Free standing modulation doped core–shell GaAs/AlGaAs hetero‐nanowires

Modulation doped AlGaAs/GaAs core–shell nanowire structures were grown by molecular beam epitaxy. A Si delta‐doping was introduced in the AlGaAs shell around the {110} facets of the GaAs core. The wires are typically highly resistive at low temperatures. However, they show a pronounced persistent photoconductivity effect indicating activation of free carriers from the delta‐doped shell to the GaAs core. The n‐type character of the channel is demonstrated by applying a back‐gate voltage. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stereoelectronic structure of MCl4-benzoyl chloride (M=Si,Ge, Sn) systems resulting from ab initio calculations and NQR 35Cl

Results of quantum-chemical calculations of MCl₄–C₆H₅COCl (M=Si, Ge, Sn) systems of 1?:?1 composition using RHF/3-21?G* and MP2/3-21?G* levels as well as those of 1?:?2 composition using the RHF/3-21?G* level have been represented. MCl₄?←?C₆H₅COCl complexes of 1?:?1 composition are energetically more advantageous. They are formed in solid state provided that the M···O distance in individual systems is considerably less than the sum of van der Waals radii of M and O and their total energies are appreciably less than the sum of total energies of components. These conditions are realized only for M=Sn. In systems of 1?:?2 compositions, calculated M···O distances are practically equal to the sum of covalent radii of M and O. Nonetheless, complexes with such composition are not formed in solid state. Total energy of the system which is lower than the sum of its components’ energies is not an indispensable condition for complex formation. The ³⁵Cl nuclear quadrupole resonance (NQR) frequencies and asymmetry parameters of the electric field gradient at the ³⁵Cl nuclei have been evaluated using the results of ab initio calculations.     

水热法合成ZnS∶Au,Cu超细X射线发光粉

报道了水热法合成的高强度ZnS∶Au,Cu超细X射线发光粉及其光致发光(PL)和X射线激发发光(XEL)的光谱特性。200℃水热处理12h直接合成样品的纳米晶粒约15nm,尺寸分布窄,分散性好,具有纯立方相的类球形结构。氩气保护下1000℃焙烧1h后的样品存在一定的团聚,但团聚后尺寸为1～2μm,为超细X射线发光粉,此时样品为纯六角相的类球形为主的结构。所有样品的PL和XEL光谱均为宽带谱,水热法直接合成样品的XEL强度最强时,样品的Cu/Zn,Au/Cu比值分别为3×10-5和2。在此比值条件下,1000℃焙烧1h样品的XEL发光最强,此时其2个峰值分别位于445和513nm,且与未焙烧前相比强度增强了10倍左右。另外通过比较PL光谱与XEL光谱特性,讨论了PL和XEL光谱的发光机理和其不同的激发机制。     

Indirect detection of the 183W and 57Fe nuclei using 119Sn‐relayed 1H,X correlation spectroscopy

Recently reported triple‐resonance Y‐relayed ¹H,X correlation experiments have been utilized to characterize ¹⁸³W and ⁵⁷Fe chemical shifts using ¹¹⁹Sn as the Y‐relaying nucleus instead of the previously used ³¹P. Application of an adaptation of Gudat's original INEPT/HMQC sequence results in a significant enhancement of the signal‐to‐noise (S/N) ratio for two‐dimensional ¹¹⁹Sn‐relayed ¹H,¹⁸³W and ¹H, ⁵⁷Fe correlation spectra with efficient detection of the transition metal nucleus in tungsten and iron complexes lacking an observable direct scalar coupling between the transition metal and any hydrogen nuclei. Strengths and shortcomings of the novel sequence and the original sequences reported by Gudat are discussed in the context of ¹¹⁹Sn‐relayed proton detection of very low frequency transition metal nuclei. Copyright © 2010 John Wiley & Sons, Ltd.     

Acidity and Catalytic Activity of Sulfate Modified Mixed Oxides of Tin and Praseodymium

A comparitive study of the acid-base properties and physico-chemical characteristics of pure SnO₂, sulfated SnO₂ and sulfated binary oxides of Sn and Pr was done. The data were correlated with the catalytic activity of the oxides towards benzoylation of toluene.     

FT-FIR-spectrum and the ground state constants of D2CO

The ground state spectrum of the formaldehyde D₂¹³CO molecule in the range from 25 to 360 cm⁻¹ has been recorded by a Fourier transform infrared spectrometer. The quantum number limits of the assigned transitions are J = 6-54 and K_a = 0-16. The data was fitted into Watson’s A- and S-reduced Hamiltonians in I^r-representation up to eighth order. The determinable constants calculated from both reductions are compared. The standard deviation of the far infrared data is 3.0 MHz. The spectroscopic constants are also calculated to high accuracy at the CCSD(T)/cc-pVQZ level of ab initio theory and agree well with the experimental ones.     

在MOCVD外延生长中Ⅴ/Ⅲ比对AlxGa1-xAs外延层中A1组分分布的影响

本实验首次观察到用MOCVD方法在GaAs基片上外延生长AlxGa1-xAs/GaAs薄膜结构时,生长中所用的Ⅴ/Ⅲ比对外延层中Al组份x的分布的影响.当Ⅴ/Ⅲ比较低时,外延层中的Al组份出现很明显的层状阶梯形三层分布:靠近基片的一层中x值较低,而在外延层的表面层中x值较高.随着生长中所用的Ⅴ/Ⅲ比增加时,Al组份的分布趋于均匀.利用GaAs的高温热分解和Ga、As离子在晶体中的热扩散及其对外延生长的影响,对这一现象作了初步解释.     

Studies on Nd:YAG Single-pass Amplifiers for High-power Q-switched Laser System

The output of Nd∶YAG single-pass laser amplifiers is studied analytically and experimentally. Methods of analysis for single-pass Nd∶YAG laser amplifier are presented. A flashlamp-pumped Q-switched Nd∶YAG oscillator/amplifier laser system has been developed with the average output power of 121.5 W.     

Study into the shape of oxide lines formed by LAO - Influence of an oxidized material

Linear oxide patterns were formed by local anodic oxidation (LAO) using an atomic force microscope (AFM) on an n-doped GaAs substrate, a 10-nm-thick titanium layer, and on shallow GaAs/AlGaAs-based heterostructures capped either with a 5-nm-thick undoped GaAs layer or a 2-nm-thick undoped InGaP layer. Each heterostructures had a 2DEG buried at a specific depth between 22 and 45 nm. LAO was performed in contact and non-contact AFM modes with the aim to explain the phenomenon of single and double line formation depending on material oxidized. The occurrence of the phenomenon was also simulated. The results showed that the occurrence of the double lines is linked with the thickness of native oxides.     

Self-averaging in complex brain neuron signals

Nonlinear statistical properties of Ventral Tegmental Area (VTA) of limbic brain are studied in vivo. VTA plays key role in generation of pleasure and in development of psychological drug addiction. It is shown that spiking time-series of the VTA dopaminergic neurons exhibit long-range correlations with self-averaging behavior. This specific VTA phenomenon has no relation to VTA rewarding function. Last result reveals complex role of VTA in limbic brain. Received 17 April 2002 / Received in final form 30 September 2002 Published online 31 December 2002