Advanced Materials

Recent advances in the burgeoning field of hydrogen-storage systems based on nanostructured materials are reviewed. In particular, metal hydride alloys, alanates, borates, inorganic nanotubes, metal–organic framework materials, and mesoporous titanium oxides are examined with regard to their potential performance as hydrogen-storage materials for use in fuel cells.

Lasing via a phase retardation defect mode is reported for the first time. The Figure shows the far-field pattern of laser emission realized by inserting an anisotropic nematic liquid crystal (LC) defect layer between polymer cholesteric LC layers. The system mimics the cuticle of Plusiotis resplendens, a beetle, using the birefringence of the anisotropic layer to achieve reflectance greater than the 50 % provided by simple cholesteric LC photonic bandgaps.

A heptafluorene lightly doped with monodisperse conjugated oligomers is used for the first demonstration of organic polarized (see Figure) light-emitting diodes utilizing Förster energy transfer. Emission of blue-green, green, red, and white light is accomplished with a turn-on voltage of < 4 V, peak polarization ratios of up to 26, integrated polarization ratios of up to 19, and luminance yields of up to 6.4 cd A^–1.

Fabrication of composite supraparticles and “Janus” particles via a novel, generic, gel trapping technique is presented. The method is based on templating particle monolayers at air–water or oil–water interfaces, followed by lifting of the particles with poly(dimethylsiloxane) and gold deposition. Replicating ordered colloid monolayers of repulsive particles at liquid surfaces allows fabrication of microporous surfaces and composite supraparticles (see Figure).

Tuning of a dye-doped cholesteric liquid crystal mirrorless laser in the violet– ultraviolet wavelength range is achieved using two different strategies for controlling the periodicity of the cholesteric helical structures. Both methods are shown produce a shift of the lasing wavelength by 30–40 nm, and smooth variation of the helical pitch can be achieved (see Figure).

A novel approach to fabricating multicomponent nanostructures is demonstrated. Atomic polarization in ferroelectric perovskites is manipulated to control electronic structure and local chemical reactivity (see Figure). Several paradigms for ferroelectric domain patterning are presented. Complex structures consisting of semiconductors, metals, and functional organic/biological molecules are produced.

Size-tunable silica nanotubes have been fabricated using a novel reverse-microemulsion-mediated sol–gel method (see Figure). This facile approach is adaptable to large-scale fabrication, and the diameter of silica nanotubes is tunable through the use of different apolar solvents. The photoluminescence spectrum of the nanotubes displays three emission bands, including a stable and strong blue light emission at 435 nm.

Crystalline organization of a methanofullerene, [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM), as achieved in various thin-film deposition techniques, is reported. Mechanically stable, and thus self-supporting, thin films obtained via fast solvent evaporation techniques are found to be composed of densely and homogeneously distributed PCBM nanocrystals with various crystallographic orientations, as shown schematically in the Figure.

Poly(3,4 ethylene dioxythiophene)/poly(styrene sulfonate) (PEDT/PSS) blends are widely used in organic light-emitting diodes (OLEDs) to enhance their performance. Using neutron reflectometry to study such blends, it is shown that substantial segregation of the blend components occurs at the surface (see Figure), a result that will have implications both for the performance and for the stability of optoelectronic devices using PEDT/PSS.

Introducing insulating phases into non-aqueous salt solution leads to remarkable conductivity enhancements, an effect ascribed to breaking up of ion pairs. This new class of electrolytes combines the high ionic conductivities of liquids and beneficial mechanical properties of soft matter (“soggy sand”; see Figure).

A new method for the parallel fabrication of carbon nanotube (CNT) electronic devices is described. Self-assembly via magnetic entrapment exploits magnetic fields generated within micrometer- or nanometer-scale devices for the controlled deposition of susceptible species onto predefined contacts (see Figure). The fabrication of carbon nanotube field-effect transistors (CNFETs) using this method are discussed.

A novel zeolite–ceramic composite monolith with a hierarchical bimodal (nano-, macroscale) pore system (see Figure) has been prepared by coating the internal cell walls of microcellular polymer-derived ceramic foams (cell size ∼ 8 μm) with a thin layer of MFI-type zeolite. Applications are envisaged as microreactors and microseparator devices for catalysis and adsorption/ separation because of the material's unique microstructural design.

Poly(phenylenevinylene) polymers, functionalized with either amine or carboxyl acid groups (see Figure), have been synthesized and assembled using layer-by-layer deposition. This hydrogen-bonding-driven self-assembly produces conjugated polymer multilayer films with molecular-level controlled thickness and architecture. The approach enables the creation of robust thin films via a thermally induced cross-linking reaction.

Diverse chain foldings of a semi-conjugated polymer are examined with submolecular resolution and molecular models (see Figure) are proposed. The ordering process observed directly supports the hypothesis that the polymer would show a preference to adsorb “straight” on a nanotube surface. The results provide important reference for the interaction mode between polymers and nanotubes.

Oriented ZnS nanobelt arrays and ZnS multicore microcables consisting of oriented nanowire bundles with sheaths (see Figure) have been synthesized via a controlled thermal process. Both the ZnS nanowires and nanobelts are single crystals grown along the [001] axis. The special structures of the oriented assemblies of ZnS one-dimensional nanostructures may have potential applications in nanoelectronics and photonics.

Thin films of organic metals generally exhibit activated conductivity caused by intergrain conduction barriers. One of the few examples of thin films showing truly metallic character is TTF[Ni(dmit)₂]₂. Films electrodeposited on silicon substrates remain metallic down to 12 K in spite of their polycrystalline morphology (see Figure).

Fluoropolymer composite films with nanoporous morphology and dielectric constants less than 2.0, consisting of plasma-polymerized allylpentafluorobenzene (pp-APFB) nanospheres and magnetron sputter-deposited poly(tetrafluoroethylene) (s-PTFE) dense layers, have been prepared on hydrogen-terminated Si(100) (H-Si) substrates (see Figure).