Advanced Functional Materials

Carbon nanotube (CNT) interconnections have been made by electron-beam-induced deposition of amorphous C, using a scanning tunneling microscope inside a transmission electron microscope. CNT structures (simple tube/tube connections, crossed and T-junctions, zigzag structures, and nanotube networks; see Figure and cover), have been successfully made with a high degree of control; electrical and mechanical properties are measured in situ.

Macroscopic mesoporous CdS and CdTe thin films have been templated by lyotropic liquid crystals with extensive hydrogen bonding and nanoscale hydrophilic compartments (see Figure). These nanotemplated semiconductor films contain a hexagonal array of 2.5 nm pores, 7 nm center-to-center, that extend in an aligned fashion perpendicular to the substrate.

Polyether-grafted ZnO nanoparticles with tunable and stable photoluminescence have been synthesized and isolated. The resulting gels can dissolve lithium salts to form stable, solid polymer electrolytes (SPEs, see Figure) with superior properties compared with the traditional composite SPEs.

The superconductors NbTi and Nb₃Sn have been synthesized directly from oxide precursors (Nb₂O₅–TiO₂, Nb₂O₅–SnO₂) by electro-deoxidation, where the oxygen in the oxide is ionized and dissolves in a molten salt. The microstructure of Nb₃Sn is shown in the Figure. The superconducting transitions of the materials produced by electro-deoxidation are similar to those of superconductors produced by alloying the metallic elements.

Cell-independent, controlled engineering of biomimetic scaffolds can be achieved by rapid fluid removal from hyperhydrated collagen gel via plastic compression. Mechanically strong native collagen with controllable nano- and microscale biomimetic structures (see Figure) can be produced over a timescale of minutes rather than the conventional days and weeks, providing promise for rapid production of biomaterials and patient-customized tissues.

Polysaccharide multilayer film degradation can be tuned via control of film crosslinking and component molecular weight. Studies performed in vitro and in vivo confirm that films of chitosan/hyaluronan layers are more resistant to enzymatic and macrophage degradation when crosslinked (see Figure: right panel, increased crosslinking, scale bar 110 μm) or formed with medium-molecular-weight chitosan. Applications as biodegradable coatings are envisaged.

Rubrene and α-naphthylphenylbiphenyl diamine can form a uniform, fused organic solid solution (FOSS) after processing at high pressures and temperatures. FOSSs with different dopant concentrations show different thermal behavior (see Figure). An interesting, two-melting-point phenomenon can be found when the concentration of rubrene is between 2 and 20 %. Organic light-emitting diodes with single-color and white-light emission have been conveniently fabricated using this method.

One-dimensional bundles of ZnSe nanowires have been successfully synthesized by the thermal treatment of a ribbon-like precursor at 600 °C (see Figure), which was prepared by a mixed solvothermal route. The cooperative action of the mixed solvents may be responsible for the morphology of the resulting products.

Optically active polyaniline thin films (see Figure) have been prepared electrochemically on indium tin oxide substrates in the presence of aniline oligomers. The electrochemical synthesis is enantioselective, and the films produced have homogeneous thicknesses.Varying the experimental parameters allows tuning of the optical activity, adsorption properties, crystallinity, and morphologies of the polyaniline thin films.

Deep-blue organic light-emitting diodes have been realized using new blue-light-emitting materials that contain an anthracene molecule as the main backbone and 1,2-diphenylstyryl and tetraphenylsilane as the side units. The electroluminescence spectra of the non-doped devices show a narrow emission band, almost perfectly matching the “standard” blue (see Figure). Thin films of the material are of high quality and thermally stable.

Poly(imide-siloxane)s with controllable surface energy are used as gate dielectrics to investigate the effects on pentacene morphology and field-effect- transistor characteristics. Three-dimensional growth of pentacene and interconnection between pentacene grains is observed on a low-surface-energy dielectric (see Figure, width = 2.5 μm), leading to a high-mobility pentacene field-effect transistor.

Nanopapillae of TiO₂ have been produced on the surface of kaolinite particles (see Figure) via a hydrothermal method. The wettability of the nanopapilla particles in silicone oil shows a marked improvement compared to that of kaolinite, resulting in a high electrorheological activity. This improvement may be attributed to enhanced dielectric properties of the nanopapilla-particle fluid.

Copper grids with controlled pore sizes (90–260 nm) are fabricated via electroless deposition with colloidal-crystal films providing the template. Films with 100 nm pores, such as that shown in the Figure, are flexible and can be rolled. Three-dimensionally ordered porous copper materials have also been prepared using a similar method.

Carbon nanotube (CNT) interconnections have been made by electron-beam-induced deposition of amorphous C, using a scanning tunneling microscope inside a transmission electron microscope. CNT structures (simple tube/tube connections, crossed and T-junctions, zigzag structures, and nanotube networks; see Figure and cover), have been successfully made with a high degree of control; electrical and mechanical properties are measured in situ.

Excellent dispersion of single-walled CNTs in poly(ethylene oxide)—manifested in the sigmoidal dependence of the reduced viscosity (η_r*) and its dramatic increase with increasing CNT content (see Figure)—using a lithium-based anionic surfactant is reported. Fitting to a percolation model implies a percolation threshold of 0.09 wt.-% and an effective CNT aspect ratio of ≥650.

Light out-coupling efficiency is an important parameter that needs to be optimized for the full exploitation of organic light emitting diodes (OLEDs; see Figure). OLED designs optimized for light out-coupling for three different light-emitting material systems are theoretically investigated, and the effect of photoluminescence quantum yield (PLQY) is examined. The high efficiency of dendrimer-based OLEDs is due to their high PLQY.

Hollow tin dioxide microspheres (see Figure) can be synthesized by simple heat treatment of a mixture composed of tin chloride and resorcinol–formaldehyde gel. The microspheres can be utilized as lithium-battery anode material, since they exhibit high discharge capacities and high coulombic efficiency, and the simple sphere-synthesis method is applicable to a broad range of materials, including zirconia and ceria.

Porous silicon multilayer structures (see Figure) with pore sizes tunable from 20 to 120 nm have been electrochemically synthesized from n+ silicon substrates. A macroporous silicon microcavity with a stable, uniform macroporous structure fabricated using this technique is used as an optical biosensor for detecting macromolecules, thus opening the door for large-molecule sensing applications with porous silicon.

Zintl phases are candidates for efficient thermoelectric materials. Chemical tuning of the electronic and thermal transport properties of Zintl phase Ca_xYb_1–xZn₂Sb₂ (see Figure) is demonstrated. Increasing x transforms the material from a metal to a semiconductor, while intermediate compositions exhibit reduced thermal conductivity and therefore enhanced thermoelectric figures of merit.

Quantum dot devices: Optimization of the electroluminescence efficiency of near-infrared light-emitting PbS–MEH-PPV large-area, solution-cast nanocomposite devices is reported. An external electroluminescence quantum efficiency of 0.27% is obtained (see Figure), corresponding to an internal efficiency of 1.9%. The best devices are limited by the internal radiative efficiency of the nanocrystals (NCs).

A Ge-doped methacrylate hybrimer shows photosensitivity that depends on the wavelength of the illuminating light. Short-wavelength light produces concave shapes (see Figure) mainly owing to photopolymerization and Ge-related densification. A convex shape is fabricated by long-wavelength UV irradiation, as only photomigration occurs. This may lead to fabrication of micro-optical devices without the need for etching or developing steps.

Composite nanowires of liquid crystalline epoxy/polyaniline (LCE/PANI) nanorods (see Figure) have been prepared by a temperature-gradient method using an anodic aluminum oxide (AAO) membrane as the template. The fabricated composite nanowires show improved electrical and thermal properties over the LCE/PANI composite monolith. The synthetic methodology provides a convenient way of producing thermoset nanomaterials using an AAO template.

Quasi-aligned Eu²⁺-doped wurtzite ZnS nanowires (see Figure) with novel optical properties can be synthesized using a vapor-deposition method. The doped nanowires have a modulated composition and crystal structure and a high density of defects. The nanowires exhibit a high initial luminescence intensity that is long-lasting and of a suitable emitting color (green) for applications in displays and nanodevices.