Advanced Materials

The ability to resolve structures over short distances is of critical importance, and this review shows how NMR can be used to understand interfacial interactions, and how these interactions affect self-assembly and structural evolution on a nanometer scale. The results give rise to a model (see Figure) for functional monolayers on mesoporous supports, from which materials properties can be predicted.

Moving light-emitting diodes are envisioned as a possible application of the work presented here. Microfabricated bilayer hinges of polypyrrole (PPy) and gold are used to rotate and position silicon plates. This is achieved by electrochemically altering the oxidation state of PPy, thus causing a significant volume change in the polymer. It is demonstrated that power can be supplied to devices on the silicon plates, which is illustrated by the use of a PPy “pixel”, which changes color as the potential is varied.

3D protein crystals as precursors to advanced materials? The work presented here demonstrates the feasibility of this. It is shown that lysozyme crystals infused with fluorescent surfactants can be etched (see Figure), drilled, and deposited via photochemical micromachining, opening up possibilities for the use of protein crystals and layers in catalysts, biosensors, or data storage devices.

Efficient, pure red-emitting polymer light-emitting diodes have been prepared by doping blue-emitting conjugated polymers with europium β-diketonate complexes (see Figure). The pure red emission is required for full color displays, while the blends are also expected to be useful as emitters in photonic crystals due to their narrow monochromaticity (see also cover).

Carbon single-walled nanotubes (SWNTs) need to be highly pure for use in various applications. A simple and efficient purification method is reported here to provide >98 % pure SWNTs (see Figure) in a three-step process involving dilute nitric acid reflux and oxidation in air. The method also provides an analytical technique for the determination of the SWNT content in the crude and purified samples.

Well-defined surfactant self-assembled microstructures are needed to take full advantage of the templating strategy for the generation of nanoparticles with controlled size and anisometry, as demonstrated here. For example, at a high concentration of Cu(AOT)₂ (AOT = bis(ethylhexyl)sulfosuccinate) in isooctane–water the micellar supra-aggregate depicted in the Figure forms, and spherical copper metal particles are produced.

Transparent sheets of epoxy resin reinforced with nylon-4,6 nanofibers are described. The 30–200 nm diameter fibers, obtained by electrospinning from formic acid solutions, are reported to provide significant improvement in strength and stiffness to the epoxy film. The Figure is a scanning electron microscopy image of the electrospun nanofibers.

Superhydrophobic surfaces are those with a water contact angle greater than 150°, and these are desired for various industrial products. Such surfaces have been achieved for boehmite and silica films, which were roughened by the sublimation of aluminum acetylacetonate during calcination. The Figure shows the shape of a water droplet on such a silica film.

The surface morphology and topography of hyperbranched films of PAA (poly(acrylic acid)) are investigated. These grafted films are reported to be significantly smoother than films formed by other methods, e.g., spin-coating. The Figure is a schematic representation of 2-PAA—the film after it has undergone two activation/grafting/hydrolysis cycles.

High-performance pentacene insulated-gate field-effect transistors (IGFETs, see Figure) are described. It is demonstrated that, if barium strontium titanate is used as the gate insulator, pentacene IGFETs with a mobility similar to that of a-Si:H thin film transistors, good current modulation, and excellent subthreshold slopes at an operating voltage of about 5 V can be fabricated.

Oligothiophenes with enhanced light emission properties in the solid state are presented. Historically, light-emitting diodes (LEDs) based on oligothiophenes have shown poor efficiency, a low photoluminescence quantum yield in the solid state contributing to this behavior. Here, it is reported that new polyfunctionalized oligothiophenes display photoluminescence quantum efficiencies in the solid state of up to 37 %, a value comparable to that of organic luminescent materials, such as poly(p-phenylenevinylene), currently under investigation for application in LEDs.

Dark opaque blue to transmissive light blue is the color transition that poly(3,4-propylenedioxythiophene) (PProDOT) and derivatives undergo when switched from their reduced to oxidized state. These polymers are very promising electrochromic materials—they exhibit more rapid switching, lower oxidation potentials, and greater stability than polythiophene. The synthesis and optical properties of the disubstituted derivative PProDOT–Me₂ are reported and compared with those of PProDOT–Me and PProDOT. Substitution is found to improve the polymer's electrochromic properties.

The efficient photoinduced charge transfer between conducting polymers and C₆₀ makes this donor–acceptor system an attractive target for use in optoelectronic devices. These authors have fabricated a semiconducting polymer photodiode synthesized from a new C₆₀-doped semiconducting disilanylene–thienylene polymer. The device shows high spectral sensitivity that extended far below the absorption edge into the near infrared region. The absorption spectra are analyzed and discussed in terms of the polymer structure, and in terms of electron–hole mobilities.

Couplers, filters, and polarization separators are some of the applications of azobenzene-based amorphous polymers such as the one shown in the Figure. The photoinduced orientation and randomization processes in films of these materials are discussed as is their possible exploitation in reversible optical storage, waveguides, and gratings, and as photorefractive materials.

The possibility of carrying out chemical synthesis on a microchip is one of the driving forces for the preparation of channel-patterned microsurfaces. It is shown that parallel wettable stripes can be placed onto an otherwise hydrophobic surface without noticeably affecting the surface topography. Water deposited onto the surface then gives stable cylindrical channels up to a certain thickness, after which droplets form (see Figure).