The kinetics of homogeneous nucleation of silver nanoparticles stabilized by polymers

The formation of Ag nanoparticles synthesized by homogeneous nucleation, stabilized by polymers (PVA and PVP) was monitored by UV–Vis spectrophotometry and transmission electron microscopy. Our aim was to differentiate between the two main phases of particle formation, i.e. nucleation and growth and to characterize their rates with the help of appropriate kinetic equations. Time resolved spectrophotometric measurements revealed that particle formation is an autocatalytic process: a slow, continuous nucleation phase (3–5 min) is followed by a rapid, autocatalytic growth phase where the maximal particle size is 5–7 nm. By freezing the reaction mixture, the process of particle growth can be followed from 5 to 40 min on TEM pictures. The first order rate constants were calculated and they are strongly depend on the polymer concentration. If the growing particles are attached by PEI to the surface of a solid support, the formation of silver nanoparticles can also be followed by atomic force microscopy (AFM) and we can control the particle growth on mica surface. The cross section analysis of the pictures show, that the particle growing process can be also monitored at solid–liquid interface.     

Cooperative effect of La-doping and template on surface photoelectron characteristics of mesoporous nano-TiO2

A mesoporous La-doped nano-TiO₂ was prepared by the sol-gel method using polyethylene glycol as the template. A combination of surface photovoltaic (SPV) and photoacoustic technologies, aided by laser Raman technology, was used to probe the photoexcited charge transfer transition behaviors in the surface space charge region of the sample. The results confirm that the lanthanum doping was responsible for inhibiting the transformation from the anatase to rutile form and grain growth, thus strengthening the microstructure that was formed after removal of the template clearly. The experiment reveals that appropriate La-doping resulted in an obvious increase in the SPV response. By contrast, the remainder of the template had a somewhat negative effect on the SPV response of the samples. The effect of both the La-doping and the removal of the template on the nonradiative de-excitation process of the main band-gap can be negligible, in spite of the nonradiative de-excitation processes of the sub-band-gap of the La-undoped sample, in which the surface states possessed the donor characteristic, being more obvious than that of the La-doped samples. The results showed that the electron-phonon interactions on the surfaces resulting from the nonradiative de-excitation process were closely dependent on the effect of quantum confinement of the mesoporous nano-TiO₂.     

On the structure and volume growth of submanifolds in Riemannian manifolds

The aim of this note is two-fold. First, we investigate the relations between the volume growth of a submanifold and its second fundamental form. In the second part, we discuss the relations between the index of some Schrödinger operators and the structure of a submanifold, and prove some one-end theorems.     

Growth of Pt thin films on WSe2

The morphology and structure of Pt deposited on a WSe₂(0 0 0 1) van der Waals surface have been investigated by reflection high energy electron diffraction and scanning tunneling microscopy. At room temperature, the initial growth is characterized by the formation of three-dimensional fcc Pt islands with (1 1 1) orientation. In contrast, at higher temperatures of about 450 °C the formation of a novel chemically ordered Pt-Se alloy is observed. Based on the diffraction patterns, a tetragonal DO₂₂-type structure of a Pt₃Se compound is suggested. With increasing Pt thickness, this chemically ordered alloy disappears and an additional superstructure occurs, which is accompanied by the coalescence of the islands. The observed superstructure is attributed to a strong Se diffusion towards the growth surface, forming most likely a PtSe₂ alloy with the CdI₂-type layered structure on the top surface. Due to the lateral lattice mismatch between the Pt(1 1 1) layers and the PtSe₂(1 1 1) top layer, a Moiré pattern with a period of 1.1 nm is created, which might be used as a long-range atomic pattern for further nanostructure growth.     

Oscillatory zoning caused by oscillating surface relaxations

Oscillatory zoning is a spatial variation in the composition of minerals. It has been observed in many different minerals and a variety of mechanisms have been proposed to explain it. We propose an equilibrium model of oscillatory zoning in which the variations in composition stabilise a ferroelastic phase. This results in a sinusoidal variation in composition. We expect that this mechanism could account for oscillatory zoning found in minerals with oscillatory surface relaxations. Received 9 March 1999     

吴大猷先生点评《经典力学》

文章介绍了吴大猷先生对《经典力学》中一些基本问题的意见以及他的近期工作。吴先生认为,Hamilton原理是一个物理原则,而不仅仅是数学变分问题。他还对“齐次问题”和“猝变运动”作了讨论。     

Method for independent control of particle size and distance in rhodium epitaxy on TiO2(110)-(1×2) surface An STM study

A method for independent control of the particle size and distance is presented for rhodium epitaxy on TiO₂(110)-(1×2) surface. The real space imaging of the surface morphology was performed by scanning tunneling microscopy. The amount of the deposited rhodium was checked by Auger electron spectrometry. The method consists of two steps: (i) evaporation of 0.001–0.050 ML equivalent of rhodium at room temperature with a post-annealing at 1100 K (“seeding”); (ii) post-deposition of rhodium for growing of the Rh nanoparticles formed in step (i) (“growing”). The mechanism of this procedure is based on the large difference of the surface diffusion coefficient between Rh adatoms and Rh nanocrystallites larger than 1–2 nm. In the first step the average distance between the metal particles is controlled in the range 5–200 nm, the second step determines the particles size (2–50 nm). This work demonstrates that the diffusion processes of metal nanoparticles of different sizes and the growing modes of the crystallites can be studied in detail by application of seeded surfaces.     

非晶氧化钛薄膜形成过程中钛离子能量对表面结构影响的机理

研究了非晶氧化钛薄膜沉积过程中入射钛离子能量对表面结构形成机理以及薄膜特性的影响.模拟结果表明,通过提高入射钛离子能量,可以有效降低成膜表面粗糙度,从而减小薄膜表面的光学散射损耗.研究发现,当入射离子能量提高后,薄膜生长模式从"岛"状生长过渡到了"层"状生长,且离子入射点附近的平均扩散系数也有显著增加,这有利于形成更加平整的高质量薄膜表面.     

Growth and spectral properties of Yb:Ca0.28Ba0.72Nb2O6 disordered crystal

The growth, XRD patterns, spectral properties, and fluorescence decays of Yb:Ca_0.28Ba_0.72Nb₂O₆ (Yb:CBN) with doping concentration of 1 at.% and 5 at.% were studied. The peak absorption cross-section and the emission cross-section were calculated. Larger Stark splitting of Yb:CBN offers the prospect of the quasi-four level laser operation.     

LIA注入器发射度增长机理研究及其抑制措施

根据直线感应加速器(LIA)的特点采用了轴对称直流模型,结合E.P.Lee的理论导出了发射度增长的微分方程,提出了一种直线感应加速器发射度增长的机理,根据该机理计算了从均匀分布到高斯分布的演变过程中发射度增长的大小,指出直线感应加速器(LIA)注入器阶段束流发射度增长主要是由于非线性效应导致的束刨面电流密度分布变化引起的.根据研究结果,提出了相应的抑制发射度增长的措施.