Advanced Materials

The current–voltage characteristics of triethylsilylethynyl anthradithiophene (TES ADT) thin-film transistors can be improved dramatically by a simple and straightforward solvent-vapor annealing process after device fabrication. Exposing the transistors to dichloroethane vapor for 2 min induces structural rearrangement and crystallization of TES ADT (see figure and cover). This procedure results in drastic increases in on currents; the saturation mobility increases by two orders of magnitude and the current–voltage hysteresis is largely eliminated.

A novel interpenetrating electrode structure is synthesized by sequential assembly of nanostructured components throughout the interconnected macropores of an inverse opal carbon monolith. Separation of electrodes by a thin polymer electrolyte film prevents hard shorts and permits intercalation and shuttling of Li ions between electrodes (see figure and inside cover).

A terahertz all-optical modulator is realized by combining the surface-plasmon-related extraordinary optical transmission phenomenon of subwavelength hole arrays and the transient properties of molecules. As shown in the figure, transmission of the signal beam (red) is modulated after excitation of the hybrid structure by the pump beam (green). This modulator may lead to ultrafast all-optical active plasmonic devices.

An epitaxially grown Na_xCoO₂ film can be peeled off from its α-Al₂O₃ substrate by dissolving the interfacial layer between the film and the substrate in water, as shown in the figure. The freestanding film thus formed can be pasted on an amorphous glass substrate and used for growing an epitaxial ZnO film. This method can be used to impart atomic regularity to amorphous glass, polymer, and metal surfaces.

Composites of poly(dimethylsiloxane) and gold nanoparticles (see figure) with an adsorbed monolayer of 1–alkanethiols afford a versatile reversible color system through treatment with solvents that swell the polymer. Even dichroism can be created, which can be converted again to optically isotropic states. Upon heat treatment irreversible thermochromic effects emerge.

Ferroelectric single–crystalline PbZr_0.2Ti_0.8O₃ thin films, free from extended defects, are grown by pulsed laser deposition onto vicinal SrTiO₃(001) single crystals. The PbZr_0.2Ti_0.8O₃ films are strained and exhibit enhanced tetragonality, c/a ≈ 1.06. They have a remnant polarization, P_r ≈ 110 μC cm^–2, dielectric constant, ϵ₃₃ ≈ 90, and piezoelectric coefficient, d₃₃, up to 50 pm V^–1 (see figure).

Close-packed and non-close-packed colloidal photonic crystals of silica spheres have been imaged in real space to reveal their 3D structure. Although the lattice spacings are of the order of a micrometer, these crystals can also be characterized in reciprocal space using small-angle X-ray scattering (see figure). After infiltration with silicon (see inset), the internal 3D structure of these photonic crystals can only be probed using X-ray scattering.

Aptamer-linked smart materials responsive to a combination of any two analytes, including adenosine, cocaine, and K⁺, are demonstrated. The effect of small molecules on the properties of the nanomaterials can be either highly cooperative or non-cooperative, which can be described as logic gates with chemical inputs and colorimetric output (see figure).

The self-healing anticorrosion effect of layer-by-layer (LbL) assembled nanoreservoirs (polyelectrolyte-coated nanoparticles) embedded in a hybrid coating deposited onto an Al alloy is investigated (see figure). The corrosion inhibitor, benzotriazole, is entrapped in the polyelectrolyte at the LbL assembly step; its release is initiated by local pH changes near the corrosion-damaged zones in the alloy. The nanoreservoirs increase long-term corrosion protection of the substrate and provide effective inhibitor storage and its prolonged release on demand.

Air-stable n-channel organic transistors are fabricated using a newly synthesized soluble fullerene derivative. The air-stable nature of this molecule allows the realization of complementary circuits under ambient conditions without encapsulation. As shown in the figure, the I–V characteristics of the devices are retained even after exposure to air for a week. To the best of our knowledge, this is the first demonstration of an air-stable electron-transporting fullerene-based molecule.

UV random lasing in p–i–n ZnO-based heterojunction diodes is achieved. The UV emission originates from the use of an intrinsic ZnO–SiO₂ nanocomposite layer; the use of ZnO powders can improve the electrical-to-optical conversion efficiency of the heterojunction. The patterned ZnO clusters in the SiO₂ matrix enhance the quality of the random media (see figure) thus sustaining the random lasing action. If low-index, p-doped, wide-bandgap materials are used as the hole-injection layer, strong coherent random lasing could be achieved.

High-aspect-ratio vertically aligned carbon nanofibers are functionally transferred from a growth substrate suitable for high-temperature processing to a substrate with electronic functionality. Nanofiber arrays are partially embedded within a polymer membrane that is then separated from the growth substrate and transferred to a new substrate that provides electrical interfacing to discrete nanofiber elements (see figure).

Highly pure membranes of double–walled carbon nanotubes (DWCNTs; see figure) have been prepared by combining a purification process with the competition characteristics of water and ethanol interactions with the surface of CNTs. The resulting ultrathin films consist of a single layer of CNTs and they show an enhanced tensile strength. The method can also easily be expanded to other types of nanotubes.

Micromembranes integrating nanometer-scale Pd films (see figure) with complex oxide methanol-reforming catalysts demonstrate high-purity hydrogen generation. Washcoating of an 8:1 LaNi_0.95Co_0.05O₃/Al₂O₃ catalyst serves the dual purposes of preventing thin-film disintegration upon direct exposure to methanol while providing improved selectivity and conversion of methanol to hydrogen through partial oxidation.

Macroporous, free-standing carbon thin films with a perfectly defined number of pore layers are shown to be obtainable by applying a chemical vapor infiltration process to a colloidal crystal template. Raman spectroscopy indicates that the deposited carbon is partially graphitizable, and the usefulness of these membranes as electrodes is demonstrated. The membrane in the figure is approximately 4.3 μm thick.

Stable and continuous gold-coated fibrous mats, which may be used as electrodes for electrochemical experiments or for fabrication of electronic devices, have been fabricated. The formation of conductive gold films on flexible electrospun poly(methyl methacrylate) ultrafine fibrous mat substrates (see figure) is described, and the positive effect of heat treatment demonstrated.

An atomically flat pentacene monomolecular layer remarkably improved the crystallinity of C₆₀ films, thus enhancing the field-effect mobilities in C₆₀ transistors (FETs) (see figure). They showed a four to five times better performance over devices with C₆₀ films grown without a pentacene buffer. Molecular-wetting-controlled substrates can thus offer a general solution to the fabrication of high-performance crystalline organic devices.

Nanoindentation experiments are used to fabricate in a controlled manner arrays of micrometer and sub-micrometer magnetic structures at the surface of nonmagnetic, atomically ordered, intermetallic sheets with composition Fe–40 % Al (at %). The figure shows atomic (top) and magnetic (bottom) force microscopy images of these fabricated structures. The ferromagnetism arises from the deformation-induced atomic disorder in the intermetallic material.

The current–voltage characteristics of triethylsilylethynyl anthradithiophene (TES ADT) thin-film transistors can be improved dramatically by a simple and straightforward solvent-vapor annealing process after device fabrication. Exposing the transistors to dichloroethane vapor for 2 min induces structural rearrangement and crystallization of TES ADT (see figure and cover). This procedure results in drastic increases in on currents; the saturation mobility increases by two orders of magnitude and the current–voltage hysteresis is largely eliminated.

Organization of flakes into hollow, curved architectures (see figure) by means of an “aggregation-then-growth” process is described, providing in addition a fine example of a V³⁺ nanostructure. The new-phased VOOH hollow “dandelions” are shown to increase very effectively the capacity of lithium-ion intercalation/deintercalation in batteries, especially at low temperatures.

Ultrathin layers of the versatile spinel oxide NiFe₂O₄ can be conductive or insulating depending on the growth conditions. Conductive NiFe₂O₄ (NFO) electrodes are inserted into conventional magnetic tunnel junctions containing La_2/3Sr_1/3MnO₃/SrTiO₃ (LMSO/STO) bilayers_, and insulating and ferrimagnetic NiFe₂O₄ layers are used as barriers to define a spin filter (see figure). The ability of such heterostructures to highly spin-polarize electrons injected from a non-magnetic electrode is demonstrated.

A very large drop in dielectric constant upon application of small magnetic fields is observed at room temperature for LuFe₂O₄ (see figure). Such behavior is unprecedented and indicates a strong coupling of spins and electric dipoles at room temperature. This behavior of LuFe₂O₄ is apparently related to its ferroelectricity, which occurs through the highly unusual mechanism of Fe²⁺ and Fe³⁺ ordering.

Using water-assisted chemical vapor deposition, aligned CNT–ZnO (CNT: carbon nanotube) heterojunctions are formed on a Zn substrate. The aligned CNT heterojunctions with intimately connected ZnO particles/rods show interesting optoelectronic properties of practical significance. This cost-effective and efficient approach can be scaled up for the production of large-area heterojunction arrays (see figure).

The hydroxyl end group of poly(vinyl pyrrolidone) (PVP) has been utilized for the first time to reduce AgNO₃ in water, generating the Ag triangular nanoplates shown in the figure via kinetic control. PVP is widely used in the chemical synthesis of many types of colloidal nanocrystals, where it is mainly considered as a steric stabilizer or capping agent that protects the product from agglomeration. It is believed that this new strategy can be extended to other noble metals and polymers.

A novel interpenetrating electrode structure is synthesized by sequential assembly of nanostructured components throughout the interconnected macropores of an inverse opal carbon monolith. Separation of electrodes by a thin polymer electrolyte film prevents hard shorts and permits intercalation and shuttling of Li ions between electrodes (see figure and inside cover).

A photopolymerized nematic liquid-crystal gel, which is visible-light absorbing and electron-donating, is used to template a nanoporous surface. This forms a distributed interface to an overlying electron acceptor in the device. An electron-blocking layer lies between the gel and bottom electrode (see figure). An open-circuit voltage of 1.1 V and a power conversion efficiency of 0.6% are obtained for a monochromatic irradiance of 45 mW cm^–2.

A phage display method has been used for the first time to identify peptides that bind to, and induce the rapid formation of, a pure crystalline binary metal oxide compound, CaMoO₄, at room temperature from a soluble aqueous precursor solution (see figure). This demonstration opens the door to the biosculpting (peptide patterning, then localized peptide-induced mineralization) of functional synthetic crystalline multicomponent compounds onto or with low-temperature or chemically dissimilar materials.

An energy-storage device consisting of polypyrrole (pPy) doped with indigo carmine (IC) and 2,2'-azinobis (3-ethylbenzothiazoline-6-sulfonate) (ABTS) has been fabricated. These redox-active conducting polymers (see figure) form the basis of a battery that depends on the faradaic reactions of the redox-active dopants, and performs better than conventional batteries and ultracapacitors at high power density.

Efficient and almost pure white-light emission is obtained by combining singlet and triplet emission in a single-polymer device. The polymers used in this work include a benzothiadiazole group attached to a fluorene backbone and an iridium complex attached to the side chain (see figure). White-light emission with CIE color coordinates of (0.32, 0.44) and a luminance efficiency of 6.1 cd A^–1 is obtained.