From amorphous to crystalline silicon nanoclusters: structural effects on exciton properties

Synchrotron x-ray absorption spectroscopy (XAS) and electron spin resonance (ESR) experiments were performed to determine, in combination with Raman spectroscopy and x-ray diffraction (XRD) data from previous reports, the structure and paramagnetic defect status of Si-nanoclusters (ncls) at various intermediate formation stages in Si-rich Si oxide films having different Si concentrations (y = 0.36-0.42 in Si(y)O(1-y)), fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition and isochronally (2 h) annealed at various temperatures (T(a) = 900-1100 °C) under either Ar or (Ar + 5%H(2)) atmospheres. The corresponding emission properties were studied by stationary and time dependent photoluminescence (PL) spectroscopy in correlation with the structural and defect properties. To explain the experimental data, we propose crystallization by nucleation within already existing amorphous Si-ncls as the mechanism for the formation of the Si nanocrystals in the oxide matrix. The cluster-size dependent partial crystallization of Si-ncls at intermediate T(a) can be qualitatively understood in terms of a 'crystalline core-amorphous shell' Si-ncl model. The amorphous shell, which is invisible in most diffraction and electron microscopy experiments, is found to have an important impact on light emission. As the crystalline core grows at the expense of a thinning amorphous shell with increasing T(a), the PL undergoes a transition from a regime dominated by disorder-induced effects to a situation where quantum confinement of excitons prevails.     

Theoretical study of structural, mechanical and spectroscopic properties of boehmite (γ-AlOOH)

The structural, mechanical and spectroscopic properties of boehmite (AlOOH polymorph) were investigated by means of first-principle density functional theory (DFT) and semiempirical density functional based tight binding (DFTB) methods. Apart from a marginal underestimation of interlayer hydrogen bond distances the DFT method well reproduces the experimental equilibrium low-pressure structure. For the DFTB method similar good agreement was obtained for lattice parameters, however bond lengths and angles showed a larger deviation from experiment in comparison to DFT results. The experimental spectrum of the OH stretching region was interpreted by means of the calculated frequencies within the frame of the harmonic approximation and by calculating the power spectra of the hydroxyl groups obtained from molecular dynamics simulations. Using the latter approach, the strong coupling between the individual OH modes was demonstrated. Isostatic structural compression of the boehmite structure was performed in order to obtain the bulk modulus and the dependence of the vibrational spectrum on the pressure. The DFT method gives a value of 97 GPa in the athermal limit. Comparison with available bulk moduli for other AlOOH polymorphs reveals that boehmite shows the highest compression, for which mainly a strong shortening mechanism of interlayer hydrogen bonds is responsible. The DFT method also described correctly the dependence of the OH stretch frequencies upon compression resulting in a strong red shift. Although good performance is observed for the low-pressure region, the DFTB method is not found to be suitable for high-pressure studies in cases such as boehmite.     

MQD--multiplex-quadrature detection in multi-dimensional NMR

With multiplex‐quadrature detection (MQD) the tasks of coherence selection and quadrature separation in N‐dimensional heteronuclear NMR experiments are merged. Thus the number of acquisitions required to achieve a desired resolution in the indirect dimensions is significantly reduced. The minimum number of transients per indirect data point, which have to be combined to give pure‐phase spectra, is thus decreased by a factor (3/4)^N?1. This reduction is achieved without adjustable parameters. We demonstrate the advantage by MQD 3D HNCO and HCCH‐TOCSY spectra affording the same resolution and the same per‐scan sensitivity as standard phase‐cycled ones, but obtained in only 56 % of the usual time and by resolution improvements achieved in the same amount of time.     

Qualitative Analysis of Gradient-Type Systems with Oscillatory Nonlinearities on the Sierpi′nski Gasket

Under an appropriate oscillating behavior either at zero or at infinity of the nonlinear data, the existence of a sequence of weak solutions for parametric quasilinear systems of the gradient-type on the Sierpi′nski gasket is proved. Moreover, by adopting the same hypotheses on the potential and in presence of suitable small perturbations, the same conclusion is achieved. The approach is based on variational methods and on certain analytic and geometrical properties of the Sierpi′nski fractal as, for instance, a compact embedding result due to Fukushima and Shima.     

科学技术名词规范化发展与现状

@@@@文章介绍了全国科学技术名词审定委员会的历史和成果,阐述了科技名词审定的原则和术语学研究的进展,还介绍了海峡两岸科技名词对照工作的成果与未来的两岸名词对照工作的计划。     

Synthesis and Crystal Structure of a Novel Co(II)-Nitronyl Complex [Co(IM2-Py)(PCA)(N_3)_2]

1 INTRODUCTION The design and synthesis of molecule-based mag-netic materials is one of the major subjects of mate-rials science. Nitronyl nitroxides acting as useful pa-ramagnetic building blocks have been extensively usedto assemble molecular magnetic materials in the pastfew years[1～5]. However, their weakly basic charac-ter strongly limits their coordination ability. Mean-while, the azide anion is a versatile ligand which canlink the transition metal atoms with different coor-dinatio…     

机械化学法制备的Al/Ce复合氧化物负载NiMo催化剂的加氢脱硫活性(英文)

将Al和/或Ce的硝酸盐与NH_4HCO_3球磨混合,采用机械化学法制备了Al_2O_3-CeO_2载体(其中Ce含量为1-10 wt%),再以杂多钼酸盐(NH_4)_4NiMo_6O_(24)为活性金属Ni和Mo的前驱体,通过浸渍法制备NiMo_6/Al_2O_3-CeO_2催化剂,并将其用于1-苯并噻吩(BT)的加氢脱硫(HDS)反应中.运用化学分析、X射线衍射、N_2吸附-脱附法、H_2程序升温还原、NH_3程序升温脱附和X射线光电子能谱对催化剂的物化性质进行了表征.结果表明,催化剂的酸性随着载体中Ce含量的增加而下降.Ce修饰的Al_2O_3载体表面的NiS/MoS原子比下降了2倍.NiMo_6/Al_2O_3催化剂表现出最高的HDS活性(以单位重量催化剂计),其酸性和表面MoO_xS_y物种的浓度(Mo~(5+)浓度)也最高;当催化剂中Ce含量逐渐增加到10 wt%以上时,其活性逐渐将为0.与浸渍法制得的参比催化剂相比,前驱体和催化剂组成相同的情况下,采用机械化学法制得的所有催化剂活性明显提高.     

二茂铁基化合物的ESR研究进展

二茂铁基化合物因具有独特的氧化还原、电、光、磁和催化性能,而被广泛研究与应用.电子自旋共振(ESR)技术是观察物质顺磁性最直接的手段,在研究二茂铁基化合物的电子结构、电子转移和分子间相互作用等方面具有独特的优势.ESR技术还具有灵敏度高、样品处理简单、直接检测不破坏样品等优点.该文综述了二茂铁基化合物的ESR研究进展.     

活性自由基聚合法制备多壁碳纳米管表面槲皮素分子印迹聚合物及其应用

为了制备能有效分离富集药草中槲皮素的固相萃取柱,以丙烯酰胺(AM)修饰的碳纳米管为载体,三硫代碳酸酯(DBTTC)为可逆加成-断裂链转移剂(RAFT试剂),槲皮素为模板,甲基丙烯酸(MAA)为功能单体,乙二醇二甲基丙烯酸酯(EGDMA)为交联剂,乙腈为致孔剂,制备了槲皮素分子印迹聚合物,采用红外光谱、扫描电镜和热重分析对印迹材料进行表征,通过高效液相色谱(HPLC)研究聚合物的吸附性能和对底物的特异性识别能力。结果表明,通过活性自由基聚合法合成的多壁碳纳米管表面槲皮素分子印迹聚合具有更好的形态结构和吸附性能,且对槲皮素有很好的特异性识别能力。     

Remarks on Thurston’s Construction of Pseudo-Anosov Maps of Riemann Surfaces

It is well known that certain isotopy classes of oseudo-Anosov maos on a Riemann surface S of non-excluded type can be defined through Dehn twists tα and tβ along simple closed geodesics α and β on S,respectively. Let G be the corresponding Fuchsian group acting on the hyperbolic plane H so that H/G≌S.For any point α∈S,define S = S／{α}.In this article, the author gives explicit parabolic elements of G from which he constructs pseudo-Anosov classes on S that can be projected to a given pseudo-Anosov class on S obtained from Thurston＇s construction.