Showing papers in "Nature Materials in 2006"
TL;DR: A density functional theory-based, high-throughput screening scheme that successfully uses these strategies to identify a new electrocatalyst for the hydrogen evolution reaction (HER), which is found to have a predicted activity comparable to, or even better than, pure Pt, the archetypical HER catalyst.
Abstract: The pace of materials discovery for heterogeneous catalysts and electrocatalysts could, in principle, be accelerated by the development of efficient computational screening methods. This would require an integrated approach, where the catalytic activity and stability of new materials are evaluated and where predictions are benchmarked by careful synthesis and experimental tests. In this contribution, we present a density functional theory-based, high-throughput screening scheme that successfully uses these strategies to identify a new electrocatalyst for the hydrogen evolution reaction (HER). The activity of over 700 binary surface alloys is evaluated theoretically; the stability of each alloy in electrochemical environments is also estimated. BiPt is found to have a predicted activity comparable to, or even better than, pure Pt, the archetypical HER catalyst. This alloy is synthesized and tested experimentally and shows improved HER performance compared with pure Pt, in agreement with the computational screening results.
3,134 citations
TL;DR: In this article, the influence of polymer regioregularity (RR) on the molecular nanostructure, and hence on the resulting material properties and device performance has been investigated, which can be attributed to enhanced optical absorption and transport resulting from the organization of P3HT chains and domains.
Abstract: Low-cost photovoltaic energy conversion using conjugated molecular materials has become increasingly feasible through the development of organic ‘bulk heterojunction (BHJ)’ structures1,2,3,4,5,6,7, where efficient light-induced charge separation is enabled by a large-area donor–acceptor interface2,3. The highest efficiencies have been achieved using blends of poly(3-hexylthiophene) (P3HT) and a fullerene derivative8,9,10,11,12, but performance depends critically on the material properties and processing conditions. This variability is believed to be influenced by the self-organizing properties of P3HT, which means that both optical13,14 and electronic15,16 properties are sensitive to the molecular packing. However, the relationship between molecular nanostructure, optoelectronic properties of the blend material and device performance has not yet been demonstrated. Here we focus on the influence of polymer regioregularity (RR) on the molecular nanostructure, and hence on the resulting material properties and device performance. We find a strong influence of RR on solar-cell performance, which can be attributed to enhanced optical absorption and transport resulting from the organization of P3HT chains and domains. Further optimization of devices using the highest RR material resulted in a power conversion efficiency of 4.4%, even without optimization of electrodes7.
2,268 citations
TL;DR: New semiconducting liquid-crystalline thieno[3,2-b ]thiophene polymers are reported on, the enhancement in charge-carrier mobility achieved through highly organized morphology from processing in the mesophase, and the effects of exposure to both ambient and low-humidity air on the performance of transistor devices.
Abstract: Organic semiconductors that can be fabricated by simple processing techniques and possess excellent electrical performance, are key requirements in the progress of organic electronics. Both high semiconductor charge-carrier mobility, optimized through understanding and control of the semiconductor microstructure, and stability of the semiconductor to ambient electrochemical oxidative processes are required. We report on new semiconducting liquid-crystalline thieno[3,2-b ]thiophene polymers, the enhancement in charge-carrier mobility achieved through highly organized morphology from processing in the mesophase, and the effects of exposure to both ambient and low-humidity air on the performance of transistor devices. Relatively large crystalline domain sizes on the length scale of lithographically accessible channel lengths (∼200 nm) were exhibited in thin films, thus offering the potential for fabrication of single-crystal polymer transistors. Good transistor stability under static storage and operation in a low-humidity air environment was demonstrated, with charge-carrier field-effect mobilities of 0.2–0.6 cm2 V−1 s−1 achieved under nitrogen.
2,011 citations
TL;DR: A two-step electrode design consisting of the electrochemically assisted template growth of Cu nanorods onto a current collector followed by electrochemical plating of Fe3O4 is used, which demonstrates a factor of six improvement in power density over planar electrodes while maintaining the same total discharge time.
Abstract: All battery technologies are known to suffer from kinetic problems linked to the solid-state diffusion of Li in intercalation electrodes, the conductivity of the electrolyte in some cases and the quality of interfaces. For Li-ion technology the latter effect is especially acute when conversion rather than intercalation electrodes are used. Nano-architectured electrodes are usually suggested to enhance kinetics, although their realization is cumbersome. To tackle this issue for the conversion electrode material Fe3O4, we have used a two-step electrode design consisting of the electrochemically assisted template growth of Cu nanorods onto a current collector followed by electrochemical plating of Fe3O4. Using such electrodes, we demonstrate a factor of six improvement in power density over planar electrodes while maintaining the same total discharge time. The capacity at the 8C rate was 80% of the total capacity and was sustained over 100 cycles. The origin of the large hysteresis between charge and discharge, intrinsic to conversion reactions, is discussed and approaches to reduce it are proposed. We hope that such findings will help pave the way for the use of conversion reaction electrodes in future-generation Li-ion batteries.
1,933 citations
TL;DR: This dense carbon-nanotube material is advantageous for numerous applications, and here it is demonstrated its use as flexible heaters as well as supercapacitor electrodes for compact energy-storage devices.
Abstract: Shape-engineerable and highly densely packed single-walled carbon nanotubes and their application as super-capacitor electrodes
1,851 citations
TL;DR: A new class of ultrasonic metamaterials consisting of an array of subwavelength Helmholtz resonators with designed acoustic inductance and capacitance with an effective dynamic modulus with negative values near the resonance frequency is reported.
Abstract: The emergence of artificially designed subwavelength electromagnetic materials, denoted metamaterials, has significantly broadened the range of material responses found in nature. However, the acoustic analogue to electromagnetic metamaterials has, so far, not been investigated. We report a new class of ultrasonic metamaterials consisting of an array of subwavelength Helmholtz resonators with designed acoustic inductance and capacitance. These materials have an effective dynamic modulus with negative values near the resonance frequency. As a result, these ultrasonic metamaterials can convey acoustic waves with a group velocity antiparallel to phase velocity, as observed experimentally. On the basis of homogenized-media theory, we calculated the dispersion and transmission, which agrees well with experiments near 30 kHz. As the negative dynamic modulus leads to a richness of surface states with very large wavevectors, this new class of acoustic metamaterials may offer interesting applications, such as acoustic negative refraction and superlensing below the diffraction limit.
1,562 citations
TL;DR: It is demonstrated that the switching behaviour is an intrinsic feature of naturally occurring dislocations in single crystals of a prototypical ternary oxide, SrTiO3, and to be related to the self-doping capability of the early transition metal oxides.
Abstract: The great variability in the electrical properties of multinary oxide materials, ranging from insulating, through semiconducting to metallic behaviour, has given rise to the idea of modulating the electronic properties on a nanometre scale for high-density electronic memory devices. A particularly promising aspect seems to be the ability of perovskites to provide bistable switching of the conductance between non-metallic and metallic behaviour by the application of an appropriate electric field. Here we demonstrate that the switching behaviour is an intrinsic feature of naturally occurring dislocations in single crystals of a prototypical ternary oxide, SrTiO(3). The phenomenon is shown to originate from local modulations of the oxygen content and to be related to the self-doping capability of the early transition metal oxides. Our results show that extended defects, such as dislocations, can act as bistable nanowires and hold technological promise for terabit memory devices.
1,561 citations
TL;DR: In this article, the authors present methods, systems and system components for transferring, assembling and integrating features and arrays of features having selected nanosized and/or microsized physical dimensions, shapes and spatial orientations.
Abstract: The present invention provides methods, systems and system components for transferring, assembling and integrating features and arrays of features having selected nanosized and/or microsized physical dimensions, shapes and spatial orientations. Methods of the present invention utilize principles of ‘soft adhesion’ to guide the transfer, assembly and/or integration of features, such as printable semiconductor elements or other components of electronic devices. Methods of the present invention are useful for transferring features from a donor substrate to the transfer surface of an elastomeric transfer device and, optionally, from the transfer surface of an elastomeric transfer device to the receiving surface of a receiving substrate. The present methods and systems provide highly efficient, registered transfer of features and arrays of features, such as printable semiconductor element, in a concerted manner that maintains the relative spatial orientations of transferred features.
1,305 citations
TL;DR: In this article, the authors examined the microscopic distribution of charge and ions across the (001) LaAlO3/SrTiO3 interface, and they found that there is a fundamental asymmetry between the ionically compensated AlO2/srO/TiO2 interface, both in interfacial sharpness and charge density.
Abstract: A central goal of modern materials physics and nanoscience is the control of materials and their interfaces to atomic dimensions. For interfaces between polar and nonpolar layers, this goal is thwarted by a polar catastrophe that forces an interfacial reconstruction. In traditional semiconductors, this reconstruction is achieved by an atomic disordering and stoichiometry change at the interface, but a new option is available in multivalent oxides: if the electrons can move, the atoms do not have to. Using atomic-scale electron energy loss spectroscopy, we have examined the microscopic distribution of charge and ions across the (001) LaAlO3/SrTiO3 interface. We find that there is a fundamental asymmetry between the ionically compensated AlO2/SrO/TiO2 interface, and the electronically compensated AlO2/LaO/TiO2 interface, both in interfacial sharpness and charge density. This suggests a general strategy to design sharp interfaces, remove interfacial screening charges, control the band offset and, hence, markedly improve the performance of oxide devices.
1,294 citations
TL;DR: A new oxalic-acid-based anodization process for long-range ordered alumina membranes that establishes a new self-ordering regime with interpore distances, (Dint)=200–300 nm, allowing 2,500–3,500% faster oxide growth with improved ordering of the nanopores.
Abstract: Nanoporous anodic aluminium oxide has been widely used for the development of various functional nanostructures. So far these self-organized pore structures could only be prepared within narrow processing conditions. Here we report a new oxalic-acid-based anodization process for long-range ordered alumina membranes. This process is a new generation of the so-called "hard anodization" approach that has been widely used in industry for high-speed fabrication of mechanically robust, very thick (>100 microm) and low-porosity alumina films since the 1960s. This hard anodization approach establishes a new self-ordering regime with interpore distances, (D(int))=200-300 nm, which have not been achieved by mild anodization processes so far. It offers substantial advantages over conventional anodization processes in terms of processing time, allowing 2,500-3,500% faster oxide growth with improved ordering of the nanopores. Perfectly ordered alumina membranes with high aspect ratios (>1,000) of uniform nanopores with periodically modulated diameters have been realized.
1,269 citations
TL;DR: In this paper, the authors describe an experimental demonstration of current-induced magnetic reversal of nanopillars with perpendicular anisotropy and high coercive fields, and the best results are observed for Co/Ni multilayers, which have higher giant magnetoresistance values and spin-torque efficiencies than Co/Pt multilayer.
Abstract: Devices that show a magnetic anisotropy normal to the film surface hold great promise towards faster and smaller magnetic bits in data-storage applications. We describe an experimental demonstration of current-induced magnetic reversal of nanopillars with perpendicular anisotropy and high coercive fields. The best results are observed for Co/Ni multilayers, which have higher giant magnetoresistance values and spin-torque efficiencies than Co/Pt multilayers. The reference layers were designed to have significantly higher anisotropy allowing a complete current-field phase diagram of the free-layer reversal to be explored. The results are compared to micromagnetic modelling of the free layer that, depending on the bias current and applied field, details regions of irreversible magnetic switching, coherent and incoherent spin waves, or static non-uniform magnetization states. This ability to manipulate high-anisotropy magnetic elements could prove useful for a range of spintronic applications.
TL;DR: This three-component system exhibits a high photocatalytic activity, far exceeding those of the single- and two-component systems, as a result of vectorial electron transfer driven by the two-step excitation of TiO2 and CdS.
Abstract: Natural photosynthesis, which achieves efficient solar energy conversion through the combined actions of many types of molecules ingeniously arranged in a nanospace, highlights the importance of a technique for site-selective coupling of different materials to realize artificial high-efficiency devices1. In view of increasingly serious energy and environmental problems, semiconductor-based artificial photosynthetic systems consisting of isolated photochemical system 1 (PS1), PS2 and the electron-transfer system have recently been developed2,3. However, the direct coupling of the components is crucial for retarding back reactions to increase the reaction efficiency. Here, we report a simple technique for forming an anisotropic CdS–Au–TiO2 nanojunction, in which PS1(CdS), PS2(TiO2) and the electron-transfer system (Au) are spatially fixed. This three-component system exhibits a high photocatalytic activity, far exceeding those of the single- and two-component systems, as a result of vectorial electron transfer driven by the two-step excitation of TiO2 and CdS.
TL;DR: A cross-disciplinary review of the essential characteristics of excitons in nanoscience is presented, highlighting the importance of quantum dots, conjugated polymers, carbon nanotubes and photosynthetic light-harvesting antenna complexes.
Abstract: Nanoscale systems are forecast to be a means of integrating desirable attributes of molecular and bulk regimes into easily processed materials. Notable examples include plastic light-emitting devices and organic solar cells, the operation of which hinge on the formation of electronic excited states, excitons, in complex nanostructured materials. The spectroscopy of nanoscale materials reveals details of their collective excited states, characterized by atoms or molecules working together to capture and redistribute excitation. What is special about excitons in nanometre-sized materials? Here we present a cross-disciplinary review of the essential characteristics of excitons in nanoscience. Topics covered include confinement effects, localization versus delocalization, exciton binding energy, exchange interactions and exciton fine structure, exciton-vibration coupling and dynamics of excitons. Important examples are presented in a commentary that overviews the present understanding of excitons in quantum dots, conjugated polymers, carbon nanotubes and photosynthetic light-harvesting antenna complexes.
TL;DR: This work demonstrates the first observation of electrical control of antiferromagnetic domain structure in a single-phase multiferroic material at room temperature with high resolution images, indicating a strong coupling between the two types of order.
Abstract: Multiferroic materials, which offer the possibility of manipulating the magnetic state by an electric field or vice versa, are of great current interest. In this work, we demonstrate the first observation of electrical control of antiferromagnetic domain structure in a single-phase multiferroic material at room temperature. High-resolution images of both antiferromagnetic and ferroelectric domain structures of (001)-oriented multiferroic BiFeO3 films revealed a clear domain correlation, indicating a strong coupling between the two types of order. The ferroelectric structure was measured using piezo force microscopy, whereas X-ray photoemission electron microscopy as well as its temperature dependence was used to detect the antiferromagnetic configuration. Antiferromagnetic domain switching induced by ferroelectric polarization switching was observed, in agreement with theoretical predictions.
TL;DR: The synthesis of the crystalline aluminosilicate materials with tunable mesoporosity and strong acidity has potentially important technological implications for catalytic reactions of large molecules, whereas conventional mesoporous materials lack hydrothermal stability and acidity.
Abstract: Zeolites are a family of crystalline aluminosilicate materials widely used as shape-selective catalysts, ion exchange materials, and adsorbents for organic compounds. In the present work, zeolites were synthesized by adding a rationally designed amphiphilic organosilane surfactant to conventional alkaline zeolite synthesis mixtures. The zeolite products were characterized by a complementary combination of X-ray diffraction (XRD), nitrogen sorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The analyses show that the present method is suitable as a direct synthesis route to highly mesoporous zeolites. The mesopore diameters could be uniformly tailored, similar to ordered mesoporous silica with amorphous frameworks. The mesoporous zeolite exhibited a narrow, small-angle XRD peak, which is characteristic of the short-range correlation between mesopores, similar to disordered wormhole-like mesoporous materials. The XRD patterns and electron micrographs of the samples taken during crystallization clearly showed the evolution of the mesoporous structure concomitantly to the crystallization of zeolite frameworks. The synthesis of the crystalline aluminosilicate materials with tunable mesoporosity and strong acidity has potentially important technological implications for catalytic reactions of large molecules, whereas conventional mesoporous materials lack hydrothermal stability and acidity.
TL;DR: It is shown how the morphology of drop-deposited nanoparticle films is controlled by evaporation kinetics and particle interactions with the liquid–air interface, and this self-assembly mechanism produces monolayers with exceptional long-range ordering that are compact over macroscopic areas, despite the far-from-equilibriumevaporation process.
Abstract: When a drop of a colloidal solution of nanoparticles dries on a surface, it leaves behind coffee-stain-like rings of material with lace-like patterns or clumps of particles in the interior. These non-uniform mass distributions are manifestations of far-from-equilibrium effects, such as fluid flows and solvent fluctuations during late-stage drying. However, recently a strikingly different drying regime promising highly uniform, long-range-ordered nanocrystal monolayers has been found. Here we make direct, real-time and real-space observations of nanocrystal self-assembly to reveal the mechanism. We show how the morphology of drop-deposited nanoparticle films is controlled by evaporation kinetics and particle interactions with the liquid-air interface. In the presence of an attractive particle-interface interaction, rapid early-stage evaporation dynamically produces a two-dimensional solution of nanoparticles at the liquid-air interface, from which nanoparticle islands nucleate and grow. This self-assembly mechanism produces monolayers with exceptional long-range ordering that are compact over macroscopic areas, despite the far-from-equilibrium evaporation process. This new drop-drying regime is simple, robust and scalable, is insensitive to the substrate material and topography, and has a strong preference for forming monolayer films. As such, it stands out as an excellent candidate for the fabrication of technologically important ultra thin film materials for sensors, optical devices and magnetic storage media.
TL;DR: A one-phase method that combines the advantages of microscope projection photolithography and microfluidics to continuously form morphologically complex or multifunctional particles down to the colloidal length scale is reported, offering unprecedented control over particle size, shape and anisotropy.
Abstract: Precisely shaped polymeric particles and structures are widely used for applications in photonic materials, MEMS, biomaterials and self-assembly. Current approaches for particle synthesis are either batch processes or flow-through microfluidic schemes that are based on two-phase systems, limiting the throughput, shape and functionality of the particles. We report a one-phase method that combines the advantages of microscope projection photolithography and microfluidics to continuously form morphologically complex or multifunctional particles down to the colloidal length scale. Exploiting the inhibition of free-radical polymerization near PDMS surfaces, we are able to repeatedly pattern and flow rows of particles in less than 0.1 s, affording a throughput of near 100 particles per second using the simplest of device designs. Polymerization was also carried out across laminar, co-flowing streams to generate Janus particles containing different chemistries, whose relative proportions could be easily tuned. This new high-throughput technique offers unprecedented control over particle size, shape and anisotropy.
TL;DR: Recent advances and future prospects of light-emitting field-effect transistors are explored, with particular emphasis on organic semiconductors and the role played by the material properties, device features and the active layer structure in determining the device performances.
Abstract: Field-effect transistors are emerging as useful device structures for efficient light generation from a variety of materials, including inorganic semiconductors, carbon nanotubes and organic thin films. In particular, organic light-emitting field-effect transistors are a new class of electro-optical devices that could provide a novel architecture to address open questions concerning charge-carrier recombination and light emission in organic materials. These devices have potential applications in optical communication systems, advanced display technology, solid-state lighting and electrically pumped organic lasers. Here, recent advances and future prospects of light-emitting field-effect transistors are explored, with particular emphasis on organic semiconductors and the role played by the material properties, device features and the active layer structure in determining the device performances.
TL;DR: The utility of these polymer-coated Bi2S3 nanoparticles for enhanced in-vivo imaging of the vasculature, the liver and lymph nodes in mice is shown and these nanoparticles and their bioconjugates are expected to become an important adjunct to in vivo Imaging of molecular targets and pathological conditions.
Abstract: Nanomaterials have become increasingly important in the development of new molecular probes for in vivo imaging, both experimentally and clinically. Nanoparticulate imaging probes have included semiconductor quantum dots, magnetic and magnetofluorescent nanoparticles, gold nanoparticles and nanoshells, among others. However, the use of nanomaterials for one of the most common imaging techniques, computed tomography (CT), has remained unexplored. Current CT contrast agents are based on small iodinated molecules. They are effective in absorbing X-rays, but non-specific distribution and rapid pharmacokinetics have rather limited their microvascular and targeting performance. Here we propose the use of a polymer-coated Bi(2)S(3) nanoparticle preparation as an injectable CT imaging agent. This preparation demonstrates excellent stability at high concentrations (0.25 M Bi(3+)), high X-ray absorption (fivefold better than iodine), very long circulation times (>2 h) in vivo and an efficacy/safety profile comparable to or better than iodinated imaging agents. We show the utility of these polymer-coated Bi(2)S(3) nanoparticles for enhanced in vivo imaging of the vasculature, the liver and lymph nodes in mice. These nanoparticles and their bioconjugates are expected to become an important adjunct to in vivo imaging of molecular targets and pathological conditions.
TL;DR: A scalable chemical vapour deposition method is presented to synthesize FeCo/single-graphitic-shell nanocry crystals that are soluble and stable in water solutions and point to the potential of using these nanocrystals for integrated diagnosis and therapeutic (photothermal-ablation) applications.
Abstract: Nanocrystals with advanced magnetic or optical properties have been actively pursued for potential biological applications, including integrated imaging, diagnosis and therapy. Among various magnetic nanocrystals, FeCo has superior magnetic properties, but it has yet to be explored owing to the problems of easy oxidation and potential toxicity. Previously, FeCo nanocrystals with multilayered graphitic carbon, pyrolytic carbon or inert metals have been obtained, but not in the single-shelled, discrete, chemically functionalized and water-soluble forms desired for biological applications. Here, we present a scalable chemical vapour deposition method to synthesize FeCo/single-graphitic-shell nanocrystals that are soluble and stable in water solutions. We explore the multiple functionalities of these core-shell materials by characterizing the magnetic properties of the FeCo core and near-infrared optical absorbance of the single-layered graphitic shell. The nanocrystals exhibit ultra-high saturation magnetization, r1 and r2 relaxivities and high optical absorbance in the near-infrared region. Mesenchymal stem cells are able to internalize these nanoparticles, showing high negative-contrast enhancement in magnetic-resonance imaging (MRI). Preliminary in vivo experiments achieve long-lasting positive-contrast enhancement for vascular MRI in rabbits. These results point to the potential of using these nanocrystals for integrated diagnosis and therapeutic (photothermal-ablation) applications.
TL;DR: In this paper, a new experimental method based on a fusible coating was presented, which showed that the temperature rise, over a few nanoseconds, can be as high as a few thousand kelvin; nevertheless, the temperature rises did not seem to control the shear-band thickness.
Abstract: At ambient temperature the plastic flow shown by metallic glasses is localized into shear bands1,2. This localization and the liquid-like features seen on fracture surfaces are consistent with shear softening in the bands. The extent to which this softening is a result of local heating has remained controversial, with estimates of the local temperature rise ranging from less than 0.1 kelvin to a few thousand kelvin3,4,5,6,7,8,9,10,11. Here we present a new experimental method based on a fusible coating, which shows that the temperature rise, over a few nanoseconds, can be as high as a few thousand kelvin; nevertheless, the temperature rise does not seem to control the shear-band thickness. It is important to understand the mechanisms of shear banding and associated softening because these are the principal factors limiting structural applications of bulk metallic glasses, which have some attractive mechanical properties such as high yield strength12,13.
TL;DR: In this article, a 3D composite with carbon-nanotube forests is proposed to improve the in-plane fracture toughness, hardness, delamination resistance, inplane mechanical properties, damping, thermo-elastic behavior, and thermal and electrical conductivities.
Abstract: Traditional fibre-reinforced composite materials with excellent in-plane properties fare poorly when out-of-plane through-thickness properties are important1. Composite architectures with fibres designed orthogonal to the two-dimensional (2D) layout in traditional composites could alleviate this weakness in the transverse direction, but all of the efforts1,2 so far have only produced limited success. Here, we unveil an approach to the 3D composite challenge, without altering the 2D stack design, on the basis of the concept of interlaminar carbon-nanotube3,4 forests that would provide enhanced multifunctional properties along the thickness direction. The carbon-nanotube forests allow the fastening of adjacent plies in the 3D composite. We grow multiwalled carbon nanotubes on the surface of micro-fibre fabric cloth layouts, normal to the fibre lengths, resulting in a 3D effect between plies under loading. These nanotube-coated fabric cloths serve as building blocks for the multilayered 3D composites, with the nanotube forests providing much-needed interlaminar strength and toughness under various loading conditions. For the fabricated 3D composites with nanotube forests, we demonstrate remarkable improvements in the interlaminar fracture toughness, hardness, delamination resistance, in-plane mechanical properties, damping, thermoelastic behaviour, and thermal and electrical conductivities making these structures truly multifunctional.
TL;DR: In this paper, the authors used X-ray diffraction rocking curves to provide direct evidence for highly oriented crystals at the critical buried interface between the polymer and the dielectric where the current flows in thin-film transistors.
Abstract: Thin films of polymer semiconductors are being intensively investigated for large-area electronics applications such as light-emitting diodes, photovoltaic cells and thin-film transistors. Understanding the relationship between film morphology and charge transport is key to improving the performance of thin-film transistors. Here we use X-ray diffraction rocking curves to provide direct evidence for highly oriented crystals at the critical buried interface between the polymer and the dielectric where the current flows in thin-film transistors. Treating the substrate surface with self-assembled monolayers significantly varies the concentration of these crystals. We show that the polymer morphology at the buried interface can be different from that in the bulk of the thin films, and provide insight into the processes that limit charge transport in polythiophene films. These results are used to build a more complete model of the relationship between chain packing in polymer thin-films and charge transport.
TL;DR: This work demonstrates the use of dual-beam focused ion beam–scanning electron microscopy to make a complete three-dimensional reconstruction of a solid-oxide fuel-cell electrode, and calculates critical microstructural features such as volume fractions and surface areas of specific phases, three-phase boundary length, and the connectivity and tortuosity of specific subphases.
Abstract: The drive towards increased energy efficiency and reduced air pollution has led to accelerated worldwide development of fuel cells. As the performance and cost of fuel cells have improved, the materials comprising them have become increasingly sophisticated, both in composition and microstructure. In particular, state-of-the-art fuel-cell electrodes typically have a complex micro/nano-structure involving interconnected electronically and ionically conducting phases, gas-phase porosity, and catalytically active surfaces. Determining this microstructure is a critical, yet usually missing, link between materials properties/processing and electrode performance. Current methods of microstructural analysis, such as scanning electron microscopy, only provide two-dimensional anecdotes of the microstructure, and thus limited information about how regions are interconnected in three-dimensional space. Here we demonstrate the use of dual-beam focused ion beam-scanning electron microscopy to make a complete three-dimensional reconstruction of a solid-oxide fuel-cell electrode. We use this data to calculate critical microstructural features such as volume fractions and surface areas of specific phases, three-phase boundary length, and the connectivity and tortuosity of specific subphases.
TL;DR: This letter reports, for the first time, on ultra-long single-crystal ZnAl2O4 spinel nanotubes fabricated through a spinel-forming interfacial solid-state reaction of core-shell ZnO–Al 2O3 nanowires involving the Kirkendall effect.
Abstract: There is a deep interest in methods to fabricate hollow nanocrystals for potential application as high-efficiency catalysts or drug-delivery agents. Tubular one-dimensional nanocrystals have been prepared for a wide variety of materials, including semiconductors1,2, metals3,4, ferroelectrics5,6 and magnetite7. They can be produced by rolling up layered materials or via an axial growth in a rolled-up form8,9,10, coating pores in templates11 or by eliminating the core of a core-shell nanowire1,7. The Kirkendall effect, a classical phenomenon in metallurgy12, was recently applied to explain the formation of hollow spherical nanocrystals13,14,15,16,17. Although the experimental demonstration and theoretical treatment mainly concern binary compounds and planar interfaces or nanoscale spherical interfaces, the fabrication route provided by the Kirkendall effect should be generic, and should also work for high-aspect-ratio hollow cylinders (that is, nanotubes) or even more complex superstructures. In this letter, we report, for the first time, on ultra-long single-crystal ZnAl2O4 spinel nanotubes (total diameter: ∼40 nm, wall thickness: ∼10 nm) fabricated through a spinel-forming interfacial solid-state reaction of core-shell ZnO–Al2O3 nanowires involving the Kirkendall effect. Our results simultaneously represent an extension of applying the Kirkendall effect in fabricating hollow nano-objects from zero-dimensional to multidimensional, and from binary to ternary systems.
TL;DR: The construction of a hydrogel entirely from branched DNA that are three-dimensional and can be crosslinked in nature is reported, allowing the hydrogels to be tailored for specific applications such as controlled drug delivery, tissue engineering, 3D cell culture, cell transplant therapy and other biomedical applications.
Abstract: DNA is a remarkable polymer that can be manipulated by a large number of molecular tools including enzymes. A variety of geometric objects, periodic arrays and nanoscale devices have been constructed. Previously we synthesized dendrimer-like DNA and DNA nanobarcodes from branched DNA via ligases. Here we report the construction of a hydrogel entirely from branched DNA that are three-dimensional and can be crosslinked in nature. These DNA hydrogels were biocompatible, biodegradable, inexpensive to fabricate and easily moulded into desired shapes and sizes. The distinct difference of the DNA hydrogel to other bio-inspired hydrogels (including peptide-based, alginate-based and DNA (linear)-polyacrylamide hydrogels) is that the crosslinking is realized via efficient, ligase-mediated reactions. The advantage is that the gelling processes are achieved under physiological conditions and the encapsulations are accomplished in situ-drugs including proteins and even live mammalian cells can be encapsulated in the liquid phase eliminating the drug-loading step and also avoiding denaturing conditions. Fine tuning of these hydrogels is easily accomplished by adjusting the initial concentrations and types of branched DNA monomers, thus allowing the hydrogels to be tailored for specific applications such as controlled drug delivery, tissue engineering, 3D cell culture, cell transplant therapy and other biomedical applications.
TL;DR: Here it is explained why In-containing (Al,In,Ga)N bulk films exhibit a defect-insensitive emission probability, and it is concluded that localizing valence states associated with atomic condensates of In–N preferentially capture holes, which have a positive charge similar to positrons.
Abstract: Group-III-nitride semiconductors have shown enormous potential as light sources for full-colour displays, optical storage and solid-state lighting. Remarkably, InGaN blue- and green-light-emitting diodes (LEDs) emit brilliant light although the threading dislocation density generated due to lattice mismatch is six orders of magnitude higher than that in conventional LEDs. Here we explain why In-containing (Al,In,Ga)N bulk films exhibit a defect-insensitive emission probability. From the extremely short positron diffusion lengths (<4 nm) and short radiative lifetimes of excitonic emissions, we conclude that localizing valence states associated with atomic condensates of In-N preferentially capture holes, which have a positive charge similar to positrons. The holes form localized excitons to emit the light, although some of the excitons recombine at non-radiative centres. The enterprising use of atomically inhomogeneous crystals is proposed for future innovation in light emitters even when using defective crystals.
TL;DR: Cationic core–shell nanoparticles that were self-assembled from a biodegradable amphiphilic copolymer offer advantages over liposomes, as they are easier to fabricate, and are more readily subject to modulation of their size and degree of positive charge.
Abstract: Non-viral gene-delivery systems are safer to use and easier to produce than viral vectors, but their comparatively low transfection efficiency has limited their applications. Co-delivery of drugs and DNA has been proposed to enhance gene expression or to achieve the synergistic/combined effect of drug and gene therapies. Attempts have been made to deliver drugs and DNA simultaneously using liposomes. Here we report cationic core-shell nanoparticles that were self-assembled from a biodegradable amphiphilic copolymer. These nanoparticles offer advantages over liposomes, as they are easier to fabricate, and are more readily subject to modulation of their size and degree of positive charge. More importantly, they achieve high gene-transfection efficiency and the possibility of co-delivering drugs and genes to the same cells. Enhanced gene transfection with the co-delivery of paclitaxel has been demonstrated by in vitro and in vivo studies. In particular, the co-delivery of paclitaxel with an interleukin-12-encoded plasmid using these nanoparticles suppressed cancer growth more efficiently than the delivery of either paclitaxel or the plasmid in a 4T1 mouse breast cancer model. Moreover, the co-delivery of paclitaxel with Bcl-2-targeted small interfering RNA (siRNA) increased cytotoxicity in MDA-MB-231 human breast cancer cells.
TL;DR: A clear relationship between the hysteresis and the middle eigenvalue of the transformation stretch tensor as predicted by the theory was observed for the first time and a new composition region of titanium-rich SMAs is identified with potential for improved control of SMA properties.
Abstract: Reversibility of structural phase transformations has profound technological implications in a wide range of applications from fatigue life in shape-memory alloys (SMAs) to magnetism in multiferroic oxides. The geometric nonlinear theory of martensite universally applicable to all structural transitions has been developed. It predicts the reversibility of the transitions as manifested in the hysteresis behaviour based solely on crystal symmetry and geometric compatibilities between phases. In this article, we report on the verification of the theory using the high-throughput approach. The thin-film composition-spread technique was devised to rapidly map the lattice parameters and the thermal hysteresis of ternary alloy systems. A clear relationship between the hysteresis and the middle eigenvalue of the transformation stretch tensor as predicted by the theory was observed for the first time. We have also identified a new composition region of titanium-rich SMAs with potential for improved control of SMA properties.
TL;DR: The phase inversion of particle-stabilized air–water systems, from air-in-water foams to water- in-air powders and vice versa is described, which has potential applications in the food, pharmaceutical and cosmetics industries.
Abstract: Small particles attached to liquid surfaces arise in many products and processes, including crude-oil emulsions and food foams and in flotation, and there is a revival of interest in studying their behaviour. Colloidal particles of suitable wettability adsorb strongly to liquid-liquid and liquid-vapour interfaces, and can be sole stabilizers of emulsions and foams, respectively. New materials, including colloidosomes, anisotropic particles and porous solids, have been prepared by assembling particles at such interfaces. Phase inversion of particle-stabilized emulsions from oil in water to water in oil can be achieved either by variation of the particle hydrophobicity (transitional) or by variation of the oil/water ratio (catastrophic). Here we describe the phase inversion of particle-stabilized air-water systems, from air-in-water foams to water-in-air powders and vice versa. This inversion can be driven either by a progressive change in silica-particle hydrophobicity at constant air/water ratio or by changing the air/water ratio at fixed particle wettability, and has not been observed in the corresponding systems stabilized by surfactants. The simplicity of the work is that this novel inversion is achieved in a single system. The resultant materials in which either air or water become encapsulated have potential applications in the food, pharmaceutical and cosmetics industries.