Effect of Eshelby twist on core structure of screw dislocations in molybdenum: atomic structure and electron microscope image simulations

This paper addresses the question as to whether the core structure of screw dislocations in Mo in the bulk can be obtained from high-resolution electron microscopy (HREM) images of such dislocations viewed end-on in a thin foil. Atomistic simulations of the core structure of screw dislocations in elastically anisotropic Mo were carried out using bond order potentials. These simulations take account automatically of the effects of the surface relaxation displacements (anisotropic Eshelby twist). They show that the differential displacements of the atoms at the surface are different with components perpendicular to the Burgers vector about five times larger than those in the middle of the foil, the latter being characteristic of the bulk. Nye tensor plots show that the surface relaxation stresses strongly affect the incompatible distortions. HREM simulations of the computed structure reflect the displacements at the exit surface, modified by interband scattering and the microscope transfer function. Nye tensor plots obtained from the HREM images show that interband scattering also affects the incompatible distortions. It is concluded that it would be very difficult to obtain information on the core structure of screw dislocations in the bulk Mo from HREM images, even under ideal experimental conditions, and that quantitative comparisons between experimental and simulated images from assumed model structures would be essential.     

Au/Ni Multilayer Transformation with Temperature: A HREM Study

Growth of Ni on (001) Au has been widely studied. Furthermore it has been shown that a structural change occurs when an Au/Ni multilayer is heated up. This transformation can be described as an ordering which is unexpected from the equilibrium phase diagram. A high-resolution electron microscopy study of the transformation of a coherent Au/Ni multilayer is presented in this paper. The transformation has been characterized by in-situ HREM and by extensive image processings made on both simulated and experimental images.     

High-resolution electron microscopy of heterostructures and interfaces

High-resolution transmission electron microscopy (HREM) allows to study a wide range of device-relevant topics in heteroepitaxial layer structures. Quantitative HREM may be used to obtain chemical information on a near-atomic scale from interfacial transition zones. The physical background is described and demonstrated on several examples in the Al _x Gal_1–x As/GaAs system. The HREM contrast of antiphase boundaries in InP grown on Si was studied by image simulations and has been compared to experimental images. Silicon carbide precipitates were identified by HREM at the homoepitaxial Si/Si interface. They stem from carbon contamination prior to Si layer growth.     

High resolution transmission electron microscopy to study very thin crystalline layers buried at an amorphous-crystalline interface

Structure characterisation of interfaces is a field of widespread application of high resolution transmission electron microscopy for its very high spatial resolution. Specimen thickness and electron optical condition have a deep influence on the high resolution electron transmission microscopy image contrast. Hence, in many cases, the real structure of the sample can be understood from experimental images only by comparison with the relevant simulation. Moreover, the understanding of the contrast variation of a few A at an interface is a task in which even the use of simulation could not produce an unequivocal solution of the experimental result. In this paper high resolution transmission electron microscopy image simulations show that two monolayers of crystalline material buried at an amorphous-crystalline interface can be successfully revealed and interpreted. The simulated images reproduce the experimental results as obtained from the Al/Si-As/n-GaAs (001) heterostructure.     

Direct observation of the CuO chains adsorbed on a ultra-thin film of Cu(110) using a high resolution electron microscope

We have, for the first time, observed directly the CuO chains adsorbed on a Cu(110) surface using a high resolution electron microscope (HREM). This observation was confirmed by a multi-slice image simulation based on electron diffraction and imaging theory. Our calculations indicate quantitatively under what conditions an adsorption layer can be observed by HREM with sufficient contrast. This study reveals the potential of HREM to investigate surface structures of a ultra-thin film with thicknesses of nanometers at atomic level.     

一种适用于非周期结构高分辨象模拟计算的新方法

通常的FFT多片层法(FFTMS)动力学衍射计算中,由于采用了边界采样点周期性延伸的假设,从而导致非周期结构高分辨象模拟的严重失真。在仔细研究这种误差成因的基础上,本文提出了一种适用于非周期结构的高分辨象模拟计算新方法,它基于实空间多片层(RSMS)象模拟计算方法,但在势投影及传播因数计算中充分考虑了非周期结构的特点,从而得到与实验符合较好的结果。 关键词：     

High-resolution transmission electron microscopy and tomographic atom probe studies of the hardening precipitation in an Al–Cu–Mg alloy

The hardening precipitation of an Al–Cu–Mg aluminium alloy designed for aeronautics was investigated using high-resolution transmission electron microscopy (HREM) and tomographic atom probe techniques. The observed precipitates clearly belong either to the Guinier–Preston–Bagaryatskii (GPB) zones type or to the so-called S-Al₂CuMg precipitation. We analysed a large number of precipitates in order to obtain statistical information on the precipitation. We focused on the structural and/or chemical composition of the different precipitates. It was found, in particular, that the very numerous GPB zones do not present a single chemical composition. Evidence is also given for the presence of two different kinds of S-precipitate/matrix orientation relationships, strongly linked to the morphology of the precipitate. The structure of the S precipitates was confirmed by direct comparison with simulated HREM images. Particular attention was paid to the nature of the S-precipitate/matrix interfaces.     

A structural model of the orthorhombic Ga7Mn5 derived from its HREM image

The base-centered orthorhombic approximant Ga7Mn5 (a = 4.547(6) nm, b = 1.256(9) nm, c = 1.482(1) nm) consisting of flattened hexagon and concave octagon (called "crown" in this paper) subunits shown by high-resolution electron microscopy (HREM) was found to coexist with the Ga-Mn decagonal quasicrystal in the Ga58Mn42 alloy. The atomic sites in the flattened hexagon subunit have been deduced recently from a structural study of the orthorhombic Ga-Mn (1/1,1/1) approximant that consists only of the hexagon subunits. Similarly, the atomic sites in the crown subunit have been deduced from another orthorhombic Ga-Mn (2/1,1/1) approximant consisting only of the crown subunits. A structural model of the orthorhombic Ga7Mn5 can thus be constructed from these structural subunits. There are 332 Ga atoms and 232 Mn atoms in a unit cell, with a composition of Mn41.2Ga58.8, satisfying the symmetry of the space group Bmm2. The simulated electron diffraction patterns, [010] HREM images, and X-ray powder diffraction pattern based on this structural model agrees mainly with the experimental observations.     

Atomic structure of boron nitride nanotubes with an armchair-type structure studied by HREM

Hexagonal networks of boron nitride (BN) nanotubes were investigated by high-resolution electron microscopy (HREM) and image simulation. From HREM images, lattice planes of {002} and hexagonal rings of a BN nanotube were confirmed. Asymmetrical layer arrangements were found, and a structure model for double-walled BN nanotube with an armchair-type structure has been proposed.     

Solving modulated structures by X-ray and electron crystallography

X-ray diffraction can be used for accurately determining not only classical, ordinary structures, but also modulated ones. For structures with weak modulations, the modulation induced satellite reflections are often hard to be observed by X-ray diffraction, but they appear clearly in electron diffraction. In these cases, X-ray diffraction will give only average structures whereas electron diffraction will yield information about the modulations. Sr(1.4)Ta(0.6)O(2.9) is a complex modulated compound with weak modulation and small modulated domains. Here we demonstrate the power of combining X-ray and electron crystallography for studying modulated structures on powders. The modulations of Sr(1.4)Ta(0.6)O(2.9) were determined from electron diffraction (SAED) and high resolution electron microscopy (HREM) images. With specially developed image processing techniques, the weak modulations were enhanced, facilitating the interpretation of HREM images in terms of atomic structure.     

3C-SiC薄膜小角晶界附近位错核心的原子组态研究

用LaB₆灯丝200 kV高分辨透射电镜拍摄了有小角晶界的3C-SiC/(001)Si 薄膜的[110]高分辨电子显微像. 用像解卷技术把本不直接反映晶体结构的实验像转化为结构像. 首先, 从完整区的结构像中分辨开间距仅为0.109 nm的Si和C原子柱; 随后按赝弱相位物体近似像衬理论, 分析像衬随晶体厚度的变化规律, 辨认出Si和C原子; 进而在原子水平上得出小角晶界附近两个复合位错的核心结构, 构建了结构模型并计算了模拟像. 实验像与模拟像的一致程度验证了结构模型的正确性. 于是, 在已知完整晶体结构的前提下, 仅从一帧实验高分辨像出发, 推演出原子的种类和位错核心的原子组态. 还讨论了3C-SiC 小角晶界的形成与晶界附近出现复合位错的关系.     

Selection rules for Bloch wave scattering for HREM imaging of imperfect crystals along symmetry axes

A Bloch wave analysis is used to investigate high-resolution electron microscope (HREM) imaging of crystals containing atomic displacements due to strain. In the absence of interband scattering, the shifts of peaks and troughs in the image will correspond to the displacements of the atoms in the exit surface. Interband scattering will shift the image peaks away from the actual atom positions and modify the apparent magnitude of the displacement identified by the observed image peak positions. By considering the case of seven-beam imaging of a cubic crystal aligned along a ?111? axis, it is shown that the symmetry of the Bloch waves leads to selection rules for the interband scattering, similar to those seen for dipole electron excitations in atoms. It is also shown that, to first order, no intraband scattering can occur.     

Structural basis for homogeneous CdS nanorods: synthesis and HREM characterization

Since the devolution of multiple efforts for producing semiconductor nanorods, the control of size and structure become critical for their applications in nanotechnology. The use of distinct methods has allowed a partial control of the size of the nanorods; however, the internal structure has been reported to be very varied, requiring the consideration of defects and twins in the interpretation of the analytical data. With the help of a solvothermal method, we report the synthesis of a homogeneous sample of CdS nanorods, with size around 50nm in length and 7nm in diameter. The structure of these rods was characterized by high-resolution transmission electron microcopy (HREM) and simulation tools for producing models and simulated images for different orientations. From the experimental and theoretical results we report the preferential axis for growing nanorods and the necessity of CdS fcc-like structures to explain all the nanorods produced in this work and in general those reported in the literature. The use of the calculated images, reported in this paper, opens an easy way to understand HREM images even for a non-specialist. PACS 61.16.Bg; 79.60.Jv; 61.46.+w; 61.50.Ah     

Magnetooptical images of inhomogeneous magnetic fields in metallic films with in-plane anisotropy

The magnetooptical (MO) images of the inhomogeneous field created by permanent magnets in magnetic metallic films with in-plane anisotropy are experimentally studied. The MO images recorded using the longitudinal Kerr effect are the superposition of two pictures, namely, a polar-sensitivity MO image and a longitudinal-sensitivity MO image. An analysis of these images after separation shows that the polar-sensitivity MO image reflects the distribution of the inhomogeneous field component that is normal to the surface of an indicator film in an analog manner. The longitudinal-sensitivity MO image reflects the angular distribution of the in-plane component of a stray field in an analog manner. The coincidence of the experimental and corresponding simulated MO images makes it possible to interpret the experimental images. In particular, it is shown that the specific features detected in the topological characteristics of the inhomogeneous field correspond to experimental singular points. Hidden magnetic images (magnetic bar codes) are shown to be visualized with metallic CoFe films. As an example, the stray field of a magnetic system made of cylindrical magnets is mapped.     

极性半导体的表面电子

有不少的极性半导休,电子与表面声学声子和体纵光学声子的耦合弱,但与表面光学声手的耦台强.本文同时考虑体纵光学声子、表面光学声子以及表面声学声予的影响,研究这类半导体的表面电予的性质,采用线性组合算符和拉格朗日乘子法,导出其有效哈密顿量和重正化质量。     

Epitaxial growth defects and interfacial structures of Cu deposited on TiO2

Epitaxial growth defects and the interfacial structure between vapor deposited Cu and TiO₂(110) have been studied by combined high-resolution electron microscopy (HREM) and image simulations. The Cu film was found to grow epitaxially with an orientation given by Cu(111)//TiO₂(110) and Cu110//TiO₂ [001]. With this relationship, there exist two equivalent domain orientations which are rotated with respect to each other by 180°. Localized misfit dislocations have not been detected, but {111} stacking faults and microtwins were observed which may occur as a result of 3-D island coalescence. HREM observations and image simulations have been used to study the interface atomic structure. The dominant interfacial structure has a stoichiometric (110) TiO₂ surface with bridging rows of O atoms and occasionally, an interfacial structure having a reduced (110) TiO₂ surface terminated by both Ti and O atoms has been observed locally.     

结合会聚束电子衍射和高分辨电子显微术来测定十四铌酸锂的结构

我们在一台有0.4nm点分辨率的分析透射电子显微镜上,结合会聚束电子衍射(CBED)和高分辨电子显微术(HREM)研究了十四铌酸锂的结构。CBED结果证明十四铌酸锂的空间群是C2/m,HREM结构象表明十四铌酸锂是由4×4氧八面体方块组成,方块中3×3通道及八面体中心的金属原子可以清楚地看到,其距离是0.38nm。结合CBED和HREM来做结构分析是发展电子晶体学的重要的一步。 关键词：     

Optical-path-length-shifting color digital holography

We propose an in-line color digital holography that can suppress a 0th-order diffraction image and a conjugate image. This technique reconstructs the image of an object from two holograms that are formed at two different distances from the object for each primary color. We numerically simulated the proposed technique and confirmed that it enabled the reconstruction of higher-quality images of an object than the Fresnel transform alone, which is the simplest reconstruction algorithm in digital holography. The effectiveness of the proposed technique was also quantitatively shown by evaluating the reconstructed images using root-mean-square errors. Thus, it was confirmed that the proposed technique was capable of recording and reconstructing both three-dimensional information and color information of an object.     

Quantitative HRTEM investigation of nanoplatelets

High resolution transmission electron microscopy experiments were performed to investigate nanoplatelets induced by ion implantation into a germanium wafer. Atomistic models were used for image simulation in order to get quantitative information from the experimental images. The geometrical phase shift analysis technique was also employed to measure the strain field induced by such defects. We discuss the limits and artefacts imposed by each approach and show how these approaches can be combined to study the atomic structure of such defects and the strain field they induce.