Advanced Functional Materials

Semiconductor nanowires with triangular cross-sections (see Figure) have been fabricated via a “top–down” process. Photoresist stripes are patterned on a surface, followed by anisotropic chemical etching. Then, continuous etching results in the formation of nanowires, which can be as long as the photoresist stripes. The wires can be transfer printed onto plastic substrates for potential use as large-area flexible electronics components.

As Advanced Functional Materials enters its fifth year, the Editor describes the rapid growth of the journal in every sense, and how it is changing to adapt to its ever-expanding author- and readership.

Blue-light-emitting para-hexaphenyl organic microrings and mutually aligned nanofibers have been investigated (see Figure; the left nanofiber is 530 nm wide). The morphology and optical properties of individual aggregates and dense bunches of aggregates can be studied. These aggregates, whose morphology and alignment can be controlled by the appropriate growth conditions and substrate surfaces, form excellent model systems for the study of sub-wavelength optics, and may become key elements of new optoelectronic devices.

A novel method of fabricating large films of photonic crystals of colloids is reported. A charge-stabilized colloidal suspension is run in a flat capillary driven by a pressure-regulated air pulse. Above a critical pressure, the capillary was filled with a single-domain texture showing angle-dependent diffraction colors and a distinct laser diffraction pattern (see Figure). This method is easily applicable to industrial processes.

Semiconductor nanowires with triangular cross-sections (see Figure) have been fabricated via a “top–down” process. Photoresist stripes are patterned on a surface, followed by anisotropic chemical etching. Then, continuous etching results in the formation of nanowires, which can be as long as the photoresist stripes. The wires can be transfer printed onto plastic substrates for potential use as large-area flexible electronics components.

A comparison between UV-visible reflectance spectra for TiO2 nanocolloid particles Degussa P25 TiO2 nanopowder, and their nitrided analogs (see Figure) is described. The nitridation of the nanocolloid which can be accomplished in seconds at room temperature provides a visible light absorbing photocatalyst.

The method of “line patterning” for producing transparent conducting-polymer patterns employs standard office equipment such as a laser printer and commercial overhead transparencies. A seven-segment liquid-crystal display (see Figure) and an 11-key keypad, prepared from a common conducting-polymer blend, are described in detail.

Aligned single crystal SnO₂ tubes (see Figure) with square-shaped cross sections of tunable size (50 nm to 0.5 μm) are synthesized on quartz substrates using a combustion chemical vapor deposition (CVD) method in an open atmosphere at 850 °C to 1150 °C.

Several ZnS nanostructures, such as nanorods (see Figure), nanowires, nanobelts, and nanosheets have been synthesized simultaneously in bulk quantities by the evaporation of ZnS nanopowders. Control of the morphology of ZnS nanostructures on substrates is achieved by regulating the substrate temperature and catalyst.

Ag-functionalized carbon molecular sieve membranes are characterized by high selectivity and high permeability for gas separations. The membranes (see Figure), prepared from a blend of sulfonated poly(ether ether ketone) with Ag⁺ as a counterion and P84 co-polyimide, possess selective bypasses around Ag-nanoclusters, where separation takes place based on the fast and selective mechanism of selective surface flow.

Surface-mediated molecular ordering (see Figure) has a substantial effect on the preferential orientations of the P3HT chains, and this effect is used to enhance the field-effect mobility. Depending on the surface properties, the P3HT nanocrystals can adopt two different orientations – parallel and perpendicular to the insulator substrate – the field-effect mobilities of which differ by more than a factor of 4 and can reach 0.28 cm² V^–1 s^–1.

Adhesion and proliferation of osteoblasts on polyelectrolyte multilayer (PEM) films (see Figure) is enhanced by crosslinking, even for films functionalized with the RGD cell-binding peptide sequence. Cell attachment after short times was enhanced by the RGD sequence, but long-term proliferation (after 8 days) was greatly improved by crosslinking, demonstrating the influence of both the chemical and the mechanical properties of the films on cell adhesion.

The three-dimensional-solar-cell concept is explored using a new approach. A nanocomposite consisting of a wide-bandgap n-type semiconducting oxide (nanocrystalline TiO₂) and a p-type visible-light-sensitive semiconductor (CuInS₂), mixed on a nanometer scale (see Figure), is used as the active material.

Poly(methyl methacrylate)/single-walled carbon nanotube (PMMA/SWNT) composites were prepared via in-situ polymerization induced by heat, UV-light irradiation, and ionizing (gamma) radiation. The composites were dissolved in methylene chloride and then cast into films. The composites (Figure, left) exhibit enhanced transparency compared with the melt-blended composite material (right), and are comparable to neat PMMA (center).

A novel garnet-like structure Li₆ALa₂Ta₂O₁₂ (A = Sr, Ba) exhibits fast, lithium ion conductivity and is found to be stable against chemical reactions with molten metallic lithium. Li₆BaLa₂Ta₂O₁₂ shows the highest ionic conductivity of 4.0 × 10^–5 S cm^–1 at 22 °C (see Figure). The direct current electrical measurements using lithium-ion blocking and reversible electrodes reveal that the electronic conductivity is very small and exhibits high electrochemical stability (≥6 V/Li) at room temperature.

Hollow silica nanospheres are synthesized by surface-initiated atom transfer radical polymerization (ATRP) of 3-(trimethoxysilyl)propyl methacrylate (TMSPM) from templating poly(vinylbenzyl chloride) (PVBC) latex nanoparticles, followed by polycondensation of tetraethoxysilane (TEOS) in ethanolic ammonia and removal of the PVBC cores by thermal decomposition (see Figure).

Biocompatible and biodegradable porous scaffolds can be synthesized by polymerization of a high-internal-phase emulsion (HIPE). The structure of an oil-in-water emulsion of a vinyl- functionalized biopolymer, such as a polysaccharide or a gelatin, is locked-in by radical-initiated polymerization, forming a highly interconnected, mechanically stable porous structure (see Figure). The size of the pores can be varied, permitting growth of murine retinal cells on and throughout the matrix.

Photoalignment films are an alternative to the rubbing technique currently used in the production of liquid-crystal displays. The photo-induced aligning capability (see Figure) of various ladder-like polysiloxane-based photo-alignment films bearing different photo reactive side chains is reported, particular attention being paid to the influence of the side-chain structure and irradiation conditions.

Gadolinium nanoparticles deposited via an inert gas evaporation method show improved stability towards oxidation. No Pd catalytic layer is required for the hydrogenation of the nanoparticles at room temperature and atmospheric pressure. The confirming resistivity measurements are shown in the Figure.

Chiral-nematic films in combination with classical circular polarizers are used as additional layers in organic light-emitting diodes (see Figure) to improve the daylight contrast while preserving the emitted light. Increased light efficiency at the expense of some daylight contrast is achieved within the reflection band of the films. Outside the band, the daylight contrast achieved is infinite.

Four-coordinate boron compounds with the general formula of BPh₂(L) (L is the N,N′-chelate ligand) and emission colors that cover the entire visible spectra are achieved and their uses in electroluminescent devices are demonstrated. The Figure shows the photoluminescence spectra of representative boron compounds.

Energy-transfer processes in optoelectronic devices consisting of conjugated organic materials are usually described in the framework of the Förster model, which relies on a point-dipole approximation. The failure of this model for densely packed films is illustrated by comparing the transfer rates to excited states characterized by various degrees of localization (see Figure).

The surface modification of TiO₂ nanostructures to incorporate nitrogen and form visible light absorbing titanium oxynitride centers is studied. Freshly prepared samples exhibited a strong emission near 560 nm (see Figure) (2.21 eV), which red shifts to 660 nm (1.88 eV) and drops in intensity after atmospheric exposure. Electron spin resonance studies identified a resonance at g = 2.0035, which increased significantly with nitridation.

Diluted magnetic semiconductor materials of type Cd_1–xMn_xS have been incorporated into the pores of mesoporous thin-film (MTF) host structures. The guest species were prepared by a wet-impregnation technique using an aqueous solution of metal acetates. Photoluminescence excitation measurements were used to show the quantum size effect exhibited by the incorporated nanostructured guest species.