Advanced Materials

The design, fabrication, and characterization of polymeric yarns (see Figure and cover) with large photonic bandgaps and omnidirectional reflectivity combined with a micro-cavity designed for near-infrared applications are reported. The reversible shift of the resonant wavelength of this single-mode optical cavity under applied strain is also demonstrated. Passive and active applications include narrow band-pass filters and low-threshold lasers.

Combinatorial studies of organic thin-film photovoltaic cells (see Figure) can identify the separate optical and electronic contributions to the photocurrent. A second charge-transfer zone is proposed to explain the observed augmentation of the short-circuit current induced by an additional TiO₂ layer. The ratio of the exciton diffusion lengths in copper phthalocyanine (CuPc) and a perylene dye (DMPTI) is estimated to be ≈ 8:5.

A new class of oligothiophenes is realized by replacing the conventional linear structure with a branched benzo[b]thiophene-based structure (see Figure). These compounds are used as active materials in light-emitting diodes exhibiting record luminances up to 10 500 cd m^–2 (see inset). Optical measurements and theoretical calculations show these performances are mainly to the distorted molecular structure.

Enzyme-catalyzed, ring-opening polymerization of biodegradable aliphatic polyesters has successfully been applied to surface-initiated polymerization (see Figure). Using lipase B as a catalyst, a thin film of poly(ϵ-caprolactone) (PCL) and poly(p-dioxanone) (PPDX) on a gold substrate (after a formation of self-assembled monolayers terminating in hydroxyl groups) is formed.

The Diels–Alder adduct of pentacene and N-sulfinylmethacrylamide is used as a soluble, photopolymerizable precursor of pentacene for fabrication of patterned thin-film transitors. The Figure shows the optical micrograph of patterned pentacene derived from the exposure of this adduct to UV radiation, followed by development in methanol and annealing at 200 °C for 90 s.

Dynamic self-assembly and metal–ligand coordination processes of porphyrin hexamers on a solid–liquid interface have been visualized with scanning tunneling microscopy. The organization of these hexamers into stable and highly organized arrays of “edge-on” or “face-on” oriented molecules can be controlled by the addition of different ditopic axial ligands (see Figure).

SiGe alloy nanowires (see Figure) have been fabricated using vapor–liquid–solid (VLS) growth with silane (SiH₄) and germane (GeH₄) gas sources. Growth conditions have been identified that produce nanowires with homogeneous alloy composition with negligible Ge coating on the wire surface. The Ge composition in the nanowire can be controlled by varying the inlet GeH₄/(GeH₄ + SiH₄) gas ratio.

Nanocrystalline, nominally undoped films of CeO_2–x undergo large (0.8–2 %) lateral expansion after local substrate removal and formation of freestanding membranes (see Figure). The structural, optical, and electrical properties are consistent with the hypothesis that substrate removal triggers disordering of oxygen vacancies leading to this expansion, which occurs irrespective of whether tensile or compressive stress existed in the films.

A computational evolutionary optimization process is adopted to screen a Eu³⁺-doped alkali earth borosilicate system in an attempt to search for red phosphors with a high luminescent efficiency when excited by soft ultraviolet irradiation. The evolutionary optimization process involves a genetic algorithm and combinatorial chemistry, which was tailored for the development of light-emitting diode (LED) phosphors. Vertical simulations and an actual synthesis have been carried out in the present investigation, and promising red phosphors for three-band white LED applications have been obtained.

An organic n-type transistor, where both insulating and active layer were processed from solution, has been produced. As the active layer the C₆₀-derivative, [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM), was used. Its electron mobility was determined to be as high as μ_e = 4.5 × 10^–3 cm² V^–1 s^–1 when calcium drain/source contacts were used. If these contacts are formed from more air-stable metals, the device performance decreases.

γ-Fe₂O₃-impregnated magnetic carbon nanotubes (CNTs) have been fabricated by the carbonization of polypyrrole (PPy) nanotubes. It is found that PPy nanotubes form graphitic structures at a low temperature (800 °C). Magnetic CNTs (see Figure) show ferromagnetic behavior even at 300 K and the main magnetic phase of the CNTs is γ-Fe₂O₃. This novel fabrication approach is expected to provide an alternative way to fabricate CNTs.

High-strength hydroxyapatite orthopaedic cements have been prepared by the addition of sodium citrate to the liquid phase, which liquified the mixture enabling high-density, low-temperature ceramics to be fabricated (see Figure, a fracture surface). Wet compressive strengths well within the range for cortical bone were measured. Following compaction at low pressures the cements were still moldable.

The up-conversion photoluminescence (PL) in films of polyfluorene (PF) doped with metallated porphyrins is reported for the first time. The dependence of the up-conversion process on the pump laser intensity and wavelength, the central metal moiety of the dopants, and the temperature is presented. Up-conversion emission is observed at pump intensities as low as 2 kW cm^–2. Comparison of the PF integral PL intensities after laser excitation by 532, 543, and 405 nm enables the discussion of the energy-transfer mechanism and the efficiency of the process.

Large quantities of Fe_1–xS (x = 0.12 and 0.1–0.08) nanowires (see Figure) have been prepared by the thermal decomposition of a hybrid precursor, Fe_1–xS(en)_0.5 (x = 0.2–0.3, en = ethylenediamine). Removal of the amine from Fe_1–xS(en)_0.5 at 200 and 300 °C under Ar flow yields Fe₇S₈ and Fe_1–xS (x ≈ 0.09) nanowires, respectively, with well-defined morphologies. The Fe₇S₈ nanowires are ferrimagnetic at room temperature.

Co-grafting an amphiphilic polypyridyl ruthenium sensitizer and 1-decylphosphonic acid on TiO₂ semiconductor nanocrystals has resulted in a long-term thermally stable dye-sensitized solar cell (see Figure) with higher than 7 % power conversion efficiency. The extraordinary stability of this high-efficiency device under both thermal stress and light soaking matches the durability criteria for outdoor applications of solar cells.

The fluorescence, including the intensity and emission peaks, and enhanced photochromism of a simple photochromic compound, bis(5-pyridyl-2-methylthien-3-yl)cyclopentene (Py-BTE) can be reversibly regulated by alternating ultraviolet/visible light irradiation and addition of Zn²⁺ and protons. A new combinational molecular logic switch with four optical outputs responding to four inputs (see Figure) is proposed.

Carbonaceous materials with high specific energy capacity are prime candidates for applications in rechargeable lithium batteries. The authors report the synthesis and characterization of ordered mesoporous carbon (CMK-3), synthesized using ordered silica as a template, with high reversible specific capacity and good charge–discharge cycle characteristics. The performance of CMK-3 is compared with that of carbon nanotubes, and its superiority is suggested to be related to the three-dimensional ordered structure of CMK-3.