Repeated temperature modulation epitaxy for p-type doping and light-emitting diode based on ZnO.

Since the successful demonstration of a blue light-emitting diode (LED), potential materials for making short-wavelength LEDs and diode lasers have been attracting increasing interest as the demands for display, illumination and information storage grow. Zinc oxide has substantial advantages including large exciton binding energy, as demonstrated by efficient excitonic lasing on optical excitation. Several groups have postulated the use of p-type ZnO doped with nitrogen, arsenic or phosphorus, and even p-n junctions. However, the choice of dopant and growth technique remains controversial and the reliability of p-type ZnO is still under debate. If ZnO is ever to produce long-lasting and robust devices, the quality of epitaxial layers has to be improved as has been the protocol in other compound semiconductors. Here we report high-quality undoped films with electron mobility exceeding that in the bulk. We have used a new technique to fabricate p-type ZnO reproducibly. Violet electroluminescence from homostructural p-i-n junctions is demonstrated at room-temperature.     

Molecular shape selectivity through multiple carbonyl-pi interactions with noncrystalline solid phase for RP-HPLC

A new approach for the synthesis of double-alkylated L-glutamide-derived stationary phases to use in RP-HPLC is described. TEM observation of lipid distearylglutamide (DSG) showed the formation of fibrous aggregates in methanol or in chloroform through intermolecular hydrogen bonding among the amide moieties while dibutylglutamide (DBG) cannot aggregate in aqueous or organic media due to its lower order of short alkyl chain. DSG and DBG were covalently bonded to silica via amino-propyl linkages. Lipid membrane analogues (e.g., DSG) attached to the silica surface have been found in noncrystalline and solid states and can form supramolecular assemblies with specific properties based on their highly ordered structures in aqueous and organic media. 13C CP/MAS NMR and suspension (in methanol)-state 1H NMR, elemental analysis, and DSC measurements were used to characterize Sil-DSG and were compared with the three other octadecyl phases, i.e., monomeric C18, polymeric C18, and silica grafted poly(octadecyl acrylate) Sil-ODA25. The chromatographic behavior of the new RP material was investigated using detailed retention studies of planar and nonplanar polyaromatic hydrocarbons (PAHs) and nonpolar aromatic positional isomers. Aspects of shape selectivity were also evaluated with Standard Reference Materials 869a, Column Selectivity Test Mixture for Liquid Chromatography. Detailed chromatographic study revealed that Sil-DSG showed extremely enhanced molecular shape selectivity compared with the other phases investigated. The higher molecular shape selectivity obtained by Sil-DSG can be explained by a carbonyl pi (present in lipid-grafted stationary phases)-benzene pi (present in guest PAHs) interaction mechanism, and these interactions are more effective for ordered carbonyl groups.     

Influence of water vapor on the reduction of cuprous chloride to copper microfibers

Reduction of cuprous chloride has been examined in various atmospheres, mostly at 600°C, to produce copper microfibers that can be applied as a filler for electromagnetic interference shielding and electron conductive paste composites. The reduction with hydrogen resulted in formation of a large amount of copper films, but fine particles and short filaments were obtained when carbon black was mixed into the raw material. The most promising copper microfibers with dimensions of several mm length and 1–10 m thickness could be produced with the system CuCl — C — H₂O using argon as a carrier gas, where H₂ and CO formed then probably reduced the vaporized CuCl molecules. Introduction of hydrogen gas into this system increased the yield of copper microfibers, though they became more bent, curved and kinked in shape. However, the morphology of the microfibers is still acceptable as a filler.     

Growth of Cu2ZnSnS4 thin films on Si (100) substrates by multisource evaporation

Cu₂ZnSnS₄ films were grown on Si (100) by vacuum evaporation using elemental Cu, Sn, S and binary ZnS as sources. X-ray diffraction patterns of films grown at different substrate temperatures indicated that polycrystalline growth was suppressed and the orientational growths were relatively induced in a film grown at higher temperatures. Tetragonal structure of Cu₂ZnSnS₄ films was confirmed by studying RHEED patterns. The existence of c-axis ([001] direction) growth, two kinds of a-axis (〈100〉 direction) growth and four kinds of {112} twins which can be classified as two symmetrical pairs is proposed. Broad emissions at around 1.45 eV and 1.31 eV were observed in the photoluminescence spectrum measured at 13 K.     

Supermagnetron plasma CVD of highly effective a-CNx:H electron-transport and hole-blocking films suited to Au/a-CNx:H/p-Si photovoltaic cells

Hydrogenated carbon nitride (a-CN_x:H) films (0-500 nm) were deposited on p-Si wafers to make Au/a-CN_x:H/p-Si photovoltaic cells using i-C₄H₁₀/N₂ supermagnetron plasma chemical vapor deposition. At a lower electrode RF power (LORF) of 50 W and an upper electrode RF power (UPRF) of 50-800 W, hard a-CN_x:H films with optical band gaps of 0.7-1.0 eV were formed. At a film thickness of 25 nm (UPRF of 500 W), the open circuit voltage and short circuit current density were 247 mV and 2.62 mA/cm², respectively. The highest energy conversion efficiency was 0.29%. The appearance of the photovoltaic phenomenon was found to be due to the electron-transport and hole-blocking effect of thin a-CN_x:H film.     

Surface-modified mesoporous silicas as recyclable adsorbents of an endocrine disrupter, bisphenol A

Surface-modified mesoporous silicas (MSs) were investigated for recyclable adsorption of an endocrine disrupter, bisphenol A (BPA). Surface-modified MSs were prepared by (i) post-synthesis surface modification of MSs using surface hydroxyl groups and organosilanes (m-MS) and by (ii) co-condensation of tetraethoxysilane and the corresponding organosilanes (d-MS). Infrared measurements indicated that organic groups mainly existed on the surface of m-MS, which resulted in a surface characterized by high hydrophobicity. Both organic groups and isolated hydroxyl groups existed on the surface of d-MS, resulting in both hydrophobicity and hydrophilicity on the surface. The amount of BPA adsorbed on surface-modified MSs per organic group was similar for m-MS and d-MS, however, the d-MS established equilibrium for BPA adsorption faster than m-MS, as measured by UV-vis spectra. A larger amount of BPA per surface area could be adsorbed on carbon materials than on the surface-modified MSs, however, the regeneration of carbon materials by washing could not be done easily. The surface-modified MSs retain adsorption capacity for BPA after several regeneration cycles, demonstrating that the surface-modified MSs are effective recyclable adsorbents of the endocrine disrupter, bisphenol A.     

Dynamical Properties of Vortices in a Bose-Einstein Condensate in a Rotating Lattice

We present simulation results of the vortex dynamics in a trapped Bose-Einstein condensate in the presence of a rotating optical lattice. Changing the potential amplitude and the relative rotation frequency between the condensate and the optical lattice, we find a rich variety of dynamical phases of vortices. In particular, when the optical lattice rotates faster than the condensate, the competition between the pinning force and the interactions by nucleated interstitial vortices leads to the melting of vortex lattice, yielding a vortex liquid phase.     

Efficiency and polarization performance of a multilayer-coated laminar grating in the 6.5-6.9-nm wavelength region

We applied a Mo/B4C multilayer coating to a laminar holographic grating with 2400 grooves/mm and a 1-m radius of curvature. By use of synchrotron soft x rays the multilayer-coated grating was evaluated to have diffraction efficiencies of 3.1% and 0.017% for s- and p-polarized radiation, respectively, at a 6.7-nm wavelength at a 45.35 degrees grazing angle of incidence in the +1 (inside) grating order. Thus the polarizance was estimated to be 98.9% at least. The zero-order peak was suppressed by the destructive interference caused by the groove profile.