掺杂Eu2O3的SrTiO3及SnO2光致发光

用具有大能隙的本征半导体(SrTiO₃及SnO₂)粉末作本体,分别掺杂1%(原子百分数)的Eu₂O₃;所得物质表现出Eu³⁺离子的线发射光谱特性,但相对发光强度及光谱形状有相当大的变化。X射线衍射结构分析显示Eu³⁺在SrTiO₃晶格里是处在间隙位置,而它在SnO₂晶格里则形成新物相Eu₂Sn₂O₇。     

Optical control of the mobility of a MODFET with a layer of self-assembled quantum dots

We report an experimental study of GaAs/Al_0.33Ga_0.67As modulation doped field effect (MODFET) transistors, in which an InAs layer of self-assembled quantum dots is placed in one of the Al_0.33Ga_0.67As barrier layers close to the two-dimensional electron gas (2DEG). We find the source–drain resistance is bistable with the two states controlled by illumination and applied gate bias. Brief illumination induces a large, persistent drop in the resistance, which can be recovered by applying a positive gate bias. Magneto-transport measurements show that while illumination causes only a relatively small change in the 2DEG density, it can greatly enhance its mobility. We suggest this is because the 2DEG mobility is limited by percolation of the electrons through the rough electrostatic potential induced by the charged dots. Illumination reduces the negative charge trapped in the dots, thus smoothing the conduction band potential, which produces a large increase in the mobility.     

Evidence for charging effects in an open dot at zero magnetic field

We have measured the low-temperature transport properties of an open quantum dot formed in a clean one-dimensional channel. At zero magnetic field, continuous and periodic oscillations superimposed upon ballistic conductance steps are observed when the conductance through the dot G exceeds 2e²/h. We ascribe the observed conductance oscillations to evidence for charging effects in an open dot. This is supported by the evolution of the oscillating features for G>2e²/h as a function of both temperature and barrier transparency.     

Effect of supercritical CO2 on the copolymerization behavior of cyclohexene oxide/CO2 and copolymer properties with DMC/Salen‐Co(III) catalyst system

The copolymerization of cyclohexene oxide (CHO) and carbon dioxide (CO₂) was carried out under supercritical CO₂ (scCO₂) conditions to afford poly (cyclohexene carbonate) (PCHC) in high yield. The scCO₂ provided not only the C1 feedstock but also proved to be a very efficient solvent and processing aid for this copolymerization system. Double metal cyanide (DMC) and salen‐Co(III) catalysts were employed, demonstrating excellent CO₂/CHO copolymerization with high yield and high selectivity. Surprisingly, our use of scCO₂ was found to significantly enhance the copolymerization efficiency and the quality of the final polymer product. Thermally stable and high molecular weight (MW) copolymers were successfully obtained. Optimization led to excellent catalyst yield (656 wt/wt, polymer/catalyst) and selectivity (over 96% toward polycarbonate) that were significantly beyond what could be achieved in conventional solvents. Moreover, detailed thermal analyses demonstrated that the PCHC copolymer produced in scCO₂ exhibited higher glass transition temperatures (T_g ～ 114 °C) compared to polymer formed in dense phase CO₂ (T_g ～ 77 °C), and hence good thermal stability. Additionally, residual catalyst could be removed from the final polymer using scCO₂, pointing toward a green method that avoids the use of conventional volatile organic‐based solvents for both synthesis and work‐up. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2785–2793     

Stark effect of negatively and positively charged excitons in semiconductor quantum wells

We study the effect of an electric field applied normal to the layers on the binding energy of charged excitons (or trions) in GaAs quantum wells. We find that, in contrast to the neutral exciton, their binding energy is sharply reduced by modest electric fields. The effect is stronger for the positively charged exciton than the negatively charged one. The ionisation of the excess carrier is explained by the field-induced polarisation of the electron and hole subband wave functions.     

Experimental evidence for a metal–insulator transition and geometric effect in a half-filled Landau level

We have measured the low-temperature transport properties of a high-mobility front-gated GaAs/Al Ga As heterostructure. By changing the applied gate voltage, we can vary the amount of disorder within the system. At a Landau level filling factor , where the system can be described by the composite fermion picture, we observe a crossover from metallic to insulating behaviour as the disorder is increased, in agreement with theory. We also report experimental evidence of geometric effect for composite fermions induced by an effective magnetic field.     

Spin dependent surface recombination in silicon p-n junctions: The effect of irradiation

We present the results of an investigation of spin dependent recombination in (100) oriented, gate controlled Si diodes irradiated by 30 keV electrons. After irradiation, recombination at the SiSiO₂ interface is increased, and saturation of the spin resonance increases the diode forward current by 5 parts in 10⁴. The results cannot be described by a conventional Shockley-Read recombination model. An alternative picture is proposed involving recombination between trapped electrons and trapped holes.