Optical properties of nanocrystalline silicon embedded in SiO2

Nanocrystalline silicon embedded SiO2 matrix has been formed by annealing the a-SiOx films fabricated by plasma enhanced chemical vapor deposition technique. Absorption and photoluminescence spectra of, the films have been studied in conjunction with micro-Raman scattering spectra. It is found that absorption presents an exponential dependence of absorption coefficient to photon energy in the range of 1.5-3.0 eV, and a sub-band appears in the range of 1.0-1.5 eV. The exponential absorption is due to the indirect band-to-band transition of electrons in silicon nanocrystallites, while the sub-band absorption is ascribed to transitions between surfaces and/or defect states of the silicon nanocrystallites. The existence of Stokes shift between absorption and photoluminescence suggests that the phonon-assisted luminescence would he enhanced due to the quantum confinement effects.

Pressure behavior of deep centers in ZnSxTe1-x alloys

We have measured photoluminescence of ZnSxTe1-x alloys (x > 0.7) at 300 K and under hydrostatic pressure up to 7 GPa. The spectra contain only a broad emission band under excitation of the 406.7 nm line. Its pressure coefficients are 47, 62 and 45 meV/GPa for x = 0.98, 0.92 and 0.79 samples, which are about 26%, 7% and 38% smaller than that of the band gap in the corresponding alloys. The Stokes shifts between emission and absorption of the bands were calculated by fitting the pressure dependence of the emission intensity, being 0.29, 0.48 and 0.13 eV for the three samples, respectively. The small pressure coefficient and large Stokes shift indicate that the emission band observed in our samples may correspond to the Te isoelectronic center in the ZnSxTe1-x alloy.

Modelling of submarine landslides and generated water waves

The slide of unstable sedimentary bodies and their hydraulic effects are studied by numerical means. A two-dimensional fluid mechanics model based on Navier-Stokes equations has been developed considering the sediments and water as a mixture. Viscoplastic and diffusion laws for the sediments have been introduced into the model. The numerical model is validated with an analytical solution for a Bingham flow. Laboratory experiments consisting in the slide of gravel mass have been carried out. The results of these experiments have shown the importance of the sediment rheology and the diffusion. The model parameters are adjusted by trial and error to match the observed “sandflow”.

Two-dimensional numerical simulations of shock waves in micro convergent-divergent nozzles

The steady two-dimensional Navier-Stokes equations with the slip wall boundary conditions were used to simulate the supersonic flow in micro convergent-divergent nozzles. It is observed that shock waves can take place inside or outside of the micronozzles under the earth environment. For the over-expanded flows, there is a boundary layer separation point, downstream of which a wave interface separates the viscous boundary layer with back air flow and the inviscid core flow. The oblique shock wave is followed by the bow shock and shock diamond. The viscous boundary layer thickness relative to the whole nozzle width on the exit plane is increased but attains the maximum value around of 0.5 and oscillates against this value with the continuous increasing of the nozzle upstream pressures. The viscous effect either changes the normal shock waves outside of the nozzle for the inviscid flow to the oblique shock waves inside the nozzle, or transfers the expansion jet flow without shock waves for the inviscid flow to the oblique shock waves outside of the nozzle. 

多相聚合物体系相分离动力学的理论和模拟

本文提出了将自洽场理论与多相流格子Boltzmann方法相结合的模型，并从该模型出发推导出了描述动量守恒的Navier-Stokes方程和扩散方程，验证了模型理论上的正确性。应用此模型，对聚合物的相分离过程进行了模拟。 首先证实了本模型最终能够得到正确的热力学平衡结果。对于高分子共混物和嵌段共聚物相分离的动力学过程。在分相各个阶段，对高分子共混物和嵌段共聚物都进行了验证。 其次，应用所提出的格子Boltzmann模型，分别对二元聚合物共混物和二嵌段共聚物的相分离后期相区增长过程进行了研究。 最后，通过模型的进一步扩展，实现了对具有复杂结构的嵌段共聚物和复杂共混物的模拟。

基质晶体的化学键性质与掺杂稀土离子发光特性间关系的研究

本文使用复杂晶体化学键理论对由Eu2+或Ce3+掺杂的复杂基质晶体的化学键性质进行了理论研究，同时对发光稀土离子的发光特性与其在所处晶体中所表现的化学键性质间的关系进行了理论上的探讨，所研究的对象是具有复杂晶体结构的系列晶态物质，其中包括Eu2+掺杂的碱土硼酸盐系列晶体、Eu2+掺杂的碱土硅酸盐系列晶体、Eu2+掺杂的碱土铝酸盐系列晶体，卤化硫硅酸饰系列晶体及Ce3+掺杂的碘化硫硅酸斓晶体等。理论计算的结果表明具有4f~(n+1) 4f ~n5d~1组态间跃迁Eu2+和Ce3+离子之所以在不同基质晶体中表现出不同的发光特性是与它在不同基质晶体中呈现出不同的化学键性质而造成的。更深入的研究发现，在晶体结构和中心离子配位环境非常相似的系列晶体中，’掺杂离子发射光的Stokes位移值与其所取代格位的平均共价性成正比的关系，这是由于Stokes位移的本质是发光离子在发光过程中向基质以光波辐射的形式传递能量，这一能量的大小是由基质的振动频率决定的，而基质的化学键性质恰恰决定着基质的振动频率。在碱土硼酸盐系列晶体中Ba2MgB2O6, BaBe2B2O6 1 Ba2LiB5Ojo和SrB4O7四种晶体所对应的点（Stokes shift vs Fc）处在同一条直线上，而SrA12B207和BaLiBO3晶体所对应的点并没有处在这条直线上。在碱土硅酸盐系列晶体中CaSiO3 1SrSiO3 , BaSiO3三种‘晶体所对应的点处于同一条直线上，而同一系列的Sr2LiSiO4F, BasS1O4Br，BaSSiO4Cl。三种晶体所对应的点却处在另一条直线上，造成这种分组的原因是由于处于同一直线上的晶体属于键性相近的晶体。进一步研究表明掺杂离子发射光的Stokes位移值不仅与所处的晶体有关，还与其在晶体中所处的不同格位的键性有关。在讨论键性与轨道劈裂能之间的关系时发现，化学键性质确实是影响劈裂能大小的一个因素，但由于还存在其它因素影响着轨道的能级劈裂，所以目前我们没能确定出化学键性质与轨道劈裂能之间的定量关系。

Accurate determination of electronic transition energy of carbon nanotubes from the resonant behavior of radial breathing modes and their overtones

The resonant Raman behavior of the radial breathing modes are very useful to analyze the electronic property of carbon nanotubes. We investigated the resonant behaviors of Stokes and anti-Stokes radial breathing mode and its overtone of a metallic nanotube, and show how to accurately determine the electronic transition energy of carbon nanotubes from radial breathing modes and their overtones. Based on the present results, the previously reported resonant Raman behavior of the radial breathing modes of SWINT bundles can be interpreted very well.

Photoluminescence of ZnS clusters in zeolite-Y

Two obvious emissions are observed from the ZnS clusters encapsulated in zeolite-Y. The emission around 355 nm is sharp and weak, locating at the onset of the absorption edge. The band around 535 nm is broad, strong and Stokes-shifted. Both the two emissions shift to blue and their intensities firstly increase then decrease as the loading of ZnS in zeolite-Y or clusters size decreases. Through investigation, the former is attributed to the excitonic fluorescence, and the latter to the trapped luminescence from surface states. The cluster size-dependence of the luminescence may be explained qualitatively by considering both the carrier recombination and the nonradiative recombination rates. Four peaks appearing in the excitation spectra are assigned to the transitions of 1S-1S, 1S-1P, 1S-1D and surface state, respectively. The excitation spectra of the clusters do not coincide with their absorption spectra. The states splitted by quantum-size confinement are detected in the excitation spectra, but could not be differentiated in the optical absorption spectra due to inhomogeneous broadening. The size-dependence of the excitation spectra is similar to that of the absorption spectra. Both the excitation spectra of excitonic and of trapped emissions are similar, but change in relative intensity and shift in position are observed.

ZnS1-xTex混晶的压力光谱

研究了ZnS1-xTex(0.02≤x≤0.3)混晶的静压光致发光谱。每块样品都观察到一个峰值比相应混晶带隙低很多的发光峰，来源于束缚在Ten(n≥2）等电子陷陆上的激子复合发光，且随压力（0．7GPa）而蓝移、发光峰的压力系数比相应混晶带边的都要小，随着Te组分的增加而减小，与混晶带隙压力系数的差别也越来越大。由于压力下与发光峰对应的吸引能量逐渐接近并超过激发光的能量，与发光峰有关的吸收效率降低，发光峰积分强度随着压力增加而减小。据此估算了Ten等电子中心的Stokes位移。发现Stokes位移随着Te组分的增加而减小。

单晶硅化学气相沉积反应器流场初探

对单晶硅化学气相沉积(CVD)反应器在沉积过程中的流场进行了初步分析。通过数值求解三维层流Navier-Stokes方程，研究了反应器内浮力效应所引起的流场对称性破坏。结果表明，由于存在浮力效应，轴对称几何体中也会发生非轴对称流场分布，从而影响单晶硅的均匀生长。

Observation of the resonant Raman behavior of individual single-walled carbon nanotubes

The different resonant Raman scattering process of single-walled carbon nanotubes (SWNTs) has been found between the Stokes and anti-Stokes sides of the radial breathing modes (RBMs), and this provides strong evidence that Raman spectra of some special diametric SWNTs are in resonance with their electronic transitions between the singularities in the one-dimensional electronic density of states in the valence and conduction bands, and other SWNTs axe beyond the resonant condition. Because of the coexistence of resonant and non-resonant Raman scattering processes for different diametric SWNTs, the relative intensity of each RBM does not reflect the proportion of a particular SWNT.

High temperature annealing behaviors of luminescent SIOx : H films

The effects of high temperature annealing on the microstructure and optical properties of luminescent SiOx:H films have been investigated. Micro-Raman scattering and IR absorption, in combination with atomic force microscopy (AFM), provide evidence for the existence of both a-Si clusters in the as-grown a-SiOx:H and Si nanocrystals in the 1170 degrees C annealed films. The dependence of optical coefficients (alpha) on photon energy (h nu) near the absorption edge (E-g) is found to follow the square root law: (alpha h nu)(1/2) proportional to (E-g - h nu), indicating that nano-Si embedded in SiO2 is still an indirect material. A comparison of the deduced absorption edge with the PL spectra shows an obvious Stokes shift, suggesting that phonons should be involved in the optical transition process.

Characterization of CdSe and CdSe/CdS core/shell nanoclusters synthesized in aqueous solution

CdSe nanoclusters overcoated with CdS shell were prepared with macapoacetic acid as stabilizer. The optical properties of CdSe nanoclusters and the influence of CdS shell on the electronic structures of CdSe cores were studied by optical absorption, photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. Based on PL and PLE results and the theoretical calculation on fine structure of bandedge exciton, a model of formation of excimer within the small clusters was proposed to explain the large Stokes shift of luminescence from absorption edge observed in PL results. (C) 2000 Elsevier Science B.V. All rights reserved.

Pressure behavior of deep centers in ZnSxTe1-x alloys

We have measured photoluminescence of ZnSxTe1-x alloys (x > 0.7) at 300 K and under hydrostatic pressure up to 7 GPa. The spectra contain only a broad emission band under excitation of the 406.7 nm line. Its pressure coefficients are 47, 62 and 45 meV/GPa for x = 0.98, 0.92 and 0.79 samples, which are about 26%, 7% and 38% smaller than that of the band gap in the corresponding alloys. The Stokes shifts between emission and absorption of the bands were calculated by fitting the pressure dependence of the emission intensity, being 0.29, 0.48 and 0.13 eV for the three samples, respectively. The small pressure coefficient and large Stokes shift indicate that the emission band observed in our samples may correspond to the Te isoelectronic center in the ZnSxTe1-x alloy.

Statistical analysis of microscale gas flows and stock price changes

First, recent studies on the information preservation (IP) method, a particle approach for low-speed micro-scale gas flows, are reviewed. The IP method was validated for benchmark issues such as Couette, Poiseuille and Rayleigh flows, compared well with measured data for typical internal flows through micro-channels and external flows past micro flat plates, and combined with the Navier-Stokes equations to be a hybrid scheme for subsonic, rarefied gas flows. Second, the focus is moved to the microscopic characteristic of China stock market, particularly the price correlation between stock deals. A very interesting phenomenon was found that showed a reverse transition behaviour between two neighbouring price changes. This behaviour significantly differs from the transition rules for atomic and molecular energy levels, and it is very helpful to understand the essential difference between stock markets and nature.