Advanced Materials

Charge-transporting molecular glasses that can be used in copiers and laser printers, organic light-emitting diodes, photovoltaic devices, and as photorefractive materials are discussed. Both p-type (hole-transporting), and n-type (electron-transporting) molecular glasses (see Figure for an oxadiazole derivative) are reviewed and an overview of current and potential applications for these materials is presented.

A protein-based solid-state electronic device that operates in air is reported; it is fabricated by interconnecting an azurin monolayer immobilized on SiO₂ with two gold nanoelectrodes. The operating mechanism is ascribed to resonant tunneling through the redox active center, combined with macroscopic polarization of the oriented self-organized protein layer. The Figure shows the molecular electrostatic potential of the oxidized protein.

A skeleton structure with a predicted bandgap between the 5th and 6th bands has been theoretically calculated (see Figure). Below the critical refractive index contrast, skeleton structures enable broader pseudogaps in the range of the 5th and 6th bands. The contrast required for this new gap is nearly the same (2.9) as that of the 8–9 gap of residual volume structures. A titania skeleton structure has been realized experimentally as shown on the cover.

An organic electronic paper display technology (see Figure and also inside front cover) is presented. The electrochromic display cell together with the addressing electrochemical transistor form simple smart pixels that are included in matrix displays, which are achieved on coated cellulose-based paper using printing techniques. The ion-electronic technology presented offers an opportunity to extend existing use of ordinary paper.

Field-emission currents from SiO₂- or MgO-coated carbon nanotube (CNT) emitters are larger than for uncoated equivalents. Moreover the coated emitters exhibit lower turn-on fields and it is shown that the MgO or SiO₂ films protect the CNT tips during field emission, which improves the lifetime stability of these emitters. The Figure shows an SEM image of a SiO₂-coated CNT emitter.

Micromolding of polymers from nanopatterned silicon wafers leads to the preparation of surfaces with well-defined nanometer-scale topography and surface chemical functionality. These types of substrates will be useful for investigations of the interactions of nanostructured interfaces with biological systems. The Figure shows the optical appearance of a nematic liquid crystal anchored on topographically patterned surfaces of a polystyrene replica.

Arrays of Au nanostructures have been fabricated based on dip-pen nanolithography and wet chemical etching, whereafter they can be functionalized with alkanethiol-capped oligonucleotides. These retain their hybridization properties on the surfaces of the nanopatterns and can react with complementary DNA or particles modified with complementary DNA.

Nanoscale conducting polymer patterns have been fabricated on modified semiconductor substrates using dip-pen nanolithography (DPN). Electrostatic interactions between water-soluble ink materials and charged substrates are the driving force for the generation of stable DPN patterns. The conducting polymers have been characterized by lateral force microscopy (see Figure) and electrochemical methods.

A high-performance micro-patterned polarizing element has successfully been fabricated by in-plane anisotropic alignment of a chromonic lyotropic liquid crystal. Although the fabrication method is quite simple, the orientational order of the polarizing element obtained is high enough for practical usage. The Figure shows a microscopy image of two crossed micro-patterned polarizing elements.

Polyaniline/multiwalled carbon nanotube composite films have been fabricated. It is shown that the nanotubes affect the free N–H environment and quinoid units along the polymer backbone. A 10-fold increase in conductivity is observed and elemental analysis indicates that the nanotubes compete with chloride ion during HCl doping (see Figure).

Cobalt-filled carbon nanotube composites (see Figure) with a high metal-filling ratio can be prepared by a simple and effective method. The length, diameter, and structure of the metal-filled nanotubes and the wall thickness of the plain carbon nanotubes can be easily controlled by the template used and synthesis conditions. Magnetic measurements reveal that the array of cobalt-filled carbon nanotubes exhibits enhanced coercivities in comparision to that of bulk Co.

Inverse opals with a photonic bandgap in the visible are highly desirable, but difficult to realize, as a combination of transparency in the visible and a high dielectric constant is required. Antimony trisulfide is a chalcogenide with the required optical properties; inverse opals based on this material have been fabricated (see Figure) and characterized.

Novel inorganic fast proton-conductors have been successfully prepared by hydrogelation of calcium phosphate glass and subsequent precipitation of the thus formed Ca(H₂PO₄)₂·H₂O hydrate crystals by heat treatment. The resulting calcium phosphate gel composites show a high conductivity of ∼ 2 × 10^–2 S/cm even at 30 °C and are mechanically ductile. The high conductivities are attributed to fast proton transfer promoted by coexistence of large amounts of acidic POH groups and water molecules.

A new electrochemical route to polythiophene (PT) films with an electrical conductivity between 800 and 1300 S cm^–1 and a mechanical strength of 135 MPa is reported. The PT polymerization was performed in a BF₃-ethyl ether solvated Lewis acid solution in the presence of the proton scavenger 2,6-di-tert-butylpyridine. The prepared PT films are shown to possess relatively regular molecular structure and inter-chain packing. The resulting microscopic structure enhances the macroscopic properties of the PT films.

Well-defined photochromic hybrid materials have been prepared by embedding spirooxazine dyes into matrices made of {(BuSn)₁₂-O₁₄(OH)₆}²⁺ nano-building blocks assembled with α,ω-carboxymethyl poly(ethylene glycol) macromonomers (see Figure). Processed as 2–3 μm thick films, these materials exhibit fast and direct photochromism.

A one-step facile synthesis of submicrometer-sized hollow silver spheres (see Figure) has been achieved in aqueous solutions by using double-hydrophilic block copolymer (DHBC)-surfactant complex micelles as removable templates. The unique silver shell structures are candidates for both fundamental research and applications.

A simple method for the fabrication of diffraction elements and arrays of micro-optical elements, using an array of microlenses, is demonstrated. The resulting diffraction patterns depend on both the period and the profile of the microstructures. This technique can be used to fabricate diffraction gratings and micro-optical elements with features that are uniform over areas > 10 cm² in a single exposure (see Figure).

Mechanically stable zeolite monoliths containing three-dimensional, ordered, closed macropores (see Figure) have been fabricated by hydrothermal treatment of nanozeolite seeded mesoporous silica spheres. The facile speed of sedimentation and digestion renders the whole process suitable for large-scale production of macroporous zeolite materials, which may be used as adsorbents and dielectric insulators.

Selectively modified mesoporous silica has been infiltrated with Pt nanoparticles by a simple in-situ reduction method. The method is based on functionalizing the inside pore channels with highly reducing Si–H bonds, which results in a preferred growth of Pt nanoclusters (about 15 wt.-% on average) along the channels of the mesoporous silica. The Figure shows a TEM image of Pt nanowires aligned along the pore channels of SBA-15.