We report on the high thermoelectric performance of p-type polycrystalline BiCuSeO, a layered oxyselenide composed of alternating conductive (Cu 2 Se 2 ) 2 À and insulating (Bi 2 O 2 ) 2 þ layers. The electrical transport properties of BiCuSeO materials can be significantly improved by substituting Bi 3 þ with Ca 2 þ . The resulting materials exhibit a large positive Seebeck coefficient of B þ 330 lV K À1 at 300 K, which may be due to the 'natural superlattice' layered structure and the moderate effective mass suggested by both electronic density of states and carrier concentration calculations. After doping with Ca, enhanced electrical conductivity coupled with a moderate Seebeck coefficient leads to a power factor of B4.74 lW cm À1 K À2 at 923 K. Moreover, BiCuSeO shows very low thermal conductivity in the temperature range of 300 (B0.9 W m À1 K À1 ) to 923 K (B0.45 W m À1 K À1 ). Such low thermal conductivity values are most likely a result of the weak chemical bonds (Young's modulus, EB76.5 GPa) and the strong anharmonicity of the bonding arrangement (Gruneisen parameter, cB1.5). In addition to increasing the power factor, Ca doping reduces the thermal conductivity of the lattice, as confirmed by both experimental results and Callaway model calculations. The combination of optimized power factor and intrinsically low thermal conductivity results in a high ZT of B0.9 at 923 K for Bi 0.925 Ca 0.075 CuSeO.

A unique dimer-type heterostructure of Ag−ZnO nanofibers with a diameter of 80−150 nm and coupled Ag nanoparticles in the size range from several to 15 nm have been fabricated by a facile electrospinning method. Rhodamine B (RhB) was employed as a representative dye pollutant to evaluate the ultraviolet (UV) photocatalytic activity of Ag−ZnO nanofibers. It was found that the heterojunction structure promoted the charge separation as well as the photon efficiency, allowing both the photogenerated electrons and holes to participate in the overall photocatalytic reaction. The optimal photocatalytic activity of Ag−ZnO nanofibers exceeded that of pure ZnO nanofibers by a factor of more than 25. A possible mechanism for the enhanced photocatalytic activity of ZnO by Ag was proposed.

No abstract

Generalized gradient approximations have been used to calculate the potential energy curves for six rare gas diatomic molecules. Several generalized gradient approximations are found to provide a good description of binding in these diatomic molecules and show a significant improvement over the local density approximation in the prediction of bond lengths and dissociation energies. It is shown here that the behavior of an exchange functional in the region of small density and large density gradient plays a very important role in the ability of the functional to describe this type of van der Waals attraction.

A general synthetic method has been developed to fabricate and assemble ferromagnetic transition metal nanofibers. By employing the novel electrospinning technique followed by subsequent heat treatment, we have successfully prepared uniform nanofibers of Fe, Co, and Ni with diameters of ∼25 nm and lengths longer than 100 µm. Using a specially designed fiber collector, we can conveniently assemble magnetic nanofibers into aligned arrays. The electrospun magnetic metal nanofibers have unique magnetic properties, with much enhanced coercivities relative to bulk materials. The outstanding features of this approach to get one-dimensional magnetic nanostructure are its simplicity, effectiveness, and ease of assembly. Therefore, electrospun magnetic nanofibers can potentially be used in fabrication of highdensity magnetic recording, magnetic sensors, flexible magnets, and spintronic devices.

Nanowires and nanotubes are important building blocks for nanoscale optoelectronics, as they can function as miniaturized devices as well as electrical interconnects.[1] Nanodevices such as field-effect transistors (FETs), [2] Schottky diodes, [3] electrical switches, [4] and memories [5] have thus far been demonstrated. As a very promising way for achieving 1D nanostructures, electrospinning has the advantages of device size reduction and more compact component density in circuits. [6,7] Electrospun nanowires exhibit several amazing characteristics, such as very large surface area to volume ratios, [8] flexibility in surface functionalities, [9] and superior mechanical performances.[10] It would be meaningful to combine the electrospinning technique with controllable ceramic nanowires assembly for device fabrication, as this would provide a simple, rapid, and cheap way to construct patterned nanoarrays and circuits. ZnO, with a large direct bandgap of 3.37 eV, is one of the most promising materials for applications such as gas sensors, [11] diodes, [12] solar cells, [13] resonators, [14] and other photoelectric devices. Meanwhile, it is highly desired to prepare 1D ZnO nanostructures doped with a selection of elements for enhancing and controlling their mechanical, electrical, and optical performance. So far, several doped 1D ZnO nanostructures have been synthesized [15][16][17][18][19][20] that are important for practical applications. Aluminum-doped zinc oxide (AZO), as a possible alternative transparent conductive oxide (TCO) to indium tin oxide (ITO), has attracted much attention because of its unique optical and electrical characteristics as well as its low cost and non-toxicity. However, there are few reports on the preparation of AZO nanostructures and their properties, although much work has been carried out on AZO films. In this communication, we report on a photoconductor device that is sensitive to illumination with below-gap light and a memory assembled by electrospinning a single AZO nanowire, both of which, to the best of our knowledge, have never been reported before. For this purpose, well-aligned and highly ordered architectures of 1D nanostructures were required. Although a number of approaches have been demonstrated to collect spatially oriented electrospun nanowires, few attempts have been made to directly obtain oriented electrospun nanowires for device fabrications. [21][22][23] Herein, by using a "Cu bridge" as the cathode, which consists of two conductive copper filaments separated by a void gap of 1 cm width (Fig. 1a), the electrospun nanowires could be uniaxially aligned over long length scales during the e-spinning process (Fig. 1b). The nanowires were then directly transferred to a silicon substrate with a 500 nm-thick thermally grown SiO 2 layer, and calcined at 550°C at a heating rate of 10°C min -1 in air for 3 h to obtain well-aligned polycrystalline AZO nanowires (Fig. 1c). Gold electrodes were then deposited on the AZO nanowires by thermal evaporation using a mask to form channe...

No abstract

Alumina/multiwalled carbon nanotube (MWNT) composites with different MWNT contents ranging from 0.5to10vol% were prepared by spark plasma sintering technique. The dc electrical conductivity and dielectric properties of the composites were investigated and percolation theory was applied to demonstrate the electrical property transition from insulator to conductor. The experimental results have shown that the electrical conductivity increased sharply as the content of MWNTs was close to percolation threshold of 0.79vol%. In the low frequency range, the dielectric constant reached as high as 5000 when the content of MWNTs was at 1.74vol% and nearly frequency independent.