Advanced Materials

Recent achievements in the synthesis of inorganic nanomaterials inside the spatially confined volume of individual micro- and submicroreactors (emulsions, micelles, organized thin films, polyelectrolyte capsules; see Figure) are presented. Particular attention is paid to polyelectrolyte capsules as confined microreactors with controlled permeability of the shell and shell engineering on the nanolevel for the tailoring of different functionalities.

[Ca₂₄Al₂₈O₆₄]⁴⁺(4e^–) is a new inorganic electride, which is air-stable even at ∼ 300 °C. Its electron field-emission properties originate from electron anions encaged in subnanometer-sized cages embedded in its crystal structure, and the emission current is controlled by Fowler–Nordheim field emission at high electric fields (see Figure). Field-emission display devices demonstrate bright light emission clearly visible under ambient light (inset).

β-Glucuronidase enzymes have been attached to a porous silicon surface through a direct silicon–carbon bond based linking system (see Figure). The attached enzymes display high activity and the photoluminescent (PL) properties and surface stability of the porous silicon are retained. Quenching of the PL is observed upon enzymatic breakdown of the substrate, leading to the possibility of a new class of chemical and biological sensors.

A maskless method for patterning substrates using surface relief structures on azobenzene-functionalized polymer is demonstrated. Surface relief structures are inscribed on a polymer film spin-coated on an indium tin oxide (ITO) substrate by exposure to an interference pattern. Etching with oxygen plasma followed by a zinc-powder-catalyzed liquid etchant yields a periodically patterned ITO substrate (see Figure).

A fabrication method for submicrometer-wide electric conductive patterns using Ag nanoparticle/polymer composites on furan-functionalized substrates (see Figure) is reported. A 180 nm wide (full width at half maximum) photofabricated pattern has been achieved, well below the wavelength of incident light. After annealing at 200 °C for 40 min, the pattern worked as an electric wire with a resistivity of ∼8 × 10^–4 Ω cm.

Poly(ethylene glycol) (PEG)-covered hybrid particles (see Figure) of calcium phosphate (CaP), and plasmid DNA (pDNA) have been prepared in a size-controllable manner in the presence of a block copolymer. The particles show a substantial colloidal stability as well as tolerance of entrapped pDNA against nuclease attack in the medium, and achieve a significantly higher gene expression compared to conventional precipitates.

Thin films of asymmetric diblock copolymers have been used as scaffolds to define an ordered array of nanometer-scale reaction vessels in which high density arrays of silicon oxide nanostructures (see Figure) are produced by exposure to silicon tetrachloride. Such site-specific silicon oxide nanostructures could have widespread uses for sensory and optoelectronic applications.

A new synthesis of the silicon-network-backbone polymer poly(methylsilyne) gives a material that is easily converted by pyrolysis to smooth continous films of stoichiometric silicon carbide (see Figure). The films are adherent to the silicon or alumina substrates, and show root mean square roughness of 169 Å over a 500 μm range. Applications in electronics are envisaged.

In nature, peptides bind metals reproducibly and selectively via specific amino acid sequences, producing nanocrystals in controlled sizes and morphologies. A novel method of growing uniform nanocrystals directly on biological nanotubes by immobilizing synthetic Ni-mineralizing peptides on the nanotubes is reported (see Figure). Control of the Ni nanocrystal size is a function of pH, enabling tunability of magnetic properties in the resulting nanotube.

Single-crystal Sb₂S₃ nanotubes with chain-like structures (see Figure) have been successfully synthesized by chemical vapor transport using sulfur as the transport agent. Detailed characterization of the nanotubes shows the growth direction of Sb₂S₃ nanotubes is determined by the crystallographic orientation of the chain-like building blocks. A mechanism explaining the formation of the nanotubes is presented.

A novel host–guest system leading to enhanced luminescence (see Figure) is investigated using a self-forming, solid-state solution of a conjugated polymer. Poly(fluorene-alt-thiophene)s are synthesized with varying ratios of 2,5- and 3,4-thiophene linkages. Small quantities of 2,5-linkages give rise to systems in which a matrix of 3,4-thiophene-linked conjugated polymer transfers electronic excitation energy to isolated, emitting 2,5-thiophene-linked conjugated segments.

A new, robust technique for preparing highly selective molecularly imprinted films (see Figure) is presented. Spin-coating is used to spread a pre-polymerization mixture on a substrate, which is subsequently cured with UV light. Important aspects of this technique include the use of a novel polymeric porogen dissolved in a low-volatility solvent to generate porous morphologies with enhanced binding capacities.

Conducting polypyrrole-coated structures with lengths of several hundreds of micrometers and diameters varying from several micrometers to less then 150 nm have been formed using biomembrane templates (see Figure). The nanotubes can be attached between Au electrode surfaces, and are stable for at least a month. Possible applications exist in the fabrication of electronic devices, inter-element wiring, and biosensors.

A nanocasting route that utilizes well-ordered mesoporous carbon as a solid template can be used to synthesize zeolite ZSM-5 with unique supermicropores. By changing the nature of the mesoporous carbon template, the textural properties of the zeolite can be varied, and as the Figure shows, the mesoporous MZSM-5 zeolites are made up of nanocrystallites.

The development of the self-organized growth of pentacence thin films on the channel region of a thin-film transistor (TFT) using surface modifications induced by organic vapor phase deposition is reported (see Figure). A bottom-contact TFT on plastic using an organic gate insulator of cross-linked poly-(4-vinylphenol) exhibited a field-effect mobility of 1.2 cm²/Vs and an on/off current ratio of ∼ 10⁷.

The intercalation of polypyrrole (ppy) into hexagonal mesostructured carbon at 6.80–12.6 wt.-% levels results in substantial shrinkage of the framework structure (Δd₁₀₀ = 0.8–1.0 nm) and the pore size (from 3.7 nm initially to 2.9–2.5 nm upon ppy binding). The surface areas (718–960 m² g^–1) and electrical conductivities (0.1–1.0 S cm^–1) for compressed powders, however, are largely retained over a wide temperature range, providing potential applications in sensing.

Self-assembled monolayer coating of silane molecules on the surface of nanometric particles of hydroxyapatite and TiO₂ has been performed using a supercritical (SC) fluid method. The performance of the SC method is compared to that of a conventional (CV) aqueous alcohol method (see Figure). SC anhydrous silanization can be used as an effective method of achieving a desired degree of surface coverage while avoiding particle agglomeration.

The incorporation of a triazole moiety in poly(phenylene vinylene) (see Figure) allows tuning of the ratio of electron flux to hole flux over a broad range. Originally hole-dominated transport can be turned to electron-dominated transport. At proper triazole content, electron and hole fluxes are balanced, and the efficiency and brightness of a device with a high work function Al cathode can be improved to the level of low work function Ca.