(acac)], yellow [(Bt)2Ir(acac)], and red [Os(bpftz)2(PPhMe2)2, OS1], respectively. In addition, combining yellowish‐green m‐(Tpm)2Ir(acac) with a blue emitter (FIrpic) and a red emitter (OS1) within a single emitting layer hosted by bipolar CzFCN, three‐color electrophosphorescent WOLEDs with high efficiencies (17.3%, 33.4 cd A−1, 30 lm W −1), high color stability, and high color‐rendering index (CRI) of 89.7 can also be realized. A new bipolar host molecule (CzFCN) configuring hole‐transporting carbazole and electron‐transporting cyano‐substituted fluorene via a saturated carbon center is utilized to realize highly efficient electrophosphorescent blue, green, yellowish‐green, yellow, red, and white devices using a common device structure.
  PubDate: 2013-01-22T07:10:27.608165-05:
  DOI: 10.1002/adfm.201202889
Origins of Low Quantum Efficiencies in Quantum Dot LEDs
- Authors: Deniz Bozyigit; Olesya Yarema, Vanessa Wood
  Pages: n/a - n/a
  Abstract: The promise for next generation light‐emitting device (LED) technologies is a major driver for research on nanocrystal quantum dots (QDs). The low efficiencies of current QD‐LEDs are often attributed to luminescence quenching of charged QDs through Auger‐processes. Although new QD chemistries successfully suppress Auger recombination, high performance QD‐LEDs with these materials have yet to be demonstrated. Here, QD‐LED performance is shown to be significantly limited by the electric field. Experimental field‐dependent photoluminescence decay studies and tight‐binding simulations are used to show that independent of charging, the electric field can strongly quench the luminescence of QD solids by reducing the electron and hole wavefunction overlap, thereby lowering the radiative recombination rate. Quantifying this effect for a series of CdSe/CdS QD solids reveals a strong dependence on the QD band structure, which enables the outline of clear design strategies for QD materials and device architectures to improve QD‐LED performance. The quantum yield (QY) of CdSe/CdS core/shell colloidal quantum dots (QD) is reduced exponentially with increasing electric field (Fqd). This reduction is more pronounced with thicker shells and is due to the reduced overlap between electron and hole wavefunctions. This has strong implications for the use of QDs in opto‐electronic applications, where electric field strengths can readily exceed 1 MV cm−1.
  PubDate: 2013-01-22T07:10:23.782021-05:
  DOI: 10.1002/adfm.201203191
Selective UV Reflecting Mirrors Based on Nanoparticle Multilayers
- Authors: J. R. Castro Smirnov; Mauricio E. Calvo, Hernán Míguez
  Pages: n/a - n/a
  Abstract: A new type of nanostructured selective ultraviolet (UV) reflecting mirror is presented. Periodic porous multilayers with photonic crystal properties are built by spin‐coating‐assisted layer‐by‐layer deposition of colloidal suspensions of nanoparticles of ZrO2 and SiO2 (electronic band gap at λ < 220 nm). These optical filters are designed to block well‐defined wavelength ranges of the UVA, UVB, and UVC regions of the electromagnetic spectrum while preserving transparency in the visible. The shielding against those spectral regions arises exclusively from optical interference phenomena and depends only on the number of stacked layers and the refractive index contrast between them. In addition, it is shown that the accessible pore network of the as‐deposited multilayer allows preparing thin, flexible, self‐standing, transferable, and adaptable selective UV filters by polymer infiltration, without significantly losing reflectance intensity, i.e., preserving the dielectric contrast. These films offer a degree of protection comparable to that of traditional ones, without any foreseeable unwanted secondary effects, such as photodegradation, increase of local temperature or, as is the case for organic absorbers, generation of free radicals, all of which are caused by light absorption. A new type of nanostructured selective ultraviolet reflecting mirrors based on the alternated deposition of layers of SiO2 and ZrO2 nanoparticles is presented. The UV blocking effect arises exclusively from optical interference phenomena and depends only on the number of stacked layers and the refractive index contrast between them.
  PubDate: 2013-01-22T07:10:15.48235-05:0
  DOI: 10.1002/adfm.201202587
Ultra‐Smooth and Ultra‐Strong Ion‐Exchanged Glass as Substrates for Organic Electronics
- Authors: Daniel Käfer; Mingqian He, Jianfeng Li, Michael S. Pambianchi, Jiangwei Feng, John C. Mauro, Zhenan Bao
  Pages: n/a - n/a
  Abstract: The introduction of new substrate materials into the world of electronics has previously opened up new possibilities for novel applications and device designs. Here, the use of ion‐exchanged sodium aluminosilicate (NAS) glass is presented as a new type of substrate that is not only highly damage resistant, but also allows the fabrication of high performance organic electronic devices. The smoothness of the NAS glass surface enables favorable growth of the semiconductor layer, enabling high charge carrier mobilities for typical organic semiconductors, such as pentacene or C60, and a polymer semiconductor. No degradation of the device performance is observed as a result of ion migration into the active device region, and no compromise in substrate strength due to the processing conditions is made. This work suggests the possibility of new, highly durable electronic devices on glass in large area format. An ion‐exchanged glass is introduced as a new substrate that is not only virtually unbreakable therefore overcoming the brittleness that Si wafers possess, but also allows fabrication of high performance organic electronic devices. The smoothness of the surface of this fusion‐drawn glass enables favorable large grain growth of the semiconductor, enabling high charge carrier mobilities of all kinds of organic semiconductors like pentacene or C60.
  PubDate: 2013-01-22T05:23:05.892867-05:
  DOI: 10.1002/adfm.201202009
An Analytical Model of Reactive Diffusion for Transient Electronics
- Authors: Rui Li; Huanyu Cheng, Yewang Su, Suk‐Won Hwang, Lan Yin, Hu Tao, Mark A. Brenckle, Dae‐Hyeong Kim, Fiorenzo G. Omenetto, John A. Rogers, Yonggang Huang
  Pages: n/a - n/a
  Abstract: Transient electronics is a class of technology that involves components which physically disappear, in whole or in part, at prescribed rates and at programmed times. Enabled devices include medical monitors that fully resorb when implanted into the human body (“bio‐resorbable”) to avoid long‐term adverse effects, or environmental monitors that dissolve when exposed to water (“eco‐resorbable”) to eliminate the need for collection and recovery. Analytical models for dissolution of the constituent materials represent important design tools for transient electronic systems that are configured to disappear in water or biofluids. Here, solutions for reactive‐diffusion are presented in single‐ and double‐layered structures, in which the remaining thicknesses and electrical resistances are obtained analytically. The dissolution time and rate are defined in terms of the reaction constants and diffusivities of the materials, the thicknesses of the layer, and other properties of materials and solution. These models agree well with the experiments for single layers of Mg and SiO2, and double layers of Mg/MgO. The underlying physical constants extracted from analysis fall within a broad range previously reported in other studies; these constants can be extremely sensitive to the morphologies of the materials, temperature, and the PH value, concentration, and properties of the surrounding liquid. Transient electronics are designed to be stable and fully functional during their lifetimes, but then completely disappear in water or biofluids at prescribed rates and at programmed times. Analytical models are established for the dissolution of constituent materials in transient electronic systems, and they provide important design tools for transient electronics.
  PubDate: 2013-01-21T03:23:04.03321-05:0
  DOI: 10.1002/adfm.201203088
A Giant Electrocaloric Effect in Nanoscale Antiferroelectric and Ferroelectric Phases Coexisting in a Relaxor Pb0.8Ba0.2ZrO3 Thin Film at Room Temperature
- Authors: Biaolin Peng; Huiqing Fan, Qi Zhang
  Pages: n/a - n/a
  Abstract: Recently, large electrocaloric effects (ECE) in antiferroelectric sol‐gel PbZr0.95Ti0.05O3 thin films and in ferroelectric polymer P(VDF‐TrFE)55/45 thin films were observed near the ferroelectric Curie temperatures of these materials (495 K and 353 K, respectively). Here a giant ECE (ΔT = 45.3 K and ΔS = 46.9 J K−1 kg−1 at 598 kV cm−1) is obtained in relaxor ferroelectric Pb0.8Ba0.2ZrO3 (PBZ) thin films fabricated on Pt(111)/TiOx/SiO2/Si substrates using a sol‐gel method. Nanoscale antiferroelectric (AFE) and ferroelectric (FE) phases coexist at room temperature (290 K) rather than at the Curie temperature (408 K) of the material. The giant ECE in such a system is attributed to the coexistence of AFE and FE phases and a field‐induced nanoscale AFE to FE phase transition. The giant ECE of the thin film makes this a promising material for applications in cooling systems near room temperature. A giant electrocaloric effect at room temperature is obtained in coexisting antiferroelectric and ferroelectric phase relaxor Pb0.8Ba0.2ZrO3 thin films. Such an effect is usually obtained only at the Curie temperature. This is therefore a promising material for applications in cooling systems near room temperature.
  PubDate: 2013-01-20T18:23:07.204401-05:
  DOI: 10.1002/adfm.201202525
Multi‐Stimuli Responsive Hydrogel Cilia
- Authors: P. J. Glazer; J. Leuven, H. An, S. G. Lemay, E. Mendes
  Pages: n/a - n/a
  Abstract: Large arrays of high aspect ratio, artificial hydrogel based cilia that can respond to multiple stimuli are produced by means of micro‐fabrication techniques. The cilia operate in aqueous solutions and are sensitive to pH, electric and/or magnetic fields. The biomimetic system combines both sensing and motility. Detection of changes in environment, such as a decrease in pH, triggers a collective response, to an external time‐dependent magnetic field. High‐aspect‐ratio responsive cilia arrays that combine, in one system, sensing and motility functions are fabricated. Based on different approaches the system can be electrically, environmentally or magnetically activated. Detection of changes in environment, such as a decrease in pH, triggers a collective cilia response, to an external time‐dependent magnetic field.
  PubDate: 2013-01-18T04:10:29.092016-05:
  DOI: 10.1002/adfm.201203212
Spray Layer‐by‐Layer Electrospun Composite Proton Exchange Membranes
- Authors: David S. Liu; J. Nathan Ashcraft, Matthew M. Mannarino, Meredith N. Silberstein, Avni A. Argun, Gregory C. Rutledge, Mary C. Boyce, Paula T. Hammond
  Pages: n/a - n/a
  Abstract: Polymer electrolyte films are deposited onto highly porous electrospun mats using layer‐by‐layer (LbL) processing to fabricate composite proton conducting membranes. By simply changing the assembly conditions for generation of the LbL film on the nanofiber mat substrate, three different and unique composite film morphologies can be achieved in which the electrospun mats provide mechanical support; the LbL assembly produces highly conductive films that coat the mats in a controlled fashion, separately providing the ionic conductivity and fuel blocking characteristics of the composite membrane. Coating an electrospun mat with the LbL dipping process produces composite membranes with “webbed” morphologies that link the fibers in‐plane and give the composite membrane in‐plane proton conductivities similar to that of the pristine LbL system. In contrast, coating an electrospun mat using the spray‐LbL process without vacuum produces a uniform film that bridges across all of the pores of the mat. These membranes have methanol permeability similar to free‐standing poly(diallyl dimethyl ammonium chloride)/sulfonated poly(2,6‐dimethyl 1,4‐phenylene oxide) (PDAC/sPPO) thin films. Coating an electrospun mat with the vacuum‐assisted spray‐LbL process produces composite membranes with conformally coated fibers throughout the bulk of the mat with nanometer control of the coating thickness on each fiber. The mechanical properties of the LbL‐coated mats display composite properties, exhibiting the strength of the glassy PDAC/sPPO films when dry and the properties of the underlying electrospun polyamide mat when hydrated. By combining the different spray‐LbL fabrication techniques with electrospun fiber supports and tuning the parameters, mechanically stable membranes with high selectivity can be produced, potentially for use in fuel cell applications. Polymer electrolyte films are deposited onto highly porous electrospun mats using layer‐by‐layer (LbL) processing to fabricate composite proton conducting membranes. Various dip and spray processing techniques are investigated for different and unique composite film morphologies. By combining the different spray‐LbL fabrication techniques with electrospun fiber supports and tuning the parameters, mechanically stable membranes with high selectivity can be produced.
  PubDate: 2013-01-18T04:10:27.085674-05:
  DOI: 10.1002/adfm.201202892
Molecularly Imprinted Polymer Grafted Porous Au‐Paper Electrode for an Microfluidic Electro‐Analytical Origami Device
- Authors: Lei Ge; Shoumei Wang, Jinghua Yu, Nianqiang Li, Shenguang Ge, Mei Yan
  Pages: n/a - n/a
  Abstract: Molecular imprinting technique is introduced into microfluidic paper‐based analytical devices (μ‐PADs) through electropolymerization of molecular imprinted polymer (MIP) in a novel Au nanoparticle (AuNP) modified paper working electrode (Au‐PWE). This is fabricated through the growth of a AuNP layer on the surfaces of cellulose fibers in the PWE. Due to the porous morphology of paper as well as the high specific surface area and conductivity of the resulting AuNP layer on the cellulose fibers, the effective surface area and the sensitivity of the Au‐PWE is enhanced remarkably. Based on this novel MIP‐Au‐PWE and the principle of origami, a microfluidic MIP‐based electro‐analytical origami device (μ‐MEOD), comprised of one auxiliary pad surrounded by four sample tabs, is developed for the detection of D‐glutamic acid in a linear range from 1.2 nM to 125.0 nM with a low detection limit of 0.2 nM. The selectivity, reproducibility, and stability of this μ‐MEOD are investigated. This μ‐MEOD would provide a new platform for high‐throughput, sensitive, specific, and multiplex assay as well as point‐of‐care diagnosis in public health, environmental monitoring, and the developing world. A molecular imprinting technique is introduced into a lab‐on‐paper device through electropolymerization of molecularly imprinted polymer in a novel Au nanoparticle (AuNP) modified paper working electrode. This is fabricated through the growth of AuNP layers on the surfaces of cellulose fibers to enhance the conductivity of the cellulose fibers in the paper and, as a result, increase the effective surface area of the electrode.
  PubDate: 2013-01-18T04:10:24.874173-05:
  DOI: 10.1002/adfm.201202785
Enhanced and Engineered d0 Ferromagnetism in Molecularly‐Thin Zinc Oxide Nanosheets
- Authors: Takaaki Taniguchi; Kazuhiro Yamaguchi, Ayako Shigeta, Yuki Matsuda, Shinya Hayami, Tetsuya Shimizu, Takeshi Matsui, Teruo Yamazaki, Asami Funatstu, Yukihiro Makinose, Nobuhiro Matsushita, Michio Koinuma, Yasumichi Matsumoto
  Pages: n/a - n/a
  Abstract: Molecularly‐thin nanosheets are ultimate two‐dimensional (2D) nanomaterials potentially giving unusual physical and chemical properties due to the strong 2D quantum and surface effects. Here, it is demonstrated that 1.5‐nm‐thick ZnO nanosheets exhibit greatly enhanced room‐temperature ferromagnetism. Saturation magnetization value of the nanosheets with intercalated dodecyl sulfate layers is approximately 100 times that of ZnO mesocrystals. Anion exchange with dodecyl phosphate layers strongly suppresses ferromagnetic ordering as a result of surface defect passivation while maintaining bulk‐like n‐type semiconducting properties, which reveals significance of interfacial states to engineer functional properties of nanosheet‐based hybrid materials. Dense integration of interfacial ferromagnetic centers in the lamellar structure gives enhanced ferromagnetism to ZnO nanosheets. Anion exchange with dodecyl phosphate layers strongly suppresses ferromagnetic ordering as a result of surface defect passivation while maintaining bulk‐like n‐type semiconducting properties.
  PubDate: 2013-01-18T04:10:17.484029-05:
  DOI: 10.1002/adfm.201202704
Functional Free‐Standing Graphene Honeycomb Films
- Authors: Shengyan Yin; Yulia Goldovsky, Moshe Herzberg, Lei Liu, Hang Sun, Yanyan Zhang, Fanben Meng, Xuebo Cao, Darren D. Sun, Hongyu Chen, Ariel Kushmaro, Xiaodong Chen
  Pages: n/a - n/a
  Abstract: Fabricating free‐standing, three‐dimensional (3D) ordered porous graphene structure can service a wide range of functional materials such as environmentally friendly materials for antibacterial medical applications and efficient solar harvesting devices. A scalable solution processable strategy is developed to create such free‐standing hierarchical porous structures composed of functionalized graphene sheets via an “on water spreading” method. The free‐standing film shows a large area uniform honeycomb structure and can be transferred onto any substrate of interest. The graphene‐based free‐standing honeycomb films exhibit superior broad spectrum antibacterial activity as confirmed using green fluorescent protein labeled Pseudomonas aeruginosa PAO1 and Escherichia coli as model pathogens. Functional nanoparticles such as titanium dioxide (TiO2) nanoparticles can be easily introduced into conductive graphene‐based scaffolds by premixing. The formed composite honeycomb film electrode shows a fast, stable, and completely reversible photocurrent response accompanying each switch‐on and switch‐off event. The graphene‐based honeycomb scaffold enhances the light‐harvesting efficiency and improves the photoelectric conversion behavior; the photocurrent of the composite film is about two times as high as that of the pure TiO2 film electrode. Such composite porous films combining remarkably good electrochemical performance of graphene, a large electrode/electrolyte contact area, and excellent stability during the photo‐conversion process hold promise for further applications in water treatment and solar energy conversion. A facile strategy to create free‐standing graphene honeycomb films is developed. The obtained free‐standing honeycomb films can be easily transferred to the substrate of interest while retaining their original sizes and structures, and exhibiting broad spectrum antibacterial activity and enhanced efficiency of photoconversion.
  PubDate: 2013-01-17T05:40:24.592337-05:
  DOI: 10.1002/adfm.201203491
Molecular Intercalation and Cohesion of Organic Bulk Heterojunction Photovoltaic Devices
- Authors: Christopher Bruner; Nichole C. Miller, Michael D. McGehee, Reinhold H. Dauskardt
  Pages: n/a - n/a
  Abstract: The phase separated bulk heterojunction (BHJ) layer in BHJ polymer:fullerene organic photovoltaic devices (OPV) are mechanically weak with low values of cohesion. Improved cohesion is important for OPV device thermomechanical reliability. BHJ devices are investigated and how fullerene intercalation within the active layer affects cohesive properties in the BHJ is shown. The intercalation of fullerenes between the side chains of the polymers poly(3,3″′‐didocecyl quaterthiophene) (PQT‐12) and poly(2,5‐bis(3‐hexadecylthiophen‐2‐yl)thieno[3,2‐b]thiophene (pBTTT) is shown to enhance BHJ layer cohesion. Cohesion values range from ≈1 to 5 J m−2, depending on the polymer:fullerene blend, processing conditions, and composition. Devices with non‐intercalated BHJ layers are found to have significantly reduced values of cohesion. The resulting device power conversion efficiencies (PCE) are also investigated and correlated with the device cohesion. The effects of molecular intercalation and thermal annealing are described on active layer cohesion in organic photovoltaics with the donor polymer poly(3,3″′‐didocecyl quaterthiophene) (PQT‐12). Increased cohesion is related to the extent of intercalation and the presence of fracture resistant domains. Correlation with annealing and device efficiency is presented.
  PubDate: 2013-01-17T05:30:21.7706-05:00
  DOI: 10.1002/adfm.201202969
Nano‐Imprinted Ferroelectric Polymer Nanodot Arrays for High Density Data Storage
- Authors: Xiang‐Zhong Chen; Qian Li, Xin Chen, Xu Guo, Hai‐Xiong Ge, Yun Liu, Qun‐Dong Shen
  Pages: n/a - n/a
  Abstract: Ferroelectric vinylidene fluoride‐trifluoroethylene copolymer [P(VDF‐TrFE)] free‐standing ultrahigh density (≈75 Gb inch−2) nanodot arrays are successfully fabricated through a facile, high‐throughput, and cost‐effective nano‐imprinting method using disposable anodic aluminum oxide with orderly arranged nanometer‐scale pores as molds. The nanodots show a large‐area smooth surface morphology, and the piezoresponse in each nanodot is strong and uniform. The preferred orientation of the copolymer chains in the nanodot arrays is favorable for polarization switching of single nanodots. The ferroelectric polymer memory prototype can be operated by a few volts with high writing/erasing speed, which comply with the requirements of integrated circuit. This approach provides a way of directly writing nanometer electronic features in two dimensions by piezoresponse force microscopy probe based technology, which is attractive for high density data storage. Ferroelectric polymer [P(VDF‐TrFE)] free‐standing nanodot arrays with ultrahigh data storage density are fabricated through nano‐imprinting. The preferred orientation of the copolymer chains in the nanodot arrays is favorable for polarization switching of single nanodots. This approach allows nanometer electronic features to be written directly in two dimensions by piezoresponse force microscopy probe based technology.
  PubDate: 2013-01-17T05:30:18.666572-05:
  DOI: 10.1002/adfm.201203042
Bench‐Top Fabrication of Hierarchically Structured High‐Surface‐Area Electrodes
- Authors: Christine M. Gabardo; Yujie Zhu, Leyla Soleymani, Jose M. Moran‐Mirabal
  Pages: n/a - n/a
  Abstract: Fabrication of hierarchical materials, with highly optimized features from the millimeter to the nanometer scale, is crucial for applications in diverse areas including biosensing, energy storage, photovoltaics, and tissue engineering. In the past, complex material architectures have been achieved using a combination of top‐down and bottom‐up fabrication approaches. A remaining challenge, however, is the rapid, inexpensive, and simple fabrication of such materials systems using bench‐top prototyping methods. To address this challenge, the properties of hierarchically structured electrodes are developed and investigated by combining three bench‐top techniques: top‐down electrode patterning using vinyl masks created by a computer‐aided design (CAD)‐driven cutter, thin film micro/nanostructuring using a shrinkable polymer substrate, and tunable electrodeposition of conductive materials. By combining these methods, controllable electrode arrays are created with features in three distinct length scales: 40 μm to 1 mm, 50 nm to 10 μm, and 20 nm to 2 μm. The electrical and electrochemical properties of these electrodes are analyzed and it is demonstrated that they are excellent candidates for next generation low‐cost electrochemical and electronic devices. A rapid, facile, and inexpensive method to fabricate hierarchically structured gold electrodes that present high surface areas is presented. The combination of vinyl film masking, stress‐driven wrinkling, and electrodeposition allows the rapid prototyping of electrode designs. The fabricated electrodes are robust, highly reproducible, perform well in electrochemical measurements, and demonstrate up to 1000% enhancements in electroactive surface area.
  PubDate: 2013-01-17T05:30:15.59833-05:0
  DOI: 10.1002/adfm.201203220
Fractal Inorganic−Organic Interfaces in Hybrid Membranes for Efficient Proton Transport
- Authors: Vasana Maneeratana; John D. Bass, Thierry Azaïs, Amaury Patissier, Karine Vallé, Manuel Maréchal, Gérard Gebel, Christel Laberty‐Robert, Clément Sanchez
  Pages: n/a - n/a
  Abstract: A facile method for preparing highly conductive hybrid organic−inorganic membranes is reported. These membranes are synthesized using an electrospinning process with a sol−gel‐based solution containing PVDF−HFP (polyvinylidenefluoride‐hexafluoropropylene), functionalized or not functionalized silicon alkoxides, and additives. Proton conduction measurements highlight that these hybrid membranes exhibit conductivity value of 101 mS/cm at 120 °C under 80% RH (relative humidity), comparable to the best Nafion measured under the same conditions. These membranes have a proton conductivity‐humidity variation close to Nafion and a modulus value higher than that for Nafion above 80 °C. Their proton conductivity value is about 15 mS/cm under 50% RH, and it constitutes one of the highest values reported. These interesting properties are related to the microstructure of the electrospun membranes that have been characterized using field emission scanning electron microscopy (FE‐SEM) and small angle neutron scattering (SANS). The electrospun membranes are made composed of a bundle of fibers surrounded by a functionalized silica network. The bundle of fibers corresponds to the assembly of small polymer fibers surrounded by small anisotropic functionalized silica domains. Coupling the reactive chemistry of the sol–gel‐based process with electrospinning allows the design of hybrid membranes with fractal hydrophobic/hydrophilic interfaces exhibiting different length scales. The design of hybrid organic−inorganic membranes using an electrospinning approach is discussed. The sulfonated silica‐based membranes exhibit proton conductivity comparable to Nafion at high temperature and low humidity and have better mechanical properties. This behavior is related to the specific microstructure of the membrane, which is reached via the electrospinning. The membrane contains polymer fibers surrounded by functionalized large silica domains.
  PubDate: 2013-01-17T05:23:22.333342-05:
  DOI: 10.1002/adfm.201202701
Location, Location, Location ‐ Strategic Positioning of 2,1,3‐benzothiadiazole Units within Trigonal Quaterfluorene‐Truxene Star‐Shaped Structures
- Authors: Colin R. Belton; Alexander L. Kanibolotsky, James Kirkpatrick, Clara Orofino, Saadeldin E. T. Elmasly, Paul N. Stavrinou, Peter J. Skabara, Donal D. C. Bradley
  Pages: n/a - n/a
  Abstract: The fused, bicyclic molecule, 2,1,3‐Benzothiadiazole (BT), has become a key ingredient in the design of new organic semiconductors for light emission and energy harvesting applications. Here, the synthesis is reported of a series of trigonal, star‐shaped compounds comprising a truxene core and three quater‐dialkylfluorene arms into each of which a BT unit is inserted sequentially at each possible position (T4BT‐A to T4BT‐E). Analysis of the resulting electronic properties shows that as a consequence of conjugative coupling to the core and the resulting symmetry there are three distinct locations for the BT unit and the influence that these locations have on light emission and other spectroscopic characteristics is discussed. The systematic variation in photophysical properties for the different structural isomers helps to clarify the influence of BT unit addition to 9,9‐dialkylfluorene chains. It also helps to establish a design template for the construction of donor‐acceptor conjugated materials with targeted properties. For T4BT‐E with a BT unit at the terminal position of each arm, the photoluminescence quantum efficiency is significantly reduced and no amplified spontaneous emission is observed under typical pumping conditions. Theoretical calculations assist in understanding the variation in behaviors among the T4BT‐X family of compounds, especially in relation to their photoluminescence decay times and the Raman scattering intensities of their dominant BT‐unit‐centred molecular vibrations. 2,1,3‐benzothiadiazole (BT) units are systematically incorporated into star‐shaped trigonal molecules comprising a truxene core and three quaterfluorene arms. Five isomers are synthesized corresponding to the symmetric insertion of a single BT unit into each of the possible positions within the arms. Three BT locations are identified (see figure) by comparison of absorption and photoluminescence (PL) spectra, supported by theoretical calculations. Additional experimental and theoretical characterizations reveal the influence of BT position on Raman, photoluminescence, and stimulated emission properties.
  PubDate: 2013-01-17T05:23:18.700527-05:
  DOI: 10.1002/adfm.201202644
Fully Patterned Low‐Voltage Transparent Metal Oxide Transistors Deposited Solely by Chemical Spray Pyrolysis
- Authors: Hendrik Faber; Benjamin Butz, Christel Dieker, Erdmann Spiecker, Marcus Halik
  Pages: n/a - n/a
  Abstract: All‐inorganic transparent thin‐film transistors deposited solely by the solution processing method of spray pyrolysis are reported. Different precursor materials are employed to create conducting and semiconducting species of ZnO acting as electrodes and active channel material, respectively, as well as zirconium oxide as gate dielectric layer. Additionally, a simple stencil mask system provides sufficient resolution to realize the necessary geometric patterns. As a result, fully functional low‐voltage n‐type transistors with a mobility of 0.18 cm2 V−1 s−1 can be demonstrated via a technique that bears the potential for upscaling. A detailed microscopic evaluation of the channel region by electron diffraction, high‐resolution and analytical TEM confirms the layer stacking and provides detailed information on the chemical composition and nanocrystalline nature of the individual layers. All‐inorganic, transparent n‐type transistors are deposited by means of the solution processing technique of chemical spray pyrolysis. The use of different precursors leads to semiconducting, conducting, and insulating materials, which are patterned with a simple stencil mask technique during the spray process, thus realizing fully functional devices. Optical, electrical and microstructural properties are investigated.
  PubDate: 2013-01-17T05:23:13.55945-05:0
  DOI: 10.1002/adfm.201202334
Self‐Assembly of π‐Conjugated Amphiphiles: Free Standing, Ordered Sheets with Enhanced Mobility
- Authors: Bhawani Narayan; Satyaprasad P. Senanayak, Ankit Jain, K. S. Narayan, Subi J. George
  Pages: n/a - n/a
  Abstract: Oligo(p‐phenylenevinylenes) (OPVs) with amphiphilic character are synthesized and their self‐assembly characteristics studied. Careful studies point at two morphologically different states of assemblies, with one being two dimensional sheets and the other as rolled tubes. This is also the first time that self‐assembled sheets are achieved for OPVs. Morphological and photo‐physical studies reveal a unique aggregate to aggregate transition between rolled tubes and two dimensional sheets, which is outlined as a more thermodynamic aggregate. The thermodynamic aggregate (2D sheet) is better ordered and consists of chromophores that are better excitonically coupled. The mobilities of these aggregates are also studied for a field effect transistor device and as expected sheets supersede rolled tubes by a couple of orders. More interestingly, the mobility values obtained for the well ordered chromophores in sheets is three orders higher than any other self‐assembled OPV previously reported. It is hypothesized that the better π interactions enforced by the amphiphilic design and the resultant supramolecular organization is a prime factor for such a remarkable rise in mobilities. An amphiphilic design and solution‐state self‐assembly of π‐conjugated systems results in free‐standing, nanostructured sheets with green fluorescence. Highly ordered, lamellar molecular organization in the sheets leads to good mobility when transferred on to a field‐effect transistor device.
  PubDate: 2013-01-17T05:23:10.986337-05:
  DOI: 10.1002/adfm.201202298
An Optimized and General Synthetic Strategy for Fabrication of Polymeric Carbon Nitride Nanoarchitectures
- Authors: Jinshui Zhang; Fangsong Guo, Xinchen Wang
  Pages: n/a - n/a
  Abstract: Nanostructured covalent carbon nitride (CN) holds great promise for artificial photosynthesis, but its nanotexturation using templating methods is restricted by the weak binding affinities of neutral silica templates towards basic precursors that are kinetically difficult to diffuse into the nanopores of the templates. This weak affinity leads to an incomplete inclusion of the CN precursors into the nanostructured silica templates, and consequently, yields a defective replica of the parent porous structures. Here, this issue is addressed through the development of an innovative synthetic strategy to facilitate the sufficient inclusion of CN precursors in silica templates, by taking advantage of the surface acidification of silica and sonication‐promoted insertion. The ordered mesoporous CN (ompg‐CN) fabricated using SBA‐15 mesozeolite as the template has been demonstrated to show a better 2D mesoporous hexagonal framework, larger surface area, and higher photocatalytic activity than that synthesized by the traditional method. This innovative strategy can in general be expanded to other silica templates with various nanostructures, enabling the creation of stable polymeric CN nanostructures with maximized material and structure functions. Based on a combination of surface acidification and sonication‐promoted insertion, an optimized and general synthetic strategy is established for the templated construction of polymeric carbon nitride nanoarchitectures with maximized material and structure functions.
  PubDate: 2013-01-16T05:10:27.264546-05:
  DOI: 10.1002/adfm.201203287
Conversion of Light to Electricity by Photoinduced Reversible pH Changes and Biomimetic Nanofluidic Channels
- Authors: Liping Wen; Ye Tian, Yongli Guo, Jie Ma, Weida Liu, Lei Jiang
  Pages: n/a - n/a
  Abstract: Inspired by living systems that have the inherent skill to convert solar energy into bioelectric signals with their light‐driven cross‐membrane proton pump, a photoelectric conversion system that can work in alkaline conditions based on photoinduced reversible pH changes by malachite green carbinol base and a smart gating hydroxide ion‐driven nanofluidic channel is demonstrated. In this system, solar energy can be considered as the only source of cross‐membrane proton motive force that induces diffusion potential and photocurrent flowing through the external circuit. The conversion performances are 0.00825% and 36%, which are calculated from the photoelectric conversion and Gibbs free energy diffusion, respectively. The results suggest that electric power generation and performance could be further optimized by selecting appropriate photosensitized molecules and enhancing the surface‐charge density as well as adopting the appropriate channel size. This facile, cost‐efficient, and environmentally friendly photoelectric conversion system has potential applications for future energy demands such as production of power for in vivo medical devices. Learning from nature has inspired the invention of intelligent materials and devices. Inspiration from the retina, which can serve as a light‐driven cross‐membrane proton pump, a photoelectric conversion system based on smart gating hydroxide ion‐driven nanochannels and photoinduced reversible pH changes is demonstrated. This photoelectric conversion system closely mimics the mechanism of the retina and shows potential applications for future energy demands.
  PubDate: 2013-01-16T05:10:22.655382-05:
  DOI: 10.1002/adfm.201203259
Cyclobutadiene–C60 Adducts: N‐Type Materials for Organic Photovoltaic Cells with High VOC
- Authors: Ggoch Ddeul Han; William R. Collins, Trisha L. Andrew, Vladimir Bulović, Timothy M. Swager
  Pages: n/a - n/a
  Abstract: New tetraalkylcyclobutadiene–C60 adducts are developed via Diels–Alder cycloaddition of C60 with in situ generated cyclobutadienes. The cofacial π‐orbital interactions between the fullerene orbitals and the cyclobutene are shown to decrease the electron affinity and thereby increase the lowest unoccupied molecular orbital (LUMO) energy level of C60 significantly (ca. 100 and 300 meV for mono‐ and bisadducts, respectively). These variations in LUMO levels of fullerene can be used to generate higher open‐circuit voltages (VOC) in bulk heterojunction polymer solar cells. The tetramethylcyclobutadiene–C60 monoadduct displays an open‐circuit voltage (0.61 V) and a power conversion efficiency (2.49%) comparable to the widely used P3HT/PCBM (poly(3‐hexylthiophene/([6,6]‐phenyl‐C61‐butyric acid methyl ester) composite (0.58 V and 2.57%, respectively). The role of the cofacial π‐orbital interactions between C60 and the attached cyclobutene group was probed chemically by epoxidation of the cyclobutene moiety and theoretically through density functional theory calculations. The electrochemical, photophysical, and thermal properties of the newly synthesized fullerene derivatives support the proposed effect of functionalization on electron affinities and photovoltaic performance. Cofacial π‐orbital interactions between the fullerene and the cyclobutene addend are shown to decrease the electron affinity and thereby increase the lowest unoccupied molecular orbital (LUMO) energy level of C60 significantly. The increased LUMO level of fullerene can be used to generate higher open‐circuit voltages and a comparable power conversion efficiency relative to the widely used P3HT/PCBM composite.
  PubDate: 2013-01-16T04:40:40.072354-05:
  DOI: 10.1002/adfm.201203251
Designed Autonomic Motion in Heterogeneous Belousov–Zhabotinsky (BZ)‐Gelatin Composites by Synchronicity
- Authors: Matthew L. Smith; Connor Slone, Kevin Heitfeld, Richard A. Vaia
  Pages: n/a - n/a
  Abstract: Critical technologies from medicine to defense are highly dependent on advanced composite materials. Increasingly there is a greater demand for materials with expanded functionality. The state of the art includes a wide range of responsive composites capable of impressive structural feats such as externally triggered shape morphing. Here a different composite concept is presented, one in which a portion of the constituent materials feed off of ambient energy and dynamically couple to convert it to mechanical motion in a cooperative, biomimetic fashion. Using a recently developed self‐oscillating gel based on gelatin and the oscillating Belousov–Zhabotinsky (BZ) reaction, a technique is demonstrated for producing continuous patterned heterogeneous BZ hydrogel composites capable of sustained autonomic function. The coupling between two adjacent reactive patches is demonstrated in an autonomic cantilever actuator which converts chemical energy into amplified mechanical motion. The design of heterogeneous BZ gels for motion using a basic finite element model is discussed. This work represents notable progress toward developing internally responsive, bio‐inspired composite materials for constructing modular autonomic morphing structures and devices. A straightforward technique for fabricating autonomic, hetero‐structured hydrogels is presented. These gels represent a novel composite concept involving internally responsive and autonomous constituent materials. Critical design parameters are established and a basic modeling approach and material functionality are demonstrated through a coupled patch actuator.
  PubDate: 2013-01-16T04:40:25.269819-05:
  DOI: 10.1002/adfm.201202769
Phase Transformations and Structural Developments in the Radular Teeth of Cryptochiton Stelleri
- Authors: Qianqian Wang; Michiko Nemoto, Dongsheng Li, James C. Weaver, Brian Weden, John Stegemeier, Krassimir N. Bozhilov, Leslie R. Wood, Garrett W. Milliron, Christopher S. Kim, Elaine DiMasi, David Kisailus
  Pages: n/a - n/a
  Abstract: During mineralization, the hard outer magnetite‐containing shell of the radular teeth of Cryptochiton stelleri undergoes four distinct stages of structural and phase transformations: (i) the formation of a crystalline α‐chitin organic matrix that forms the structural framework of the non‐mineralized teeth, (ii) the templated synthesis of ferrihydrite crystal aggregates along these organic fibers, (iii) subsequent solid state phase transformation from ferrihydrite to magnetite, and (iv) progressive magnetite crystal growth to form continuous parallel rods within the mature teeth. The underlying α‐chitin organic matrix appears to influence magnetite crystal aggregate density and the diameter and curvature of the resulting rods, both of which likely play critical roles in determining the local mechanical properties of the mature radular teeth. Dynamic mineralization in the radular teeth of Cryptochiton stelleri, investigated via microscopic and spectroscopic methods, initiates via the templated synthesis of ferrihydrite crystal aggregates along an α‐chitin matrix. These aggregates subsequently transform to magnetite via a solid‐state phase transformation, followed by magnetite crystal growth, which yields highly oriented nanorods that exhibit regionally defined geometries affecting the mechanical properties of the mature teeth.
  PubDate: 2013-01-16T04:40:22.410985-05:
  DOI: 10.1002/adfm.201202894
A Functional DNase I Coating to Prevent Adhesion of Bacteria and the Formation of Biofilm
- Authors: Jan J. T. M. Swartjes; Theerthankar Das, Shahriar Sharifi, Guruprakash Subbiahdoss, Prashant K. Sharma, Bastiaan P. Krom, Henk J. Busscher, Henny C. van der Mei
  Pages: n/a - n/a
  Abstract: Biofilms are detrimental in many industrial and biomedical applications and prevention of biofilm formation has been a prime challenge for decades. Biofilms consist of communities of adhering bacteria, supported and protected by extracellular‐polymeric‐substances (EPS), the so‐called “house of biofilm organisms”. EPS consists of water, proteins, polysaccharides and extracellular‐DNA (eDNA). eDNA, being the longest molecule in EPS, connects the different EPS components and therewith holds an adhering biofilm together. eDNA is associated with bacterial cell surfaces by specific and non‐specific mechanisms, mediating binding of other biopolymers in EPS. eDNA therewith assists in facilitating adhesion, aggregation and maintenance of biofilm structure. Here, a new method is described to prevent biofilm formation on surfaces by applying a DNase I enzyme coating to polymethylmethacrylate, using dopamine as an intermediate. The intermediate coupling layer and final DNase I coating are characterized by water‐contact‐angle measurements and X‐ray photoelectron‐spectroscopy. The DNase I coating strongly reduces adhesion of Staphylococcus aureus (95%) and Pseudomonas aeruginosa (99%) and prevents biofilm formation up to 14 h, without affecting mammalian cell adhesion and proliferation. Also agarose‐gel‐electrophoresis indicates loss of enzyme activity between 8 and 24 h. This duration however, is similar to many local antibiotic‐delivery devices, which makes it an ideal coating for biomaterial implants and devices, known to fail due to biofilm formation with disastrous consequences for patients and high costs to the healthcare system. With threatening increases in antibiotic resistance, the DNase I coating may provide a timely, potent new approach to biofilm prevention on biomaterial implants and devices. A DNase I coating using dopamine as an intermediate layer, strongly reduces bacterial adhesion and prevents biofilm formation by degradation of eDNA in the biofilm matrix causing disruption of the matrix that holds the biofilm together, without affecting mammalian cell adhesion and proliferation.
  PubDate: 2013-01-16T04:40:16.702867-05:
  DOI: 10.1002/adfm.201202927
Solution‐Processed Nickel Oxide Hole Transport Layers in High Efficiency Polymer Photovoltaic Cells
- Authors: Jesse R. Manders; Sai‐Wing Tsang, Michael J. Hartel, Tzung‐Han Lai, Song Chen, Chad M. Amb, John R. Reynolds, Franky So
  Pages: n/a - n/a
  Abstract: The detailed characterization of solution‐derived nickel (II) oxide (NiO) hole‐transporting layer (HTL) films and their application in high efficiency organic photovoltaic (OPV) cells is reported. The NiO precursor solution is examined in situ to determine the chemical species present. Coordination complexes of monoethanolamine (MEA) with Ni in ethanol thermally decompose to form non‐stoichiometric NiO. Specifically, the [Ni(MEA)2(OAc)]+ ion is found to be the most prevalent species in the precursor solution. The defect‐induced Ni3+ ion, which is present in non‐stoichiometric NiO and signifies the p‐type conduction of NiO, as well as the dipolar nickel oxyhydroxide (NiOOH) species are confirmed using X‐ray photoelectron spectroscopy. Bulk heterojunction (BHJ) solar cells with a polymer/fullerene photoactive layer blend composed of poly‐dithienogermole‐thienopyrrolodione (pDTG‐TPD) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM) are fabricated using these solution‐processed NiO films. The resulting devices show an average power conversion efficiency (PCE) of 7.8%, which is a 15% improvement over devices utilizing a poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL. The enhancement is due to the optical resonance in the solar cell and the hydrophobicity of NiO, which promotes a more homogeneous donor/acceptor morphology in the active layer at the NiO/BHJ interface. Finally, devices incorporating NiO as a HTL are more stable in air than devices using PEDOT:PSS. By replacing poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole‐transporting layers (HTLs) with solution‐processed nickel oxide (NiO), polymer photovoltaic cells with a power conversion efficiency of 7.8% are fabricated. Solar cells with NiO are more efficient and more air stable than those with PEDOT:PSS. The HTL/active layer interface plays a critical role in solar cell performance.
  PubDate: 2013-01-16T04:30:37.999186-05:
  DOI: 10.1002/adfm.201202269
Functional Polymer Brushes on Diamond as a Platform for Immobilization and Electrical Wiring of Biomolecules
- Authors: Andreas A. Reitinger; Naima A. Hutter, Andreas Donner, Marin Steenackers, Oliver A. Williams, Martin Stutzmann, Rainer Jordan, Jose A. Garrido
  Pages: n/a - n/a
  Abstract: For the biofunctionalization of electronic devices, polymer brushes can provide a route which allows combining the advantages of other commonly used approaches, such as immobilization of functional biomolecules via self assembled monolayers or coated polymer matrices: high stability and loading capacity, efficient electron transport, and excellent biocompatibility. In the work presented here, poly(methacrylic acid) brushes are prepared by self‐initiated photografting and photopolymerization on diamond electrodes. In this straightforward process no prior grafting of initiators is required since the initiation of the polymerization can be conveniently controlled by the hydrogen or oxygen termination of the diamond surface. Boron doped nanocrystalline diamond as an electrode material provides extreme chemical inertness and stability, inherent biocompatibility, and superior electrochemical properties, such as the large accessible potential window and low background currents. As a proof of concept we demonstrate the amperometric detection of glucose by polymer brushes covalently modified with the redox enzyme glucose oxidase and aminomethyl ferrocene as electron mediator. Characterization by X‐ray photoelectron spectroscopy and atomic force microscopy both indicate a high loading of the ferrocene mediator. Consistently, electrochemical cyclic voltammetry shows a multilayer equivalent loading of ferrocene and highly efficient electron transfer throughout the polymer film. Overall, functionalized polymer brushes can provide a promising platform for the immobilization and electrical wiring of biomolecules for bioelectronic and biosensing applications. Polymer brushes created by self‐initiated photografting and photopolymerization provide a straightforward route for the biofunctionalization of bioelectronic devices, where a high loading and stable immobilization of biomolecules as well as a biocompatible environment and efficient charge transfer are essential. In this report the potential of this approach is demonstrated by the example of amperometric glucose sensing with glucose oxidase‐ and ferrocene‐functionalized poly(methacrylic acid) brushes on nanocrystalline diamond electrodes.
  PubDate: 2013-01-16T04:30:33.237227-05:
  DOI: 10.1002/adfm.201202342
Solid‐Phase Synthesis of Molecularly Imprinted Polymer Nanoparticles with a Reusable Template–“Plastic Antibodies”
- Authors: Alessandro Poma; Antonio Guerreiro, Michael J. Whitcombe, Elena V. Piletska, Anthony P. F. Turner, Sergey A. Piletsky
  Pages: n/a - n/a
  Abstract: Molecularly imprinted polymers (MIPs) are generic alternatives to antibodies in sensors, diagnostics, and separations. To displace biomolecules without radical changes in infrastructure in device manufacture, MIPs should share their characteristics (solubility, size, specificity and affinity, localized binding domain) whilst maintaining the advantages of MIPs (low‐cost, short development time, and high stability) hence the interest in MIP nanoparticles. Herein, a reusable solid‐phase template approach is reported (fully compatible with automation) for the synthesis of MIP nanoparticles and their precise manufacture using a prototype automated UV photochemical reactor. Batches of nanoparticles (30–400 nm) with narrow size distributions imprinted with: melamine (d = 60 nm, Kd = 6.3 × 10−8 M), vancomycin (d = 250 nm, Kd = 3.4 × 10−9 M), a peptide (d = 350 nm, Kd = 4.8 × 10−8 M) and proteins have been produced. The instrument uses a column packed with glass beads, bearing the template. Process parameters are under computer control, requiring minimal manual intervention. For the first time, the reliable re‐use of molecular templates is demonstrated in the synthesis of MIPs (≥30 batches of nanoMIPs without loss of performance). NanoMIPs are produced template‐free and the solid‐phase acts both as template and affinity separation medium. The first example of the solid‐phase synthesis of MIP nanoparticles is reported using an immobilized template in an automated reactor. Formation of nanoMIP by photopolymerization in a column packed with the template phase is followed by washing to remove low affinity particles. High temperature elution releases the high affinity nanoMIPs and the template phase, which can be re‐used.
  PubDate: 2013-01-16T04:30:30.701017-05:
  DOI: 10.1002/adfm.201202397
Foam‐Like Behavior in Compliant, Continuously Reinforced Nanocomposites
- Authors: Brent J. Carey; Prabir K. Patra, Myung Gwan Hahm, Pulickel M. Ajayan
  Pages: n/a - n/a
  Abstract: In the pursuit of advanced polymer composites, nanoscale fillers have long been championed as promising candidates for structural reinforcement. Despite progress, questions remain as to how these diminutive fillers influence the distribution of stresses within the matrix and, in turn, influence bulk mechanical properties. The dynamic mechanical behavior of elastomer‐impregnated forests of carbon nanotubes (CNTs) has revealed distinct orientation‐dependent behavior that sheds light on these complicated interactions. When compressed along the axis of the fillers, the composite will mimic open‐cell foams and exhibit strain softening for increasing amplitudes due to the collective Euler buckling of the slender nanotubes. In contrast, the same material will behave similarly to the neat polymer when compressed orthogonal to the alignment direction of the nanotubes. However, in this orientation the material is incapable of achieving the same ultimate compressive strain due to the role that the embedded nanotubes play in augmenting the effective cross‐link density of the polymer network. Both of these responses are recoverable, robust, and show little dependency on the diameter and wall‐number of the included CNTs. Such observations give insight into the mechanics of polymer/nanoparticle interactions in nanocomposite structures under strain, and the thoughtful control of such coordinated buckling behavior opens the possibility for the development of foam‐like materials with large Poisson ratios. The dynamic mechanical compression of continuously reinforced carbon nanotube (CNT)/poly(dimethylsiloxane) composites provides insight into the mechanics of deformation in nanocomposite systems. Loaded along the nanotube axis, these composites respond similar to open‐cell forms due to a collective buckling of the CNTs. Compressed normal to nanotube alignment, the CNTs will effectively augment the cross‐link density of the elastomer network, lowering the ultimate compressive strain.
  PubDate: 2013-01-16T04:30:04.824595-05:
  DOI: 10.1002/adfm.201201999
Evaluating the Critical Thickness of TiO2 Layer on Insulating Mesoporous Templates for Efficient Current Collection in Dye‐Sensitized Solar Cells
- Authors: Aravind Kumar Chandiran; Pascal Comte, Robin Humphry‐Baker, Florian Kessler, Chenyi Yi, Md. Khaja Nazeeruddin, Michael Grätzel
  Pages: n/a - n/a
  Abstract: In this paper, a way of utilizing thin and conformal overlayer of titanium dioxide on an insulating mesoporous template as a photoanode for dye‐sensitized solar cells is presented. Different thicknesses of TiO2 ranging from 1 to 15 nm are deposited on the surface of the template by atomic layer deposition. This systematic study helps unraveling the minimum critical thickness of the TiO2 overlayer required to transport the photogenerated electrons efficiently. A merely 6‐nm‐thick TiO2 film on a 3‐μm mesoporous insulating substrate is shown to transport 8 mA/cm2 of photocurrent density along with ≈900 mV of open‐circuit potential when using our standard donor‐π‐acceptor sensitizer and Co(bipyridine) redox mediator. Different thicknesses of TiO2 are deposited by atomic layer deposition on SiO2 mesoporous templates and a critical thickness for the efficient collection of the photogenerated charge carriers in dye sensitized solar cells is identified. This work presents an alternative photoanode with one order of magnitude higher transport rate compared to the conventional nanoparticle titanium dioxide mesoporous films.
  PubDate: 2013-01-15T03:10:27.039462-05:
  DOI: 10.1002/adfm.201202956
A Printable Optical Time‐Temperature Integrator Based on Shape Memory in a Chiral Nematic Polymer Network
- Authors: Dylan J. D. Davies; Antonio R. Vaccaro, Stephen M. Morris, Nicole Herzer, Albertus P. H. J. Schenning, Cees W. M. Bastiaansen
  Pages: n/a - n/a
  Abstract: An optical and irreversible temperature sensor (e.g., a time‐temperature integrator) is reported based on a mechanically embossed chiral‐nematic polymer network. The polymer consists of a chemical and a physical (hydrogen‐bonded) network and has a reflection band in the visible wavelength range. The sensors are produced by mechanical embossing at elevated temperatures. A relative large compressive deformation (up to 10%) is obtained inducing a shift to shorter wavelength of the reflection band (>30 nm). After embossing, a temperature sensor is obtained that exhibits an irreversible optical response. A permanent color shift to longer wavelengths (red) is observed upon heating of the polymer material to temperatures above the glass transition temperature. It is illustrated that the observed permanent color shift is related to shape memory in the polymer material. The films can be printed on a foil, thus showing that these sensors are potentially interesting as time‐temperature integrators for applications in food and pharmaceutical products. An optical, irreversible temperature sensor is fabricated based on a chiral‐nematic polymer network that has an orange reflection color. After mechanical embossing of the polymer film a temporary shift to a green color is achieved, which shifts back to the original color above a certain threshold temperature. This sensor can be printed on a foil, thus displaying its potential application as a battery‐free time‐temperature integrator in food and pharmaceutical products.
  PubDate: 2013-01-15T03:10:21.689377-05:
  DOI: 10.1002/adfm.201202774
Topographically Flat Substrates with Embedded Nanoplasmonic Devices for Biosensing
- Authors: Jincy Jose; Luke R. Jordan, Timothy W. Johnson, Si Hoon Lee, Nathan J. Wittenberg, Sang‐Hyun Oh
  Pages: n/a - n/a
  Abstract: The ability to precisely control the topography, roughness, and chemical properties of metallic nanostructures is crucial for applications in plasmonics, nanofluidics, electronics, and biosensing. Here a simple method to produce embedded nanoplasmonic devices that can generate tunable plasmonic fields on ultraflat surfaces is demonstrated. Using a template‐stripping technique, isolated metallic nanodisks and wires are embedded in optical epoxy, which is capped with a thin silica overlayer using atomic layer deposition. The top silica surface is topographically flat and laterally homogeneous, providing a uniform, high‐quality biocompatible substrate, while the nanoplasmonic architecture hidden underneath creates a tunable plasmonic landscape for optical imaging and sensing. The localized surface plasmon resonance of gold nanodisks embedded underneath flat silica films is used for real‐time kinetic sensing of the formation of a supported lipid bilayer and subsequent receptor‐ligand binding. Gold nanodisks can also be embedded in elastomeric materials, which can be peeled off the substrate to create flexible plasmonic membranes that conform to non‐planar surfaces. A topographically flat and laterally homogeneous silica surface with embedded metallic nanostructures is fabricated using a template‐stripping technique to interface biomembranes with tunable plasmonic fields for label‐free optical biosensing and imaging.
  PubDate: 2013-01-15T03:10:12.795295-05:
  DOI: 10.1002/adfm.201202214
Tunable Hierarchical Metallic‐Glass Nanostructures
- Authors: Sundeep Mukherjee; Ryan C. Sekol, Marcelo Carmo, Eric I. Altman, André D. Taylor, Jan Schroers
  Pages: n/a - n/a
  Abstract: Synthesizing metallic nanostructures with control over morphology, surface chemistry, and length‐scale is important for a wide range of applications. Nanostructures having large surface area paired with suitable chemistry are particularly desirable in catalytic applications to facilitate the reaction kinetics. However, the techniques used for nanostructure synthesis are often lengthy, difficult, require expensive precursors/stabilizers, and limit the control over nanostructure morphology/chemistry. Here tuning metallic‐glass nanostructures to a wide range of morphologies, where the surface is enriched with catalytic noble metal, is reported. By combining thermoplastic nanofabrication together with electrochemical processing, hierarchical metallic nanostructures with large electrochemical surface area and high catalytic activity are synthesized. Due to the versatility in processing and independent control over multiple length‐scales, the approach may serve as a tool‐box for fabricating complex hierarchical nanostructures for wide ranging applications. Metallic‐glasses are promising for electrocatalytic applications due to their favorable chemistry and unique abilities for thermoplastic manipulation down to the nanometer length‐scale. Combination of electrochemical processing and thermoplastic nanofabrication of metallic glasses serves as a versatile toolbox for fabricating hierarchical metallic nanostructures with large surface area and tunable morphology.
  PubDate: 2013-01-14T02:30:21.560985-05:
  DOI: 10.1002/adfm.201202887
Large Electrocaloric Effect in a Dielectric Liquid Possessing a Large Dielectric Anisotropy Near the Isotropic–Nematic Transition
- Authors: Xiao‐Shi Qian; Sheng‐Guo Lu, Xinyu Li, Haiming Gu, Liang‐Chy Chien, Qiming Zhang
  Pages: n/a - n/a
  Abstract: The recent findings of large electrocaloric effects (ECEs) in ferroelectric polymers and in ferroelectric ceramic thin films have attracted great interest for developing new cooling cycles that are environmental friendly and have the potential to reach better efficiency than the existing vapor‐compression approach. Compared with these solid state ECE materials, a dielectric fluid with a large ECE can be more interesting because it may lead to new cooling cycles with simpler structures than these based on solid state ECE materials. Here it is shown that a large ECE can be realized in the liquid crystal (LC) 5CB near its nematic–isotropic (N‐I) phase transition. 5CB has a large dielectric anisotropy, which facilitates the electric‐field‐induced large polarization change. As a result, a large ECE, i.e., an isothermal entropy change of more than 23.6 J kg−1 K−1 is observed just above the N‐I transition. The electrocaloric effect (ECE) is investigated in a dielectric liquid. By exploiting the large dielectric anisotropy in a liquid crystal, 5CB, that facilitates the electric‐field‐induced large polarization change, a large ECE is observed near 39 °C, the nematic‐isotropic transition temperature region. Fluidic ECE materials could lead to easier and better design of ECE‐based cooling devices.
  PubDate: 2013-01-14T02:30:21.440451-05:
  DOI: 10.1002/adfm.201202686
Bioinspired Multifunctional Foam with Self‐Cleaning and Oil/Water Separation
- Authors: Xiyao Zhang; Zhou Li, Kesong Liu, Lei Jiang
  Pages: n/a - n/a
  Abstract: Oil/water separation is a worldwide challenge. Learning from nature provides a promising approach for the construction of functional materials with oil/water separation. In this contribution, inspired by superhydrophobic self‐cleaning lotus leaves and porous biomaterials, a facile method is proposed to fabricate polyurethane foam with simultaneous superhydrophobicity and superoleophilicity. Due to its low density, light weight, and superhydrophobicity, the as‐prepared foam can float easily on water. Furthermore, the foam demonstrates super‐repellency towards corrosive liquids, self‐cleaning, and oil/water separation properties, possessing multifunction integration. We expect that this low‐cost process can be readily and widely adopted for the design of multifunctional foams for large‐area oil‐spill cleanup. Inspired by superhydrophobic self‐cleaning lotus leaves and porous biomaterials, polyurethane foam with simultaneous superhydrophobicity and superoleophilicity is fabricated. The resulting foam exhibits super‐repellency towards corrosive liquids, self‐cleaning, and oil/water separation properties, thus possessing multifunction integration.
  PubDate: 2013-01-14T02:30:08.457978-05:
  DOI: 10.1002/adfm.201202662
Highly Ordered Helical Nanofilament Assembly Aligned by a Nematic Director Field
- Authors: Fumito Araoka; Go Sugiyama, Ken Ishikawa, Hideo Takezoe
  Pages: n/a - n/a
  Abstract: Successful alignment control of the B4 helical nanofilament assembly in a binary mixture system of bent‐shaped and rod‐shaped liquid crystals is demonstrated. The aligned nanofilament domains appear as extremely smooth and uniform stripes over millimeters. The interferometric second‐harmonic generation microscopy technique is developed and applied to these aligned domains. It is found that not only chirality, but also polarity, are preserved in a single domain formed from a single nucleus. Such an easily processed uniform bulk of the spontaneously symmetry‐broken material is intriguing in functional materials science. Successful alignment control of the B4 helical nanofilament assembly in a binary mixture system of bent‐shaped and rod‐shaped liquid crystals is presented. The aligned nanofilament domains appear as extremely smooth and uniform stripes over millimeters. Second‐harmonic generation (SHG) studies including a new technique, interferometric SHG microscopy, reveal that in these aligned domains, polarity from the nucleus and chirality are preserved.
  PubDate: 2013-01-11T04:40:24.159355-05:
  DOI: 10.1002/adfm.201201889
A Dramatic Odd–Even Oscillating Behavior for the Current Rectification and Negative Differential Resistance in Carbon‐Chain‐Modified Donor–Acceptor Molecular Devices
- Authors: Zhenhua Zhang; Chao Guo, Denise Jeng Kwong, Jie Li, Xiaoqing Deng, Zhiqiang Fan
  Pages: n/a - n/a
  Abstract: The donor–acceptor molecule is the only molecule that features a real intrinsic rectification. However, all investigations in the last decades showed that rectification behaviors of such molecules are not promising since their rectification ratio is only on the order of 10. Use of carbon chains Cn to serve as spacers is reported, along with attempts to modulate electrical behavior of the donor–acceptor molecule. Calculations using the first‐principles method show that electrical behavior is indeed altered substantively, and a particular regularity can be clearly observed, i.e., a dramatic odd–even oscillation for electronic behavior with increasing carbon‐chain length n. For models with even‐n carbon chains, the rectification ratio is small (30), and no negative differential resistance (NDR) behavior is detected, but the rectifying performance of models with odd‐n carbon chains is tremendously improved and rectification ratios on the order of 50 to 400 can be achieved, alongside a large NDR. This study thus suggests that using a suitable spacer might be an effective way to significantly boost electrical characteristics, including rectifying performance, of the donor–acceptor molecule. Tremendous improvement in rectifying performance is predicted by calculations for donor–acceptor molecular devices with odd numbers of carbon atom chains that act as spacers, to predicted values that are much higher than in all previous studies. A dramatic odd–even oscillation effect is also shown for the current rectification and negative differential resistance with increasing carbon‐chain length.
  PubDate: 2013-01-11T04:40:10.018722-05:
  DOI: 10.1002/adfm.201201790
Flexible Organic Photovoltaic Cells with In Situ Nonthermal Photoreduction of Spin‐Coated Graphene Oxide Electrodes
- Authors: Emmanuel Kymakis; Kyriaki Savva, Minas M. Stylianakis, Costas Fotakis, Emmanuel Stratakis
  Pages: n/a - n/a
  Abstract: The first reduction methodology, compatible with flexible, temperature‐sensitive substrates, for the production of reduced spin‐coated graphene oxide (GO) electrodes is reported. It is based on the use of a laser beam for the in situ, non‐thermal, reduction of spin‐coated GO films on flexible substrates over a large area. The photoreduction process is one‐step, facile, and is rapidly carried out at room temperature in air without affecting the integrity of the graphene lattice or the flexibility of the underlying substrate. Conductive graphene films with a sheet resistance of as low as 700 Ω sq−1 and transmittance of 44% can be obtained, much higher than can be achieved for flexible layers reduced by chemical means. As a proof of concept of our technique, laser‐reduced GO (LrGO) films are utilized as transparent electrodes in flexible, bulk heterojunction, organic photovoltaic (OPV) devices, replacing the traditional ITO. The devices displayed a power‐conversion efficiency of 1.1%, which is the highest reported so far for OPV device incorporating reduced GO as the transparent electrode. The in situ non‐thermal photoreduction of spin‐coated GO films creates a new way to produce flexible functional graphene electrodes for a variety of electronic applications in a process that carries substantial promise for industrial implementation. A methodology for controlled in situ reduction of spin‐cast graphene oxide nanometric films on flexible substrates and the subsequent realization of highly conductive and transparent electrodes for flexible organic photovoltaic devices is presented. The technique is compatible with temperature‐sensitive substrates in the sense that it achieves reduction of films on flexible substrates in a single step.
  PubDate: 2013-01-09T03:40:18.328588-05:
  DOI: 10.1002/adfm.201202713
Efficiency Enhancement of Organic Solar Cells by Using Shape‐Dependent Broadband Plasmonic Absorption in Metallic Nanoparticles
- Authors: Xuanhua Li; Wallace Chik Ho Choy, Haifei Lu, Wei E. I. Sha, Aaron Ho Pui Ho
  Pages: n/a - n/a
  Abstract: It is been widely reported that plasmonic effects in metallic nanomaterials can enhance light trapping in organix solar cells (OSCs). However, typical nanoparticles (NP) of high quality (i.e., mono‐dispersive) only possess a single resonant absorption peak, which inevitably limits the power conversion efficiency (PCE) enhancement to a narrow spectral range. Broadband plasmonic absorption is obviously highly desirable. In this paper, a combination of Ag nanomaterials of different shapes, including nanoparticles and nanoprisms, is proposed for this purpose. The nanomaterials are synthesized using a simple wet chemical method. Theoretical and experimental studies show that the origin of the observed PCE enhancement is the simultaneous excitation of many plasmonic low‐ and high‐order resonances modes, which are material‐, shape‐, size‐, and polarization‐dependent. Particularly for the Ag nanoprisms studied here, the high‐order resonances result in higher contribution than low‐order resonances to the absorption enhancement of OSCs through an improved overlap with the active material absorption spectrum. With the incorporation of the mixed nanomaterials into the active layer, a wide‐band absorption improvement is demonstrated and the short‐circuit photocurrent density (Jsc) improves by 17.91%. Finally, PCE is enhanced by 19.44% as compared to pre‐optimized control OSCs. These results suggest a new approach to achieve higher overall enhancement through improving broadband absorption. A broadband absorption of organic solar cells (OSCs) is realized by combining differently shaped nanomaterials exhibiting varied localized plasmonic resonances and scattering regions into the active layer. As a result, power conversion efficiency is enhanced by 19.44% compared to pre‐optimized control OSCs. With respect to the origin of this enhancement, our theoretical and experimental studies show that the broadband resonance is achieved by the simultaneous excitation of versatile plasmonic resonances, i.e., material‐, shape‐, size‐, and polarization‐dependent low‐ and high‐order plasmonic resonances.
  PubDate: 2013-01-06T18:23:10.72112-05:0
  DOI: 10.1002/adfm.201202476
Engineering of Graphene Layer Orientation to Attain High Rate Capability and Anisotropic Properties in Li‐Ion Battery Electrodes
- Authors: Amartya Mukhopadhyay; Fei Guo, Anton Tokranov, Xingcheng Xiao, Robert H. Hurt, Brian W. Sheldon
  Pages: n/a - n/a
  Abstract: Novel carbon films with different graphene layer orientations are investigated as electrode materials for Li‐ion batteries. It is demonstrated that engineering the crystallographic orientation with graphene layers oriented perpendicular to the surface substantially alters stress evolution during Li insertion. With this crystallographic orientation the intercalating/de‐intercalating Li‐ions also have direct access to the graphene interlayer spaces, resulting in higher capacity at faster electrochemical cycling, compared to carbon films with graphene layers parallel to the film surface. Electrodes with perpendicular alignment are prepared by supramolecular synthesis using either spin coating or bar coating of chromonic liquid crystal precursors into precursor organic films followed by in situ carbonization. These materials are compared with in situ stress measurements during lithiation/delithiation cycles, and the bar‐coated films exhibit a highly anisotropic stress which is consistent with long‐range alignment of the graphene layers. In contrast, the in‐plane stresses in the spin‐coated films are isotropic, which is consistent with the presence of randomly oriented domains (still with graphene layers oriented perpendicular to the surface). Overall, the use of thin film graphitic materials with controlled crystallographic orientations provides a valuable platform for investigating the impact of graphene structure on the properties of Li‐ion battery electrode materials. Engineering of graphene layer orientations to obtain vertically aligned graphene layer based thin‐film electrodes (a‐axis oriented graphitic carbon film) for Li‐ion batteries leads to considerably improved specific capacity at higher electrochemical cycling rates (improved rate capability), as compared to the more usual c‐axis oriented graphitic carbon films.
  PubDate: 2013-01-06T18:23:08.350086-05:
  DOI: 10.1002/adfm.201201128
One‐Step Macroscopic Alignment of Conjugated Polymer Systems by Epitaxial Crystallization during Spin‐Coating
- Authors: Christian Müller; Mahdieh Aghamohammadi, Scott Himmelberger, Prashant Sonar, Miquel Garriga, Alberto Salleo, Mariano Campoy‐Quiles
  Pages: n/a - n/a
  Abstract: The one‐step preparation of highly anisotropic polymer semiconductor thin films directly from solution is demonstrated. The conjugated polymer poly(3‐hexylthiophene) (P3HT) as well as P3HT:fullerene bulk–heterojunction blends can be spin‐coated from a mixture of the crystallizable solvent 1,3,5‐trichlorobenzene (TCB) and a second carrier solvent such as chlorobenzene. Solidification is initiated by growth of macroscopic TCB spherulites followed by epitaxial crystallization of P3HT on TCB crystals. Subsequent sublimation of TCB leaves behind a replica of the original TCB spherulites. Thus, highly ordered thin films are obtained, which feature square‐centimeter‐sized domains that are composed of one spherulite‐like structure each. A combination of optical microscopy and polarized photoluminescence spectroscopy reveals radial alignment of the polymer backbone in case of P3HT, whereas P3HT:fullerene blends display a tangential orientation with respect to the center of spherulite‐like structures. Moreover, grazing‐incidence wide‐angle X‐ray scattering reveals an increased relative degree of crystallinity and predominantly flat‐on conformation of P3HT crystallites in the blend. The use of other processing methods such as dip‐coating is also feasible and offers uniaxial orientation of the macromolecule. Finally, the applicability of this method to a variety of other semi‐crystalline conjugated polymer systems is established. Those include other poly(3‐alkylthiophene)s, two polyfluorenes, the low band‐gap polymer PCPDTBT, a diketopyrrolopyrrole (DPP) small molecule as well as a number of polymer:fullerene and polymer:polymer blends. Macroscopic spherulite‐like structures of the conjugated polymer poly(3‐hexylthiophene) (P3HT) grow directly during spin‐coating. This is achieved by processing P3HT or P3HT:fullerene bulk–heterojunction blends from a mixture of the crystallizable solvent 1,3,5‐trichlorobenzene and a second carrier solvent such as chlorobenzene. Epitaxial growth of the polymer on solidified solvent crystals gives rise to circular‐symmetric, spherulite‐like structures that feature a high degree of anisotropy.
  PubDate: 2013-01-03T06:20:16.509273-05:
  DOI: 10.1002/adfm.201202983
Energy Harvesting Materials: Super‐Flexible Nanogenerator for Energy Harvesting from Gentle Wind and as an Active Deformation Sensor (Adv. Funct. Mater. 19/2013)
- Authors: Sangmin Lee; Sung‐Hwan Bae, Long Lin, Ya Yang, Chan Park, Sang‐Woo Kim, Seung Nam Cha, Hyunjin Kim, Young Jun Park, Zhong Lin Wang
  Pages: 2341 - 2341
  Abstract: To construct super‐flexible and conformable nanogenerators, ZnO nanowires are grown on 18‐μm‐thick Al foil, which is used as both the electrode and the substrate. Coating the Al foil with poly(methyl methacrylate) before the growth of ZnO nanowires results in a high‐throughput process due to complete insulation between the as‐grown ZnO nanowires and the Al foil. As reported by Zhong Lin Wang and co‐workers on page 2445, this super‐flexible nanogenerator shows potential as an energy harvester from a waving flag and as a self‐powered sensor for detecting skin movement.
  PubDate: 2013-05-13T15:12:57.678752-05:
  DOI: 10.1002/adfm.201370091
Sensors: Thin‐Wall Assembled SnO2 Fibers Functionalized by Catalytic Pt Nanoparticles and their Superior Exhaled‐Breath‐Sensing Properties for the Diagnosis of Diabetes (Adv. Funct. Mater. 19/2013)
- Authors: Jungwoo Shin; Seon‐Jin Choi, Inkun Lee, Doo‐Young Youn, Chong Ook Park, Jong‐Heun Lee, Harry L. Tuller, Il‐Doo Kim
  Pages: 2342 - 2342
  Abstract: Thin‐wall assembled SnO2 tubes with a number elongated pores are synthesized via electrospinning, controlled by the variation of flow rates. On page 2357, Il‐Doo Kim and co‐workers report that these highly porous SnO2 tubes show a five‐fold higher acetone response compared with dense SnO2 fiber in a humid atmosphere that is similar to the oral cavity. The accurate detection of acetone gas in exhaled breath can provide useful information for real‐time diagnosis of diabetes.
  PubDate: 2013-05-13T15:12:57.678752-05:
  DOI: 10.1002/adfm.201370092
Contents: (Adv. Funct. Mater. 19/2013)
- Pages: 2343 - 2347
  PubDate: 2013-05-13T15:12:57.678752-05:
  DOI: 10.1002/adfm.201370094
Nanotube Arrays: In Situ Generated Gas Bubble‐Directed Self‐Assembly: Synthesis, and Peculiar Magnetic and Electrochemical Properties of Vertically Aligned Arrays of High‐Density Co3O4 Nanotubes (Adv. Funct. Mater. 19/2013)
- Authors: Guoxiu Tong; Jianguo Guan, Qingjie Zhang
  Pages: 2405 - 2405
  Abstract: The simple, versatile, and large‐scale fabrication of vertically aligned nanotube arrays (NTAs) is achieved by regulating the gas bubbles in situ generated from the thermal decomposition of melted salts. On page 2406, Jianguo Guan and co‐workers report that as‐obtained Co3O4 NTAs show unique magnetic properties and significantly enhanced electrochemical activity, and are promising for magnetic recording media, magnetic shielding, lithium‐ion batteries, and chemical sensing.
  PubDate: 2013-05-13T15:12:57.678752-05:
  DOI: 10.1002/adfm.201370095
Dendron‐Stabilized Liquid Crystalline Blue Phases with an Enlarged Controllable Range of the Photonic Band for Tunable Photonic Devices
- Authors: Seishi Shibayama; Hiroki Higuchi, Yasushi Okumura, Hirotsugu Kikuchi
  Pages: n/a - n/a
  Abstract: Liquid crystalline blue phases (BPs) show excellent potential for application in tunable photonic devices because they possess the unique optical property that the selective 3D Bragg diffraction in a visible wavelength region can be continuously shifted using an electric field. A new approach to simultaneously extend the wavelength range of field‐induced Bragg diffraction shift and the temperature range of thermodynamically stable BPs is critically needed. Here, a new BP material system is shown using a dendron molecule to extend simultaneously the two BP ranges. One is the temperature range of thermodynamically stable BPs, which is expanded from 2.1 to 4.6 °C. The other is the reversible maximum shift range of Bragg wavelength on the electric field, which is extended from 85 to 109 nm. The physical mechanism of the dendron‐stabilizing effect in BPs is discussed in terms of elastic property and orientational order of liquid crystal molecules. Liquid crystalline blue phases have great potential for advanced applications in tunable photonic devices. A new material system is developed with a dendron molecule, simultaneously enabling both the enlargement of the temperature range of thermodynamically stable blue phases and the continuously reversible maximum shift range of Bragg reflection in a visible wavelength region under an electric field.
  PubDate: 2012-12-27T04:50:33.487888-05:
  DOI: 10.1002/adfm.201202497
Core/Shell PbSe/PbS QDs TiO2 Heterojunction Solar Cell
- Authors: Lioz Etgar; Diana Yanover, Richard Karel Čapek, Roman Vaxenburg, Zhaosheng Xue, Bin Liu, Mohammad Khaja Nazeeruddin, Efrat Lifshitz, Michael Grätzel
  Pages: n/a - n/a
  Abstract: Quasi type‐II PbSe/PbS quantum dots (QDs) are employed in a solid state high efficiency QD/TiO2 heterojunction solar cell. The QDs are deposited using layer‐by‐layer deposition on a half‐micrometer‐thick anatase TiO2 nanosheet film with (001) exposed facets. Theoretical calculations show that the carriers in PbSe/PbS quasi type‐II QDs are delocalized over the entire core/shell structure, which results in better QD film conductivity compared to PbSe QDs. Moreover, PbS shell permits better stability and facile electron injection from the QDs to the TiO2 nanosheets. To complete the electrical circuit of the solar cell, a Au film is evaporated as a back contact on top of the QDs. This PbSe/PbS QD/TiO2 heterojunction solar cell produces a light to electric power conversion efficiency (η) of 4% with short circuit photocurrent (Jsc) of 17.3 mA/cm2. This report demonstrates highly efficient core/shell near infrared QDs in a QD/TiO2 heterojunction solar cell. PbSe core and PbSe/PbS core/shell quantum dots (QDs) are employed in a QD/TiO2 heterojunction solar cell. Theoretical calculations show that the wave functions of electrons and holes are extended to both materials, which contribute to the better photovoltaic performance of the PbSe/PbS core/shell than the PbSe core. The PbSe/PbS core/shell heterojunction solar cell produces a power conversion efficiency of 3.93%.
  PubDate: 2012-12-27T04:50:27.886896-05:
  DOI: 10.1002/adfm.201202322
Single‐Crystalline p‐Type Zn3As2 Nanowires for Field‐Effect Transistors and Visible‐Light Photodetectors on Rigid and Flexible Substrates
- Authors: Gui Chen; Zhe Liu, Bo Liang, Gang Yu, Zhong Xie, Hongtao Huang, Bin Liu, Xianfu Wang, Di Chen, Ming‐Qiang Zhu, Guozhen Shen
  Pages: n/a - n/a
  Abstract: Zn3As2 is an important p‐type semiconductor with the merit of high effective mobility. The synthesis of single‐crystalline Zn3As2 nanowires (NWs) via a simple chemical vapor deposition method is reported. High‐performance single Zn3As2 NW field‐effect transistors (FETs) on rigid SiO2/Si substrates and visible‐light photodetectors on rigid and flexible substrates are fabricated and studied. As‐fabricated single‐NW FETs exhibit typical p‐type transistor characteristics with the features of high mobility (305.5 cm2 V−1 s−1) and a high Ion/Ioff ratio (105). Single‐NW photodetectors on SiO2/Si substrate show good sensitivity to visible light. Using the contact printing process, large‐scale ordered Zn3As2 NW arrays are successfully assembled on SiO2/Si substrate to prepare NW thin‐film transistors and photodetectors. The NW‐array photodetectors on rigid SiO2/Si substrate and flexible PET substrate exhibit enhanced optoelectronic performance compared with the single‐NW devices. The results reveal that the p‐type Zn3As2 NWs have important applications in future electronic and optoelectronic devices. Single‐crystalline p‐type Zn3As2 nanowires are successfully synthesized and used as building blocks for high‐performance field‐effect transistors and photodetectors. By using a contact printing process, highly aligned nanowire arrays are produced to fabricate thin‐film transistors and photodetectors on both rigid and flexible substrates.
  PubDate: 2012-12-27T04:50:21.28222-05:0
  DOI: 10.1002/adfm.201202739
Synthesis of Hierarchically Porous Hydrogen Silsesquioxane Monoliths and Embedding of Metal Nanoparticles by On‐Site Reduction
- Authors: Nirmalya Moitra; Kazuyoshi Kanamori, Toyoshi Shimada, Kazuyuki Takeda, Yumi H. Ikuhara, Xiang Gao, Kazuki Nakanishi
  Pages: n/a - n/a
  Abstract: A facile synthesis of a new class of reactive porous materials is reported: hierarchically porous hydrogen silsesquioxane (HSiO1.5, HSQ) monoliths with well‐defined macropores and mesopores. The HSQ monoliths are prepared via sol‐gel accompanied by phase separation in a mild condition, and contain micrometer‐sized co‐continuous macropores and high specific surface area reaching up to 800 m2 g−1 because of the small mesopores. A total preservation of Si–H, which is always an issue of HSQ materials, is confirmed by 29Si solid‐state NMR. The HSQ monolith has then been subjected to reduction of noble metal ions to their corresponding metal nanoparticles in simple aqueous solutions under an ambient condition. The nanoparticles produced in this manner are immobilized on the HSQ monolith and are characterized by X‐ray diffraction (XRD) and high angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM). Both the bare HSQ and nanoparticles‐embedded HSQ are promising as heterogeneous catalysts, exhibiting reusability and recyclability. Surface‐active hierarchically porous monoliths consisting of hydrogen silsesquioxane (HSiO1.5) are prepared via sol‐gel accompanied by phase separation. The on‐site reduction of aqueous noble metal salts results in the formation and embedding of metal nanoparticles throughout the monoliths. Both the bare monolith and nanoparticle‐embedded monolith are promising as reusable and recyclable heterogeneous catalyst.
  PubDate: 2012-12-27T04:50:18.618664-05:
  DOI: 10.1002/adfm.201202558
Components Simulation of Viral Envelope via Amino Acid Modified Chitosans for Efficient Nucleic Acid Delivery: In Vitro and In Vivo Study
- Authors: Jing Chang; Xianghui Xu, Haiping Li, Yeting Jian, Gang Wang, Bin He, Zhongwei Gu
  Pages: n/a - n/a
  Abstract: Novel nonviral gene vectors of alkaline amino acids such as arginine‐ (Arg), histidine‐ (His), and lysine‐ (Lys) modified chitosans (AAA‐CSs) are developed to simulate the components of viral envelopes to enhance transfection efficiency. The structures of the modified chitosans are characterized using 1H NMR spectroscopy. Acid‐base titration results indicate that the modified chitosans exhibit strong buffering capacity. The morphology of the AAA‐CSs/pDNA complexes is observed by use of transmission electron microscopy and atomic force microscopy. The complexes are spherical nanoparticles with a mean size around 100 nm. Zeta potential tests reveal that the complexes are positively charged and their zeta potentials vary from +0.1 to +19.5 mV. The MTT assay and agarose gel electrophoresis demonstrate that the AAA‐CSs are non‐cytotoxic and have excellent DNA condensation and protection abilities. Cellular uptake investigation of the AAA‐CSs/pDNA complexes demonstrates that Arg‐CS and His‐CS have better cellular internalization property than the unmodified chitosan. The in vitro gene transfection is evaluated in HEK293 and NIH3T3 cell lines and in vivo transfection is carried out in tibialis anterior muscles. The results reveal that the arginine‐modified chitosan could significantly enhance gene‐transfection efficiency both in vitro and in vivo. Alkaline amino acids of arginine‐, histidine‐, and lysine‐modified chitosans (AAA‐CSs) are designed for use as nonviral gene vectors and synthesized to simulate the components of viral envelopes. The DNA condensation capacity and intracellular tracking of the AAA‐CSs are investigated. Arginine‐modified chitosan exhibited the best cellular internalization and gene‐transfection efficiency both in vitro and in vivo.
  PubDate: 2012-12-27T04:50:15.611208-05:
  DOI: 10.1002/adfm.201202503
High‐Performance Red, Green, and Blue Electroluminescent Devices Based on Blue Emitters with Small Singlet–Triplet Splitting and Ambipolar Transport Property
- Authors: Kai Wang; Fangchao Zhao, Chenguang Wang, Shanyong Chen, Dong Chen, Hongyu Zhang, Yu Liu, Dongge Ma, Yue Wang
  Pages: n/a - n/a
  Abstract: Two coordination complex emitters as well as host materials Be(PPI)2 and Zn(PPI)2 (PPI = 2‐(1‐phenyl‐1H‐phenanthro[9,10‐d]imidazol‐2‐yl)phenol) are designed, synthesized, and characterized. The incorporation of the metal atom leads to a twisted conformation and rigid molecular structure, which improve the thermal stability of Be(PPI)2 and Zn(PPI)2 with high Td and Tg at around 475 and 217 °C, respectively. The introduction of the electron‐donating phenol group results in the emission color shifting to the deep‐blue region and the emission maximum appears at around 429 nm. This molecular design strategy ensures that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) HOMO and LUMO of Be(PPI)2 and Zn(PPI)2 localize on the different moieties of the molecules. Therefore, the two complexes have an ambipolar transport property and a small singlet–triplet splitting of 0.35 eV for Be(PPI)2 and 0.21 eV for Zn(PPI)2. An undoped deep‐blue fluorescent organic light‐emitting device (OLED) that uses Be(PPI)2 as emitter exhibits a maximum power efficiency of 2.5 lm W−1 with the CIE coordinates of (0.15, 0.09), which are very close to the National Television Standards Committee (NTSC) blue standard (CIE: 0.14, 0.08). Green and red phosphorescent OLEDs (PhOLEDs) that use Be(PPI)2 and Zn(PPI)2 as host materials show high performance. Highest power efficiencies of 67.5 lm W−1 for green PhOLEDs and 21.7 lm W−1 for red PhOLEDs are achieved. In addition, the Be(PPI)2‐based devices show low‐efficiency roll‐off behavior, which is attributed to the more balanced carrier‐transport property of Be(PPI)2. Two coordination complexes with deep blue emission, small singlet–triplet splitting, and ambipolar transport properties are designed and synthesized. Efficient electroluminescent devices with the three primary colors are achieved by employing the complexes as an undoped deep‐blue emitter in organic light‐emitting devices (OLEDs) and as a host in red and green phosphorescent OLEDs.
  PubDate: 2012-12-21T03:50:10.740672-05:
  DOI: 10.1002/adfm.201202981
Candle Light‐Style Organic Light‐Emitting Diodes
- Authors: Jwo‐Huei Jou; Chun‐Yu Hsieh, Jing‐Ru Tseng, Shiang‐Hau Peng, Yung‐Cheng Jou, James H. Hong, Shih‐Ming Shen, Ming‐Chun Tang, Pin‐Chu Chen, Chun‐Hao Lin
  Pages: n/a - n/a
  Abstract: In response to the call for a physiologically‐friendly light at night that shows low color temperature, a candle light‐style organic light emitting diode (OLED) is developed with a color temperature as low as 1900 K, a color rendering index (CRI) as high as 93, and an efficacy at least two times that of incandescent bulbs. In addition, the device has a 80% resemblance in luminance spectrum to that of a candle. Most importantly, the sensationally warm candle light‐style emission is driven by electricity in lieu of the energy‐wasting and greenhouse gas emitting hydrocarbon‐burning candles invented 5000 years ago. This candle light‐style OLED may serve as a safe measure for illumination at night. Moreover, it has a high color rendering index with a decent efficiency. A sensationally‐warm but physically‐cold candle light‐style emission driven by electricity is developed in lieu of hydrocarbon‐burning in the energy‐inefficient, greenhouse gas releasing candles invented 5000 years ago. The dimmable and single‐sided emitting nature of the developed candle light‐style organic light emitting diode (OLED) provides a lighting atmosphere free from glare and obtrusion.
  PubDate: 2012-12-19T02:10:05.806391-05:
  DOI: 10.1002/adfm.201203209
Tunable Magnonic Spectra in Two‐Dimensional Magnonic Crystals with Variable Lattice Symmetry
- Authors: Susmita Saha; Ruma Mandal, Saswati Barman, Dheeraj Kumar, Bivas Rana, Yasuhiro Fukuma, Satoshi Sugimoto, YoshiChika Otani, Anjan Barman
  Pages: n/a - n/a
  Abstract: Tunable magnonic properties are demonstrated in two‐dimensional magnonic crystals in the form of artificial ferromagnetic nanodot lattices with variable lattice symmetry. An all‐optical time‐domain excitation and detection of the collective precessional dynamics is performed in the strongly magnetostatically coupled Ni80Fe20 (Py) circular dot lattices arranged in different lattice symmetry such as square, rectangular, hexagonal, honeycomb, and octagonal symmetry. As the symmetry changes from square to octagonal through rectangular, hexagonal and honeycomb, a significant variation in the spin wave spectra is observed. The single uniform collective mode in the square lattice splits in two distinct modes in the rectangular lattice and in three distinct modes in the hexagonal and octagonal lattices. However, in the honeycomb lattice a broad band of modes are observed. Micromagnetic simulations qualitatively reproduce the experimentally observed modes, and the simulated mode profiles reveal collective modes with different spatial distributions with the variation in the lattice symmetry determined by the magnetostatic field profiles. For the hexagonal lattice, the most intense peak shows a six‐fold anisotropy with the variation in the azimuthal angle of the external bias magnetic field. Analysis shows that this is due to the angular variation of the dynamical component of magnetization for this mode, which is directly influenced by the variation of the magnetostatic field on the elements in the hexagonal lattice. The observations are important for tunable and anisotropic propagation of spin waves in magnonic crystal based devices. Tunable magnonic spectra in two‐dimensional magnonic crystals with variable lattice symmetry are studied by an all‐optical time‐resolved magneto‐optical Kerr microscope. The magnonic mode profiles are interpreted in different lattices by micromagnetic simulations. A six‐fold anisotropy is observed in the hexagonal lattice due to the variation of the magnetostatic stray field on the elements.
  PubDate: 2012-12-17T10:50:19.748993-05:
  DOI: 10.1002/adfm.201202545
Layered α‐Co(OH)2 Nanocones as Electrode Materials for Pseudocapacitors: Understanding the Effect of Interlayer Space on Electrochemical Activity
- Authors: Lei Wang; Zhi Hui Dong, Zheng Gong Wang, Feng Xing Zhang, Jian Jin
  Pages: n/a - n/a
  Abstract: The effect of space accessible to electrolyte ions on the electrochemical activity is studied for a system of transition‐metal hydroxide‐based pseudocapacitors. Layered α‐Co(OH)2 with various intercalated anions is used as a model material. Three types of layered α‐Co(OH)2 with intercalated anions of dodecyl sulfate, benzoate, or nitrate, are prepared by a simple reflux and an anion‐exchange process. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations and X‐ray diffraction (XRD) data show the formation of layered α‐Co(OH)2 nanocones with interlayer spacing between adjacent Co(OH)2 single sheets of 1.6, 0.7, and 0.09 nm, corresponding to the anions as listed above. Electrochemical characterization reveals that interlayer space has a great effect on the electrochemical activity of α‐Co(OH)2 nanocones as an electrode material. For the interlayer spacing of 1.6 nm, in the case of dodecyl sulfate‐intercalated α‐Co(OH)2, the Faradaic reaction takes place more adequately than for benzoate‐ and nitrate‐intercalated α‐Co(OH)2. As a result, a higher specific capacitance and better cycling stability is obtained for the dodecyl sulfate‐intercalated α‐Co(OH)2. The electrochemical activity obviously reduces when the interlayer space decreases to 0.7 nm. Our results suggest the importance of rational designing the interlayer space of layered transition metal hydroxides for high‐performance pseudocapacitors. Three types of layered α‐Co(OH)2 nanocones with intercalated anions of dodecyl sulfate, benzoate, and nitrate, corresponding to interlayer spacings of 1.6, 0.7, and 0.09 nm, respectively, are prepared and used as model materials to investigate the effect of interlayer space on the electrochemical activity in the system of α‐Co(OH)2 nanocone‐based pseudocapacitors.
  PubDate: 2012-12-17T10:50:07.672736-05:
  DOI: 10.1002/adfm.201202786
Anion‐Doped Mixed Metal Oxide Nanostructures Derived from Layered Double Hydroxide as Visible Light Photocatalysts
- Authors: Seungho Cho; Ji‐Wook Jang, Ki‐jeong Kong, Eun Sun Kim, Kun‐Hong Lee, Jae Sung Lee
  Pages: n/a - n/a
  Abstract: Mixed metal oxide (MMO) nanostructures co‐doped uniformly by carbon and nitrogen are synthesized for the first time by annealing a terephthalate‐intercalated layered double hydroxide (LDH) under ammonia gas flow. The interlayer gallery of LDH allows effective access of NH3 and the carbon source to its crystal lattice for a uniform nitrogen and carbon doping. Such co‐doped MMO exhibit significantly red‐shifted absorption spectra to visible light region relative to pure MMO. Photoelectrochemical water oxidation and incident‐photon‐to‐current‐conversion efficiency of LDH‐derived photocatalysts demonstrate that all the visible light absorption caused by the anion doping contributes to the photocatalytic activity over the entire absorbed wavelength range of
  PubDate: 2012-12-16T18:50:18.92951-05:0
  DOI: 10.1002/adfm.201201883
Thin‐Wall Assembled SnO2 Fibers Functionalized by Catalytic Pt Nanoparticles and their Superior Exhaled‐Breath‐Sensing Properties for the Diagnosis of Diabetes
- Authors: Jungwoo Shin; Seon‐Jin Choi, Inkun Lee, Doo‐Young Youn, Chong Ook Park, Jong‐Heun Lee, Harry L. Tuller, Il‐Doo Kim
  Pages: n/a - n/a
  Abstract: Hierarchical SnO2 fibers assembled from wrinkled thin tubes are synthesized by controlling the microphase separation between tin precursors and polymers, by varying flow rates during electrospinning and a subsequent heat treatment. The inner and outer SnO2 tubes have a number of elongated open pores ranging from 10 nm to 500 nm in length along the fiber direction, enabling fast transport of gas molecules to the entire thin‐walled sensing layers. These features admit exhaled gases such as acetone and toluene, which are markers used for the diagnosis of diabetes and lung cancer. The open tubular structures facilitated the uniform coating of catalytic Pt nanoparticles onto the inner SnO2 layers. Highly porous SnO2 fibers synthesized at a high flow rate show five‐fold higher acetone responses than densely packed SnO2 fibers synthesized at a low flow rate. Interestingly, thin‐wall assembled SnO2 fibers functionalized by Pt particles exhibit a dramatically shortened gas response time compared to that of un‐doped SnO2 fibers, even at low acetone concentrations. Moreover, Pt‐decorated SnO2 fibers significantly enhance toluene response. These results demonstrate the novel and practical feasibility of thin‐wall assembled metal oxide based breath sensors for the accurate diagnosis of diabetes and potential detection of lung cancer. Electrospun fibers with wrinkled SnO2 walls composed of a number of elongated openings and pores are synthesized by a microphase separation controlled by the variation of flow rates. The unique structure enables superior acetone sensing performance due to the open pore structure, which provides fast transport and penetration of exhaled gases into the entire sensing layers.
  PubDate: 2012-12-13T16:50:03.61585-05:0
  DOI: 10.1002/adfm.201202729
Super‐Flexible Nanogenerator for Energy Harvesting from Gentle Wind and as an Active Deformation Sensor
- Authors: Sangmin Lee; Sung‐Hwan Bae, Long Lin, Ya Yang, Chan Park, Sang‐Woo Kim, Seung Nam Cha, Hyunjin Kim, Young Jun Park, Zhong Lin Wang
  Pages: n/a - n/a
  Abstract: Using an Al‐foil of thickness ≈18 μm as a substrate and electrode, a piezoelectric nanogenerator (NG) that is super‐flexible in responding to the wavy motion of a very light wind is fabricated using ZnO nanowire arrays. The NG is used to harvest the energy from a waving flag, demonstrating its high flexibility and excellent conformability to be integrated into fabric. The NG is applied to detect the wrinkling of a human face, showing its capability to serve as an active deformation sensor that needs no extra power supply. This strategy may provide a highly promising platform as energy harvesting devices and self‐powered sensors for practical use wherever movement is available. A super‐flexible and conformable piezoelectric nanogenerator (NG) based on cost‐effective Al foil is invented. The super‐flexible NG can generate output power in light air and serve as an energy harvesting device under waving motion of an attached flag due to its excellent conformability. The NGs show potential in applications as active sensors, capable of detecting slight skin movement due to its extremely low resistance to motion. This is a highly promising platform to produce energy harvesting devices and self‐powered sensors.
  PubDate: 2012-12-13T16:20:04.507962-05:
  DOI: 10.1002/adfm.201202867
In Situ Generated Gas Bubble‐Directed Self‐Assembly: Synthesis, and Peculiar Magnetic and Electrochemical Properties of Vertically Aligned Arrays of High‐Density Co3O4 Nanotubes
- Authors: Guoxiu Tong; Jianguo Guan, Qingjie Zhang
  Pages: n/a - n/a
  Abstract: A novel and versatile gas bubble induced self‐assembly technique is developed for the one‐step fabrication of vertically aligned polycrystalline Co3O4 nanotube arrays (NTAs) by the rapid thermal decomposition of Co(NO3)2·6H2O on a flat substrate. In this protocol, the in situ generation and release of gas bubbles, which can be regulated by elaborately adjusting the kinetic factors such as reaction time, decomposition temperature and pressure as well as the content of the chemically adsorbed water, play a vital role in the formation of the Co3O4 NTAs. Due to the shape anisotropy, ordered hierarchically porous structure and high surface area, the as‐obtained Co3O4 NTAs show unique magnetic properties of a low Néel temperature and a large exchange bias field, as well as an initial discharge capacity up to 1293 mAh·g−1 at 35 mA·g−1 and the retention of a charge capacity as high as 895.4 mAh·g−1 after 10 cycles. This endows them with important potential use in magnetic shielding, magnetic recording media, and lithium ion batteries, etc. Due to the simplicity of the self‐assembly method, this process is applicable to the large‐scale production of the Co3O4 NTAs, and may be extended to other materials. A versatile gas bubble‐induced self‐assembly technique based on hermal decomposition of Co(NO3)2·6H2O on a flat substrate is developed for the novel one‐step fabrication of vertically aligned arrays of Co3O4 nanotubes, which show unique magnetic properties and enhanced electrochemical activity, and thus have promising applications in magnetic shielding, energy storage, etc.
  PubDate: 2012-12-10T19:50:24.779581-05:
  DOI: 10.1002/adfm.201202747
Reconstruction of Conformal Nanoscale MnO on Graphene as a High‐Capacity and Long‐Life Anode Material for Lithium Ion Batteries
- Authors: Yongming Sun; Xianluo Hu, Wei Luo, Fangfang Xia, Yunhui Huang
  Pages: n/a - n/a
  Abstract: To tackle the issue of inferior cycle stability and rate capability for MnO anode materials in lithium ion batteries, a facile strategy is explored to prepare a hybrid material consisting of MnO nanocrystals grown on conductive graphene nanosheets. The prepared MnO/graphene hybrid anode exhibits a reversible capacity as high as 2014.1 mAh g−1 after 150 discharge/charge cycles at 200 mA g−1, excellent rate capability (625.8 mAh g−1 at 3000 mA g−1), and superior cyclability (843.3 mAh g−1 even after 400 discharge/charge cycles at 2000 mA g−1 with only 0.01% capacity loss per cycle). The results suggest that the reconstruction of the MnO/graphene electrodes is intrinsic due to conversion reactions. A long‐term stable nanoarchitecture of graphene‐supported ultrafine manganese oxide nanoparticles is formed upon cycling, which yields a long‐life anode material for lithium ion batteries. The lithiation and delithiation behavior suggests that the further oxidation of Mn(II) to Mn(IV) and the interfacial lithium storage upon cycling contribute to the enhanced specific capacity. The excellent rate capability benefits from the presence of conductive graphene and a short transportation length for both lithium ions and electrons. Moreover, the as‐formed hybrid nanostructure of MnO on graphene may help achieve faster kinetics of conversion reactions. A facile strategy is explored to prepare a hybrid material consisting of MnO nanocrystals grown on conductive graphene nanosheets. A long‐term stable nanoarchitecture of graphene‐supported ultrafine manganese oxide nanoparticles is formed upon cycling, which yields a high‐capacity and long‐life anode material for lithium ion batteries.
  PubDate: 2012-12-10T19:50:24.147242-05:
  DOI: 10.1002/adfm.201202623
Superparamagnetic Ag@Co‐Nanocomposites on Granulated Cation Exchange Polymeric Matrices with Enhanced Antibacterial Activity for the Environmentally Safe Purification of Water
- Authors: Amanda Alonso; Xavier Muñoz‐Berbel, Núria Vigués, Rosalía Rodríguez‐Rodríguez, Jorge Macanás, Maria Muñoz, Jordi Mas, Dmitri N. Muraviev
  Pages: n/a - n/a
  Abstract: Cation exchange polymeric matrices are widely used in water treatment protocols to reduce the mineral content of hard waters, even for human consumption. However, they are not antibacterial and flowing bacteria can be trapped in their structures and proliferate, thus acting as microbial contamination sources. Here, Ag@Co‐nanoparticles (Ag@Co‐NPs) with a low‐cost superparamagnetic Co0‐core and an antibacterial Ag‐shell are synthesized on granulated cation exchange polymeric matrices under soft reaction conditions. The presence of these NPs provides the final nanocomposite (NC) with additional functionalities (superparamagnetism and antibacterial activity) making it ideal for water purification applications. Ag@Co‐NPs are synthesized in situ on four cation exchange polymeric matrices containing either strong (sulfonic) or weak (carboxylic) acid functional groups homogeneously distributed (C‐type) or concentrated on an external shell (SST‐type) by the intermatrix synthesis (IMS) method. The NCs are characterized (metal content, NP size and distribution, metal oxidative state, and metal release) and evaluated for water purification applications. Ag@Co‐nanocomposites with a low cost magnetic Co core and an antibacterial Ag shell, which preserves the core integrity, are fabricated on granulated cation exchange polymeric matrices by intermatrix synthesis under soft reaction conditions. These materials present superparamagnetic and have enhanced antibacterial activity, making them ideal for water purification applications.
  PubDate: 2012-12-07T07:40:24.268476-05:
  DOI: 10.1002/adfm.201202663
Highly Stable Graphene‐Based Multilayer Films Immobilized via Covalent Bonds and Their Applications in Organic Field‐Effect Transistors
- Authors: Xiaowei Ou; Lang Jiang, Penglei Chen, Mingshan Zhu, Wenping Hu, Minghua Liu, Junfa Zhu, Huanxin Ju
  Pages: n/a - n/a
  Abstract: Highly stable graphene oxide (GO)‐based multilayered ultrathin films can be covalently immobilized on solid supports through a covalent‐based method. It is demonstrated that when (3‐aminopropyl) trimethoxysilane (APTMS), which works as a covalent cross‐linking agent, and GO nanosheets are assembled in an layer‐by‐layer (LBL) manner, GO nanosheets can be covalently grafted on the solid substrate successfully to produce uniform multilayered (APTMS/GO)N films over large‐area surfaces. Compared with conventional noncovalent LBL films constructed by electrostatic interactions, those assembled using this covalent‐based method display much higher stability and reproducibility. Upon thermal annealing‐induced reduction of the covalent (APTMS/GO)N films, the obtained reduced GO (RGO) films, (APTMS/RGO)N, preserve their basic structural characteristics. It is also shown that the as‐prepared covalent (APTMS/RGO)N multilayer films can be used as highly stable source/drain electrodes in organic field‐effect transistors (OFETs). When the number of bilayers of the (APTMS/RGO)N film exceeds 2 (ca. 2.7 nm), the OFETs based on (APTMS/RGO)N electrodes display much better electrical performance than devices based on 40 nm Au electrodes. The covalent protocol proposed may open up new opportunities for the construction of graphene‐based ultrathin films with excellent stability and reproducibility, which are desired for practical applications that require withstanding of multistep post‐production processes. Highly stable graphene‐based multilayers films are covalently immobilized on solid supports. Compared with those formulated through a noncovalent‐based method, as‐constructed covalent‐based multilayered films display much higher stability and reproducibility, suggesting a bright future for practical applications that require withstanding of multistep post‐production processes. The application of the covalent graphene‐based films as source/drain electrodes in organic field‐effect transistors is investigated.
  PubDate: 2012-12-06T07:30:27.351578-05:
  DOI: 10.1002/adfm.201202586
Arsonic Acid Self‐Assembled Monolayers Protect Oxide Surfaces from Micronewton Nanomechanical Forces
- Authors: Natalie A. LaFranzo; Joshua A. Maurer
  Pages: n/a - n/a
  Abstract: The development of new surface coatings is critical for combating wear and increasing the device lifetime in microelectromechanical systems (MEMS). Here, a class of arsonic acid self‐assembled monolayers (SAMs) is reported that form readily on oxide substrates including silicon oxide, borosilicate glass, and titanium oxide. Monolayers are easily prepared using a straightforward soaking technique, which is amenable to large‐scale commercial applications. Monolayer formation on borosilicate glass and titanium oxide is characterized using infrared spectroscopy. Monolayers on borosilicate glass, native silicon oxide and titanium oxide are evaluated with contact angle measurements, as well as wear measurements using nanoscratching experiments. On titanium oxide and borosilicate glass, monolayers prepared from hexadecylarsonic acid provide significantly greater surface protection than surfaces reacted under similar conditions with hexadecylphosphonic acid, a common modifying agent for oxide substrates. Hexadecylarsonic acid self‐assembled monolayers are prepared on glass, silicon oxide, and titanium oxide via a straight‐forward soaking method. These monolayers protect the substrates from micronewton mechanical forces applied in scanning probe microscopy nanoscratching experiments. Compared to hexadecylphosphonic acid, the arsonate shows increased reactivity and greater protection of the substrate from mechanical stress. This system shows excellent potential as a MEMS lubricant.
  PubDate: 2012-12-06T07:30:22.570607-05:
  DOI: 10.1002/adfm.201202566
Nanomaterial‐Based Fluorescent DNA Analysis: A Comparative Study of the Quenching Effects of Graphene Oxide, Carbon Nanotubes, and Gold Nanoparticles
- Abstract: A variety of nanomaterials have shown extraordinarily high quenching ability toward a broad range of fluorophores. Recently, there has been intense interest in developing new tools for fluorescent DNA analysis in solution or inside the cell based on this property, and by exploiting interactions between these nanoscale “superquenchers” and DNA molecules in the single‐stranded (ss‐) or double‐stranded (ds‐) forms. Here, a comparative study on the nanoqueching effects is performed by using a series of nanomaterials with different dimensions, i.e., gold nanoparticles (AuNPs, 0D), carbon nanotubes (CNTs, 1D), and graphene oxide (GO, 2D). The quenching efficiency, kinetics, differentiation ability, and influencing factors such as concentration and ionic strength are studied. Interestingly, GO exhibits superior quenching abilities to the other two materials in both the quenching efficiency and kinetics. As a result, a GO‐based fluorescent sensor, designed in a simple mix‐and‐detect format, can detect concentrations of DNA as low as 0.2 nM, which is better than either CNTs or AuNPs by an order of magnitude. This sensor can also differentiate single‐base mismatches much better than either CNTs‐ or AuNPs‐ based sensors. This study paves the way to better choice of nanomaterials for bioanalysis and elaborate design of biosensors for both in vitro diagnosis and in vivo bioimaging. Three nanomaterials‐based fluorescent biosensors are used for DNA detection in homogeneous solution. The quenching efficiency, kinetics, differentiation ability, and influencing factors are studied, and graphene oxide (GO) exhibits the best performance, characterized by superior quenching abilities, limit of detection, and the highest discrimination for single‐base mismatches
Synchrotron X‐Ray Scanning Tunneling Microscopy: Fingerprinting Near to Far Field Transitions on Cu(111) Induced by Synchrotron Radiation
- Abstract: The combination of the high spatial resolution of scanning tunneling microscopy with the chemical and magnetic contrast provided by synchrotron X‐rays has the potential to allow a unique characterization of advanced functional materials. While the scanning probe provides the high spatial resolution, synchrotron X‐rays that produce photo‐excitations of core electrons add chemical and magnetic contrast. However, in order to realize the method's full potential it is essential to maintain tunneling conditions, even while high brilliance X‐rays irradiate the sample surface. Different from conventional scanning tunneling microscopy, X‐rays can cause a transition of the tip out of the tunneling regime. Monitoring the reaction of the z‐piezo (the element that controls the tip to sample separation) alone is not sufficient, because a continuous tip current is obtained. As a solution, an unambiguous and direct way of fingerprinting such near to far field transitions of the tip that relies on the simultaneous analysis of the X‐ray‐induced tip and sample current is presented. This result is of considerable importance because it opens the path to the ultimate resolution in X‐ray enhanced scanning tunneling microscopy. Nanofabricated insulator‐coated smart tips are indispensible for stable tunneling conditions in synchrotron X‐ray enhanced scanning tunneling microscopy. An unambiguous and direct way of fingerprinting tunneling to far field transitions of the tip that relies on the simultaneous analysis of the X‐ray‐induced tip and sample current is presented.
Diverse Organic Field‐Effect Transistor Sensor Responses from Two Functionalized Naphthalenetetracarboxylic Diimides and Copper Phthalocyanine Semiconductors Distinguishable Over a Wide Analyte Range
- Abstract: Naphthalenetetracarboxylic diimide derivatives (octyl “8” NTCDI, dimethylaminopropyl “DMP” NTCDI) and copper phthalocyanine (CuPc) are used to form a diverse organic field‐effect transistor (OFET) sensor array. CuPc and 8‐NTCDI are p‐channel and n‐channel semiconductors, respectively, showing expected and opposing responses to analytes. DMP‐NTCDI, on the other hand, because of its ionizable side chain, shows response directions and magnitudes that are not correlated to those of the other two. The result is a distinct response pattern and unambiguous recognition ability for individual analytes. The differences are even more dramatic if the time evolution of the responses is considered. The three‐response patterns obtained from representative polar, nonpolar, acidic, and basic vapors are all different, showing the potential for this approach in rapid, low‐cost electronic detection of volatile compounds. Naphthalenetetracarboxylic diimide derivatives (octyl “8” NTCDI, dimethylaminopropyl “DMP” NTCDI) and copper phthalocyanine (CuPc) are used to form a diverse organic field‐effect transistor sensor array. The three‐response patterns obtained from representative polar, nonpolar, acidic, and basic vapors are all different, showing the potential for this approach in rapid, low‐cost electronic detection of volatile compounds.