新实验技术在材料研究中的应用讲座 第四讲 电子通道效应及其应用

一、引 言 电子通道效应是扫描电镜中的一项重要物理现象,它的发现进一步扩大了扫描电镜在材料科学和金属物理中的应用. 对电子与晶体相互作用的研究表明,入射电子被晶体的散射几率是同它相对于(hkl)晶面的入射角有关.在某些入射方向下,电子被散射的几率较大(相当于禁道),而在另一些入射方向下,电子被散射的几率较小(相当于通道),这种现象称为电子通道效应. 在扫描电镜中,应用电子通道效应所产生的花样和显微象称为电子通道花样和通道显微象.由于上述花样和显微象能反映晶体的位向、晶体的表面状态以及晶体的不完整性,因此,它在材料科学和金…     

电子通道花样的几何测量和解释

在扫描电镜中,应用电子通道效应所产生的花样称为电子通道花样(简写为ECP).作为ECP的应用和分析基础,首先必须对ECP进行几何测量和结晶学注释.本文作为1982年第11卷第12期《物理》讲座栏内“电子通道效应及其应用”[1]一文在实验方法上的补充,以便对这个专门的问题进行系统的阐述. 一、ECP的成象几何关系 在文献[1]中曾系统地阐述了ECP的成象几何关系的特点.根据这些特点,并引入等效投射距离R这个参数,则可以画出ECP的成象等效几何关系,如图1所示. 在图1中,两个相交的电子通道带依次属于(hikili)晶面和(hikili)晶面,这两个晶面的晶…     

振子强度密度理论研究——类锂离子3s态到p通道跃迁振子强度密度

以类锂离子3s态到p通道光跃迁为例,在单通道理论框架下统一处理从一个初态(3s)到一个通道(p通道)中无数个激发态(包括束缚态np和连续态εp)的光跃迁过程。具体地根据相对论性和非相对论性原子自洽场理论,分别计算了类锂离子3s态到p通道跃迁的有关振子强度密度,阐明了其相对论效应以及振子强度密度随原子序数变化的标度规律。根据本文所计算的振子强度密度,可方便地获得3s跃迁至所有np束缚激发态的振子强度。 关键词：     

Fe-10%Cr铁素体合金中氢对辐照诱起偏析的影响

Fe-10％Cr铁素体合金经室温注氢后,在620-720K进行电子辐照时,会出现一种新的共格偏析现像。与此偏析相对应,在电子衍射图像中可以观察到明亮的条纹衍射。分析结果表明,这种偏析为沿<100>方向的Cr元素的针状共格偏析,其长度约为100nm,直径为1至2个原子尺寸。氢元素和电子辐照产生的纯点缺陷是形成这种偏析的必要条件。 关键词：     

隧道结的发光效应

一、基本原理 根据量子力学原理,如果被真空或绝缘物分开的二电极间间隙足够窄,窄到仅有几个原子直径的距离,这时即使电子没有足够的能量“越过”由这个间隙形成的势垒,由于电子的波动性它们也能经“隧道”穿过势垒.这种现象称作电子隧道效应. 观察电子隧道效应的实验模型是隧道结:一层约20A厚的绝缘物夹在两层金属薄膜之间.当在这两层金属膜之间加上小的偏压 V时,便有电子隧道穿过绝缘层形成隧道电流,此时电子获得能量eV. 隧道电子通过绝缘层有两条通道:弹性通道和非弹性通道.在弹性通道中,电子隧穿获得的能量没有损失;在非弹性通道中,电…     

纯元素厚试样的标识X射线强度比

本文在15—30kV加速电压下测定了电子激发的八种纯元素(从铝到锗)的K系X射线强度和四种纯元素(从镍到锗)的L系X射线强度。利用经过我们修正并简化的“完全扩散”电子扩散模型,计算出了这些元素的标识X射线强度比值。新模型本身包含了背散射效应,由模型给出的台阶状X射线分布函数,得出了各元素的吸收修正因子,还考虑了连续谱的荧光效应。计算值与实验值相符合,从而验证了模型的可靠性。 关键词：     

随机驱动等离子体电流

本文探索波驱动等离子体电流的一种新机制,称之为随机驱动效应。在相干波与磁化等离子体相互作用的系统中,当波的强度超过某个阈值时,速度空间中的一些共振区将互相重迭。共振区的重迭可导致粒子的随机运动,引起速度空间中的扩散现象。如果在电子平行于磁场方向的速度分布中,这种重迭区是不对称的,这样,就可驱动纵向等离子体电流。 关键词：     

CIDNP及其在光化学中的应用

化学诱导动态核极化(Chemically Induced Dynamic Nuclear Polarization,简称CIDNP)是一种在化学反应体系中观察到的NMR谱线强度反常的现象(常称之为极化谱,包括增强吸收和发射等)。1967年由Bargon和Fischer^[1] (西德)以及Ward和Lawler^[2] (美国)首先发现的。由于他们所研究的反应体系中涉及自由基,因此最初认为这种谱线强度反常现象是由于自由基中间体中的电子-核交叉弛豫引起的。这种机理通常称为:Overhauser型的动态核极化(Dynamic Nnclear Polarization简称DNP)。但很快就发现它不足以解释实验得到的结果,而CIDNP的命名却是这样因袭下来了。     

非共面索末菲参量对三重微分截面的影响

考虑末通道He⁺中剩余电子的屏蔽效应之后,在非共面几何条件下利用3种不同的修正方案,详细地比较并研究了不同索末菲参量在非共面双对称几何条件下对He原子 (e, 2e) 反应中三重微分截面所产生的影响.讨论了产生这种影响的物理本质. 关键词： 非共面 双对称几何条件 三重微分截面 索末菲参量     

调制掺杂GaAs/AlGaAs 2DEG材料持久光电导及子带电子特性研究

在掺Si的GaAs/AlGaAs二维电子气(2DEG)结构中，得到μ_2K=1.78×10^{6cm2/(V·s)的高迁移率.在低温(2K)和高磁场(6T)的条件下，对样品进行红 光辐照，观察到持久光电导(PPC)效应，电子浓度在光照后显著增加.通过整数量子霍尔效应 (IQHE)和Shubnikov-de Haas (SdH)振荡的测量，研究了2DEG的子带电子特性.样品在低温光 照后2DEG中第一子带和第二子带的电子浓度同时随电子总浓度的增加而增加；而且电子迁移 率也明显提高.同时，通过整数霍尔平台的宽度对光照前后电子的量子寿命变短现象作了理 论分析. 关键词： 二维电子气 量子霍尔效应 SdH振荡 持久光电导效应}     

Complete characterization of an (Sb2O3)n/SWNT inclusion composite

The detailed inclusion crystallography of a one-dimensional valentinite Sb₂O₃ crystal incorporated within a helical (21, ?8) single-walled carbon nanotube (SWNT) was identified from a phase image that was recovered via a modified object wave restoration scheme. A detailed analysis of asymmetric fringe contrast in the tube walls provided strong evidence for the chiral sense of the tube itself. Due to the good agreement of the observed wall periodicity with the determined absolute focus values and power spectra obtained from single-pixel line traces along both tube walls, we were able to determine the chiral sense of the SWNT and the tilt angle of the Sb₂O₃/SWNT composite relative to the electron beam. The angle between the optimum $\left\langle {10\bar 1} \right\rangle $ viewing direction of the crystal fraction and the tube axis, which is aligned with the $\left\langle {4\bar 12} \right\rangle $ direction of the Sb₂O₃ crystal, is 78.3°. Since small deviations from this viewing direction make an insignificant difference to the observed contrast, a tube inclination of 15° is plausible for both the Sb₂O₃ crystal and the assigned (21, ?8) SWNT, which is the mirror image of a (13, 8) SWNT.     

Growth of AgInSe<Subscript>2</Subscript> on Si(100) substrate by thermal evaporation technique

AgInSe₂ films were prepared by a thermal evaporation technique onto Si(100) substrates at a pressure of 10⁻⁵ mbar. Structural and optical properties of films deposited at 300 and 473 K have been investigated. The film composition was studied by energy dispersive analysis through X-rays. X-ray diffraction patterns indicate that AgInSe₂ films have chalcopyrite structure with strong preferred orientation in the (112) direction. Average vertical crystallite size of 25 nm was observed. The optical energy gaps of 1.20 and 1.90 eV were obtained due to the fundamental absorption edge and a transition originating from crystal field splitting, respectively. Field emission scanning electron microscopy shows loosely packed grains of spherical symmetry with some facets.     

静电场对α-LiIO3单晶透射光的影响

沿α-LiIO₃单晶z轴向施加静电场。当沿z轴方向通光时,几乎观察不出晶体透射像的变化;当偏离z轴方向通光时,透射像上显现出一些花纹。本文还作了空间频谱分析,显微观察和光衍射。 关键词：     

Branched crystal morphology of linear polyethylene crystallized in a two-dimensional diffusion-controlled growth field

The branched crystal morphology of linear polyethylene formed at various temperatures from thin films has been studied by atomic-force microscopy (AFM), transmission electron microscopy (TEM), electron diffraction (ED) pattern and polymer decoration technique. Two types of branched patterns, i.e. dendrite and seaweed patterns, have been visualized. The fractal dimension d _f = 1.65 of both dendrite and some of seaweed patterns was obtained by using the box-counting method, although most of the seaweed patterns are compact. Selected-area ED patterns indicate that the fold stems tilt about 34.5^° around the b-axis and polymer decoration patterns show that the chain folding direction and regularity in two (200) regions are quite different from each other. Because of chain tilting, branched crystals show three striking features: 1) the lamella-like branches show two (200) regions with different thickness; 2) the crystals usually bend towards the thin region; 3) the thick region grows faster by developing branches, thus branches usually occur outside the thick region. The branched patterns show a characteristic width w, which gives a linear relationship with the crystallization temperature on a semilogarithmic plot. Received 15 March 2002 and Received in final form 29 April 2002     

Unique crystal growth with crystal axis rotation in multi-ferroic β′-(Sm,Gd)2(MoO4)3 narrow lines patterned by lasers in glass

The morphology and orientation of multi-ferroic β′-(Sm,Gd)₂(MoO₄)₃ crystals in the lines patterned by laser irradiations in 3Sm₂O₃–18.25Gd₂O₃–63.75MoO₃–15B₂O₃ (mol%) glass were examined using transmission electron microscope (TEM) observations in order to clarify the origin of the appearance of periodic birefringence in an atomic-scale level. The crystallographic direction of crystals in the lines was determined from selected area electron diffraction (SAED) patterns in TEM observations, and it was demonstrated that a gradual rotation of crystallographic axes takes place along the crystal growth direction (laser scanning direction). The unique crystal growth with a spiral-like rotation of the crystallographic axes is due to the gradual rotation of (MoO₄)²⁻ tetrahedral units in β′-(Sm,Gd)₂(MoO₄)₃ crystals. The laser-induced crystallization, in which the region and direction of crystal growth are restricted artificially to a narrow space, provides a new crystal growth engineering.     

马氏体时效钢的时效结构

本文利用透射电子显微相和电子衍射相对18Ni马氏体时效钢中沉淀强化相和时效结构的结晶学性质进行了研究。利用数个不同晶带的衍射相,一致地鉴别出同时存在Ni₃Mo,Ni₃Ti沉淀相和弥散的逆转变奥氏体,并且确定了它们相对马氏体基体的取向关系。证明了沉淀相、逆转变奥氏体与基体之间,密排面和密排方向都相互平行。指出在透射电子显微相中看到的魏氏型沉淀结构,实际上是由棒状沉淀相构成的三维空间格条结构,其中棒状沉淀相的长度方向平行于基体的四个〈111〉型方向。所观察到的扩张位错和位错塞积的真实性需要更多的实验证实。从结构观点,讨论了沿位错线的沉淀过程,并提出了钴钼交互作用的可能机制。     

Compton scattering study of orthorhombic sulphur

The Compton profiles of orthorhombic sulphur crystals have been measured the scattering vector perpendicular to the crystal planes (110) and (001) using 60 keV radiation from a ²⁴¹Am source. Significant anisotropy was observed in the electron momentum distribution. Fourier transformed Compton profiles were compared with theoretical autocorrelation functions for a S₈ ring molecule. It is shown that the observed anisotropy can be only partially interpreted in terms of intermolecular interactions. The strong correlation, observed in the direction perpendicular to the plane of the S₈ ring indicates a meaningful ring — ring interaction in the orthorhombic sulphur crystal.     

Fabrication of high quality two-dimensional photonic crystal mask layer patterns

This work discusses the fabrication of two-dimensional photonic crystal mask layer patterns. Photonic crystal patterns having holes with smooth and straight sidewalls are achieved by optimizing electron beam exposure doses during electron beam lithography process. Thereafter, to precisely transfer the patterns from the beam resist to the SiO₂ mask layer, we developed a pulse-time etching method and optimize various reaction ion etching conditions. Results show that we can obtain high quality two-dimensional photonic crystal mask layer patterns.     

偏振对飞秒激光辐照LiF晶体的影响

飞秒激光聚焦到LiF晶体内部, 晶体的加工形貌随偏振改变. 实验表明, 偏振方向平行于<110> 晶向时, 加工起点到表面的距离是<100>偏振下的1.08 倍; 而<110>偏振下加工终点到表面的距离是<100> 偏振下的1.01 倍. 为了解释加工形貌的偏振依赖, 建立了逆韧致辐射、雪崩电离和无辐射跃迁的模型, 首先, 价带电子通过强场电离和雪崩电离, 从激光中吸收能量跃迁到导带, 该过程用电子密度演化方程和傍轴非线性薛定谔方程描述, 求解方程得到导带电子密度; 其次, 导带电子通过无辐射跃迁过程释放能量给晶格, 由能量守恒计算出晶格温度沿激光传播方向的分布; 最后, 晶格温度超过熔点以上的区域被加工. 模拟结果显示, <110>偏振下加工起点到表面的距离是<100> 偏振下的1.03倍, 而<110>偏振下加工终点到表面的距离是<100>偏振下的0.981 倍, 与实验结果基本一致. 虽然Z扫描技术测量的非线性折射率随偏振方向变化, 但是非线性折射率的变化趋势与实验结果相反. 模拟和实验证明逆韧致辐射导致加工形貌随偏振变化.     

单轴应力场下钨中氦扩散行为的分子动力学模拟研究

本文采用分子动力学方法研究了沿<100>及<111>晶向的单轴应变对钨中单个氦原子扩散的影响。结果表明,应变会使得金属钨材料发生相变,且引起相变的临界应变随温度升高而减小。相变起始的应变在达到抗拉强度的应变附近。计算结果表明,拉应变使得单个氦原子在钨中的扩散系数发生骤降,在不同应变下扩散系数变化平缓。沿<100>晶向氦扩散系数随应变的增大而线性减小,而<111>晶向则出现了震荡变化趋势。研究结果表明,沿<100>晶向应变达到+0.15%时阿纽列斯方程不再适用,而沿<111>晶向应变大+5%阿纽列斯方程仍然适用;沿<111>晶向随应变增加氦扩散激活能减小,说明应变使得单个氦原子在钨中迁移性增强。