Thickness-dependent magnetic domain structures of Co ultra-thin film investigated by scanning transmission X-ray microscopy

Thickness-dependent magnetic domain structure of ultrathin Co wedge films (0.3 nm–1.0 nm) sandwiched by Pt layers was investigated by scanning transmission x-ray microscopy (STXM) employing X-ray magnetic circular dichroism (XMCD), utilizing elliptically polarized soft x-rays and electromagnetic fields, with a spatial resolution of 50 nm. The magnetic domain images measured at the Co L₃ edge showed the evolution of the magnetic domain structures from maze-like form to the bubble-like form as the perpendicular magnetic field was applied. The asymmetric domain expansion of a 500 nm-scale bubble domain was also measured when the in-plane and perpendicular external magnetic field were applied simultaneously.     

Characterization of LiBC by phase-contrast scanning transmission electron microscopy

LiBC was used as a model compound for probing the applicability of phase-contrast (PC) imaging in an aberration-corrected scanning transmission electron microscope (STEM) to visualize lithium distributions. In the LiBC structure, boron and carbon are arranged to hetero graphite layers between which lithium is incorporated. The crystal structure is reflected in the PC-STEM images recorded perpendicular to the layers. The experimental images and their defocus dependence match with multi-slice simulations calculated utilizing the reciprocity principle. The observation that a part of the Li positions is not occupied is likely an effect of the intense electron beam triggering Li displacement.     

Ratio-contrast imaging of dual-energy absorption for element mapping with a scanning transmission X-ray microscope

The detection of chemical mapping with a spatial resolution of 30 nm has been achieved with a scanning transmission X‐ray microscope at the Shanghai Synchrotron Radiation Facility. For each specimen, two absorption images were scanned separately with energies E₁ and E₂: E₁ was focused on the absorption edge of the chosen element and E₂ was focused below the edge. A K‐edge division method is proposed and applied to obtain the element mapping. Compared with the frequently used K‐edge subtraction method, this ratio‐contrast method is shown to be more accurate and sensitive in identifying the elements of interest, where the definition of the contrast threshold is simple and clear in physics. Several examples are presented to illustrate the effectiveness of the method.     

The image potential in scanning transmission electron microscopy and scanning tunneling microscopy

Abstract

We sketch developments in the theory of the self-energy of charged particles moving near condensed matter surfaces. Some applications to experimental results from spectroscopy with electrons localized in microprobe beams and to electrons tunneling across a gap between two metals are considered.     

Structure and mass analysis by scanning transmission electron microscopy.

In the scanning transmission electron microscope (STEM) an electron beam of a few angstroms diameter is raster scanned over a thin sample and the scattered electrons are sequentially measured for each sample element irradiated. The mass, the elemental composition and the structure of a protein can be simultaneously assessed if all detector systems of the STEM are used. Aspects affecting the accuracy of the mass measurement technique and the demands placed on the instrument's dark-field detector system are outlined. In addition, the influences of some sample preparation techniques are noted and the mass-loss induced at ambient temperatures by the incidence of 80kV electrons on various biological samples is reported. Finally, the importance of the STEM for the structural analysis of proteins is documented by examples.     

Quantitative atomic resolution scanning transmission electron microscopy

Complete understanding of atomic resolution high-angle annular dark-field (Z-contrast) images requires quantitative agreement between simulations and experiments. We show that intensity variations can be placed on an absolute scale by normalizing the measured image intensities to the incident beam. We construct fractional intensity images of a SrTiO3 single crystal for regions of different thickness up to 120 nm. Experimental images are compared directly with image simulations. Provided that spatial incoherence is taken into account in the simulations, almost perfect agreement is found between simulation and experiment.     

2D atomic mapping of oxidation states in transition metal oxides by scanning transmission electron microscopy and electron energy-loss spectroscopy

Using a combination of high-angle annular dark-field scanning transmission electron microscopy and atomically resolved electron energy-loss spectroscopy in an aberration-corrected transmission electron microscope we demonstrate the possibility of 2D atom by atom valence mapping in the mixed valence compound Mn3O4. The Mn L(2,3) energy-loss near-edge structures from Mn2+ and Mn3+ cation sites are similar to those of MnO and Mn2O3 references. Comparison with simulations shows that even though a local interpretation is valid here, intermixing of the inelastic signal plays a significant role. This type of experiment should be applicable to challenging topics in materials science, such as the investigation of charge ordering or single atom column oxidation states in, e.g., dislocations.     

Contamination mitigation strategies for scanning transmission electron microscopy

Modern scanning transmission electron microscopy (STEM) enables imaging and microanalysis at very high magnification. In the case of aberration-corrected STEM, atomic resolution is readily achieved. However, the electron fluxes used may be up to three orders of magnitude greater than those typically employed in conventional STEM. Since specimen contamination often increases with electron flux, specimen cleanliness is a critical factor in obtaining meaningful data when carrying out high magnification STEM. A range of different specimen cleaning methods have been applied to a variety of specimen types. The contamination rate has been measured quantitatively to assess the effectiveness of cleaning. The methods studied include: baking, cooling, plasma cleaning, beam showering and UV/ozone exposure. Of the methods tested, beam showering is rapid, experimentally convenient and very effective on a wide range of specimens. Oxidative plasma cleaning is also very effective and can be applied to specimens on carbon support films, albeit with some care. For electron beam-sensitive materials, cooling may be the method of choice. In most cases, preliminary removal of the bulk of the contamination by methods such as baking or plasma cleaning, followed by beam showering, where necessary, can result in a contamination-free specimen suitable for extended atomic scale imaging and analysis.     

Electron tomography algorithms in scanning transmission electron microscopy

Different weighting filters used in tomographic algorithms are investigated. Algorithms for the recovery of tomographic structures are produced for general application in graphic processors. Software for a Carl Zeiss Libra 200FE microscope that enables us to record a series of projections in the STEM mode is developed. Calculation experiments show the improvement in tomogram quality when using cosine and 1/z ² filters instead of a linear filter at high noise levels for objects larger than 25 points.     

Z衬度扫描透射电子显微术的前沿进展

文章介绍了Z衬度扫描透射电子显微术(Z-scanning transmission electron microscopy，Z-STEM，Z为原子序数)的最新进展：Z-STEM可以直接“观察”到晶体中原子的真实位置，Z衬度图像的分辨率在经过球差校正后可达0．6A；在利用Z衬度成像技术对材料的阴极荧光(cathodoluminescence，CL)性质的研究中，首次观察到了“死层”(dead layer)的存在．然后，文章以半导体与结晶氧化物界面结构、Al72Ni20Co8十角形准晶结构以及SrTiO3晶界结构为例，具体介绍了Z衬度成像在测定物质结构与化学组成方面独特的优势。     

Characterisation of InAs/GaAs short period superlattices using column ratio mapping in aberration-corrected scanning transmission electron microscopy

The image processing technique of column ratio mapping was applied to aberration-corrected high angle annular dark field (HAADF) images of short period MBE (molecular beam epitaxy) grown InAs/GaAs superlattices. This method allowed the Indium distribution to be mapped and a more detailed assessment of interfacial quality to be made. Frozen-phonon multislice simulations were also employed to provide a better understanding of the experimental column ratio values. It was established that ultra-thin InAs/GaAs layers can be grown sufficiently well by MBE. This is despite the fact that the Indium segregated over 3-4 monolayers. Furthermore, the effect of the growth temperature on the quality of the layers was also investigated. It was demonstrated that the higher growth temperature resulted in a better quality superlattice structure.     

二维原子晶体的低电压扫描透射电子显微学研究

二维原子晶体材料,如石墨烯和过渡金属硫族化合物等,具有不同于其块体的独特性能,有望在二维半导体器件中得到广泛应用.晶体中的结构缺陷对材料的物理化学性能有直接的影响,因此研究结构缺陷和局域物性之间的关联是当前二维原子晶体研究中的重要内容,需要高空间分辨率的结构研究手段.由于绝大部分二维原子晶体在高能量高剂量的电子束辐照下容易发生结构损伤,利用电子显微方法对二维原子晶体缺陷的研究面临诸多挑战.低电压球差校正扫描透射电子显微(STEM)技术的发展,一个主要目标就是希望在不损伤结构的前提下对二维原子晶体的本征结构缺陷进行研究.在STEM下,多种不同的信号能够被同步采集,包括原子序数衬度高分辨像和电子能量损失谱等,是表征二维原子晶体缺陷的有力工具,不但能对材料的本征结构进行单原子尺度的成像和能谱分析,还能记录材料结构的动态变化.通过调节电子束加速电压和电子辐照剂量,扫描透射电子显微镜也可以作为电子刻蚀二维原子晶体材料的平台,用于加工新型纳米结构以及探索新型二维原子晶体的原位制备.本综述主要以本课题组在石墨烯和二维过渡金属硫族化合物体系的研究为例,介绍低电压扫描透射电子显微学在二维原子晶体材料研究中的实际应用.     

High-resolution transmission and scanning electron microscopy of boride-nitride nanostructured films

The results of electron-microscopic studies of grain boundaries and the structure of fractures of titanium boride-and nitride-based films obtained by nonreactive magnetron sputtering are considered. The chemical and phase composition of the films is analyzed with the help of Auger electron spectroscopy and microscopic electron diffraction analysis. The structure of boundaries and the presence of amorphous inclusions, dislocations, and other structural distortions are discussed and the nature of the deformation under indentation is considered.     

Aberration-corrected scanning transmission electron microscopy for complex transition metal oxides

Lattice,charge,orbital,and spin are the four fundamental degrees of freedom in condensed matter,of which the interactive coupling derives tremendous novel physical phenomena,such as high-temperature superconductivity(high-T_c SC) and colossal magnetoresistance(CMR) in strongly correlated electronic system.Direct experimental observation of these freedoms is essential to understanding the structure-property relationship and the physics behind it,and also indispensable for designing new materials and devices.Scanning transmission electron microscopy(STEM) integrating multiple techniques of structure imaging and spectrum analysis,is a comprehensive platform for providing structural,chemical and electronic information of materials with a high spatial resolution.Benefiting from the development of aberration correctors,STEM has taken a big breakthrough towards sub-angstrom resolution in last decade and always steps forward to improve the capability of material characterization;many improvements have been achieved in recent years,thereby giving an indepth insight into material research.Here,we present a brief review of the recent advances of STEM by some representative examples of perovskite transition metal oxides;atomic-scale mapping of ferroelectric polarization,octahedral distortions and rotations,valence state,coordination and spin ordering are presented.We expect that this brief introduction about the current capability of STEM could facilitate the understanding of the relationship between functional properties and these fundamental degrees of freedom in complex oxides.     

扫描透射离子显微成像系统的建立和实验研究

 基于上海应用物理研究所的扫描质子微探针系统,对扫描透射离子显微成像（STIM）技术进行了仿真分析和实验研究,建立了基于中值滤波方法的数据处理程序,获得了大鼠股骨切片的面密度分布和薄壁小球的离轴STIM图像。理论分析和实验结果显示：由在轴探测数据的中值滤波结果可以计算出定量的面密度分布信息,由能量窗滤波的离轴探测数据则可以给出高分辨的定性图像,这在薄样品、强束流、以及质子激发X荧光（PIXE）分析同时进行的STIM分析中尤为有用;增加获取的事件数、选择合适的滤波方法可以使测量误差远小于阻止本领计算带来的误差。该方法可以用于测量几十μm尺度样品的绝对质量以及辅助PIXE技术进行元素定量分布分析。     

扫描透射离子显微成像系统的建立和实验研究

基于上海应用物理研究所的扫描质子微探针系统,对扫描透射离子显微成像（STIM）技术进行了仿真分析和实验研究,建立了基于中值滤波方法的数据处理程序,获得了大鼠股骨切片的面密度分布和薄壁小球的离轴STIM图像。理论分析和实验结果显示：由在轴探测数据的中值滤波结果可以计算出定量的面密度分布信息,由能量窗滤波的离轴探测数据则可以给出高分辨的定性图像,这在薄样品、强束流、以及质子激发X荧光（PIXE）分析同时进行的STIM分析中尤为有用;增加获取的事件数、选择合适的滤波方法可以使测量误差远小于阻止本领计算带来的误差。该方法可以用于测量几十μm尺度样品的绝对质量以及辅助PIXE技术进行元素定量分布分析。     

Combined surface-focused acoustic microscopy in transmission and scanning ultrasonic holography

Employment of ultrasound techniques in nondestructive testing may require identification of the acoustic modes contributing to imaging. Such identification can be achieved, with some restrictions, by time-of-flight analysis. Another approach is acoustic holography that reveals the propagation properties of any selected mode. In anisotropic media, the propagation features are distinct and allow for a reliable classification of the selected mode. Both techniques were applied for classification of bonded, disbonded, and weakly bonded areas in directly bonded semiconductor wafers.     

Three-dimensional mapping of cholinergic molecules by confocal laser scanning microscopy in sea urchin larvae.

Confocal laser scanning microscopy (CLSM) was used to examine molecules related to the cholinergic neurotransmission system and detected at all the larval stages of Paracentrotus lividus, by histochemical and immunohistochemical methods. CLSM, providing spatial resolution of the cells located both at the larval surface and at depth, allows a complete mapping in a three-dimensional volumetric frame. At early larval stages acetylcholinesterase- as well as choline acetyltransferase-like molecules were found mainly in the gut wall cells, and along the ciliary bands of the arms, together with muscarinic acetylcholine receptors. At perimetamorphic stages, cholinergic molecules were present in the ciliate strands along the arms, in the larval body and in the rudiment. At metamorphosis, positivity to cholinergic molecules translocated to the juvenile, where a high frequency of mAChR- and ChAT-like positive cells was found.     

Video-frequency scanning transmission electron microscopy of moving gold nanoparticles in liquid

Immobilized gold nanoparticles were imaged in a liquid containing water and 50% glycerol with scanning transmission electron microscopy (STEM). The specimen was enclosed in a liquid compartment formed by two silicon microchips with electron transparent windows. A series of images was recorded at video frequency with a spatial resolution of 1.5nm. The nanoparticles detached from their support after imaging them for several seconds at a magnification of 250,000. Their movement was found to be much different than the movement of nanoparticles moving freely in liquid as described by Brownian Motion. The direction of motion was not random-the nanoparticles moved either in a preferred direction, or radially outwards from the center of the image. The displacement of the gold nanoparticles over time was three orders of magnitude smaller than expected on the basis of Brownian Motion. This finding implies that nanoscale objects of flexible structure or freely floating, including nanoparticles and biological objects, can be imaged with nanoscale resolution, as long as they are in close proximity to a solid support structure.     

Characterization of compositional particles in eye shadow powder by scanning electron microscope and X-ray mapping

We developed an analysis method to evaluate the size and shape of particles of eye shadow cosmetics by applying the scanning electron microscopy (SEM) combined with the energy-dispersive spectroscopy (EDS). Backscattered electron (BE) and X-ray mapping images from the SEM-EDS measurements were interpreted to identify the size and shape of powder particles according to different factors such as 9 size and 5 shape indices established in this work. By using the developed method, we could classify the inorganic and organic particles of the eye shadow powder by composition, which can contribute to improving the product quality.