Youqi Tang

Preparation of II-VI group semiconductor nanowire arrays by dc electrochemical deposition in porous aluminum oxide templates

II-VI group compound semiconductors such as CdS, CdSe, and CdTe nanowire arrays have been prepared by direct current electrodeposition in porous anodic aluminum oxide template from nonaqueous electrolyte. SEM and TEM results show that these nanowires have a highly anisotropic structure of aligned nanowires with diameters of 15-200 nm, which are consistent with the diameters of the templates used. Electron diffraction and HREM investigations demonstrate that the crystalline structures of these nanowires are uniform hexagonal single crystal. This approach can be used to fabricate single crystal nanowire arrays of a wide range of semiconductors and other materials.

Preparation of colloidal solutions of thin platinum nanowires

Colloidal solutions of thin platinum nanowires having lengths of 10 to 70 nm and diameters of 1.5–3 nm were prepared by reducing K2PtCl4 with hydrogen in a glycol solution containing K2C2O4 and preformed Pt seeds. TEM, HRTEM, XRD and UV-Vis absorption spectroscopy were used to study the structure of the obtained nanowires and the effect of preparation conditions, such as pH value, temperature, preformed Pt nanoseeds and stabilizing agents, on the morphology of the products. The mechanism for forming the thin Pt nanowires in the absence of a 1D structure template was discussed based on the experimental results. Stable colloidal solutions of mercaptan-modified Pt nanowires were prepared for the first time by transferring the prepared thin Pt nanowires into toluene containing octadecylmercaptan.

A study on the monolayer dispersion of tungsten oxide on anatase

The monolayer dispersion of tungsten oxide on the surface of anatase has been studied by BET surface area measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transformation infrared spectroscopy (FTIR) and Raman spectroscopy. The results reveal that for the anatase supported tungsten oxide samples, tungsten oxide species can be highly dispersed on the surface of the support and the dispersion state of tungsten oxide species is strongly related to the loading amount of tungsten oxide. In addition, it is found that tungsten oxide can retard the anatase–rutile transformation. When the loading of WO3 is below its monolayer dispersion capacity, the onset temperature of the anatase–rutile transformation is strongly affected by the dispersed tungsten oxide. Crystalline WO3 appears when the WO3 loading is higher than the monolayer dispersion capacity but has little effect on the phase transformation of the samples.

Intense and stable blue-violet emission from porous silicon modified with alkyls

An intense blue-violet band, centered at 3.0 eV, has been observed besides the red-orange band of the photoluminescence (PL) from the chemically modified porous silicon (PS) samples. The PS samples were formed on 30–50 Ω cm p-type (100) Si wafers by anodic etching at large current densities and then treated with 5-cyano-1-pentynes in toluene solution at 110–120 °C. The intensities of the blue-violet lights increase with the increasing of the etching current densities, while the peak energies are hardly changed. The Fourier transform infrared spectra of the modified samples show that the alkyls have been bonded to the surfaces of the nanometer crystallite silicon particles. The decays of the blue-violet emission with a lifetime about 0.5 μs are different from the nanosecond lifetimes of the blue emissions in the oxidized PS samples reported before, and the 20 μs lifetimes of the concomitant orange PL bands. A carbon-compounds-related luminescence center has been proposed as the origin of this blue-violet P...

Diffusing behavior of MoO3 on Al2O3 and SiO2 thin films

By means of XPS, SIMS, synchrotron radiation excited total-reflection X-ray fluorescence, AES etc., the diffusion process of MoO3 on stable Al2O3 and SiO2 oxide thin films was investigated. After thermal treatment, MoO3 formed a monolayer or a submonolayer on the flat surface of the thin films. The diffusion capacity and the diffusion rate on these two kinds of films differ significantly. Besides the support used, several factors, such as the heating temperature, the heating time and the ambience influence the diffusion process. A possible explanation to all the phenomena is the combination of surface diffusion and transportation via gas phase.

Ordered porous films of organically-modified silica prepared by a two-step replicating process

Abstract An ordered porous film of vinyl-modified silica was prepared by a two-step replication of the honeycomb structure of anodic porous alumina. Preparation of the negative porous structure of anodic porous alumina followed by the formation of the positive structure with vinyl-modified silica resulted in a honeycomb structure of vinyl-modified silica. The silica film has nearly uniform, closely packed nanoholes with an average diameter of 198 nm and length of 11 μm. The porous silica film was characterized by using the X-ray photoelectron spectroscopy (XPS) and FT-IR techniques.

Optical absorption property of oxidized free-standing porous silicon films*

We have systematically studied the evolution of the optical absorption of free-standing PS films during thermal oxidation at 200°C in air. Our experiment results show the evolution of transmission curve is quite complicated, which red-shifts first and then blue-shifts during thermal oxidation. At the same time, the transmission at the low energy decreases first and then increases. We propose an explanation as follows: (1) the energy gap associated with each crystallite should increase during thermal oxidation process, due to the quantum confinement effect; (2) the energy gap should decrease with an increase in oxygen termination atoms. Both the increasing of the gap due to the quantum confinement effect and the decreasing of the gap due to the Si-O bond formation cause a complicated evolution of optical absorption.

STRUCTURAL STUDIES ON THE SUPERCONDUCTING PHASE Bi4Sr4Ca2Cu4O2−δ

Very recently H. Maeda et al.1 reported a high Tc superconducting material of the composition BiSrCaCu2Oχ that makes a feature of containing no rare earth metal. We are interested in the structural and chemical aspects of this new class of materials.2 We have prepared a homogeneous sample for the superconducting phase in the Bi-Sr-Ca-Cu-O system, and have determined its crystal structure by powder XRD method. At a very late stage we noticed the structural work by a Bellcore group.3 Fortunately our work has quite a number of distinguishable features.

PREPARATIVE AND STRUCTURAL STUDIES ON THE SUPERCONDUCTING PHASE YBa2Cu3O7−d

Reasonably homogeneous samples were obtained by following the formula Y_(0.334)Ba_(0.666)CuO_(3-y) andusing citric acid as a complex-forming agent to improve the dispersion of the metal oxides. We havealso found that the variant condition in preparative procedure leads to variant deviation from tetrago-nality and that more pronounced deviation gives better superconducting properties. The crystal structure determined from X-ray powder diffraction diagrams confirms the stoichiom-etry of the metal ions and gives the positions of the oxygen atoms. The orthorhombic or pseudo-tetragonal unit cell with a=3.893, b= 3.813 and c= 11.687 A contains YBa_2Cu_3O_7. It is composed ofthree perovskite cubes with three copper atoms at their corners, the two barium atoms at the centers ofthe top and bottom cubes, and the yttrium at the center of the unit cell. The oxygen atoms sit on centersof cube edges. The positions (0 0 1/2) are vacant. The vacancies represented by (0(1/2)0) cause the ar-rangement of atoms to lose tetragonal symmetry, and have far-reaching effect for this phase. The yttriumand barium atoms are coordinated in a cuboctahedron respectively with four equatoria oxygen atoms at(0 0 1/2) and two at (0 1/2 0) removed. In the light of bond valence theory we can infer from the in-teratomic distances Y-O, Ba-O and Cu-O that the occupancy of the oxygen atoms at (1/2 0 0) is wellbelow unity while the positions (0 0 1/2) are vacant, the formula of the phase is YBa_2Cu_3O_(7-d), thefraction of Cu(Ⅱ) and (Ⅲ) for the set of copper at (0 0 0) is respectively d/0.5 and (1-d/0.5), and0.2d0.4. The other set of copper is a normal bivalent one.

Correlation between the H+ Concentration in the Electrolyte and the Photoluminescence of Porous Silicon

We have investigated the photoluminescence (PL) properties of porous silicon (PS) samples prepared in a 20% HF–ethanol electrolyte with different H+ concentration by varying the added HCl or NH3 · H2O concentration in the range of 0–6 mol/l at a constant anodic current density. The PL spectra continuously blueshift with increasing NH3 · H2O concentration or decreasing HCl concentration in electrolyte. Meanwhile, the infrared absorption associated with Si–O gradually increases with the NH3 · H2O concentration. Raman scattering results indicate that the average particle sizes of the PS samples have no obvious evolution with varying H+ concentration in electrolyte. These experimental results strongly support that light emission of PS originates from luminescent centers.