Experimental study of annular bright field (ABF) imaging using aberration-corrected scanning transmission electron microscopy (STEM)

Experimental parameters used in the annular bright field (ABF) imaging method were tested using images simulated with the multislice method. Images simulated under identical conditions were found to agree well with experimental images. The ABF technique was shown to be relatively insensitive to the sample thickness and the defocus. In experimental ABF images, atomic columns exhibited dark contrast over a wide range of specimen thickness and defocus values, from 10 to 70 nm and ?20 to +20 nm, respectively. A series of diffraction patterns at atomic columns, obtained using the diffraction imaging method, exhibited higher intensities in their central regions (0–11 mrad) for light elements and in their peripheral regions (11–22 mrad) for heavy elements. The results indicated that the contrast of light elements is enhanced by subtraction of the central region of the transmitted beam, since this is blocked by a circular mask in the ABF-STEM technique. Thus, the overall contrast of light elements is greatly improved, allowing them to be clearly visualized.     

Performance and early applications of a versatile double aberration-corrected JEOL-2200FS FEG TEM/STEM at Aalto University

Applications relevant to carbon based nano-materials have been explored using a newly installed JEOL-2200FS field emission gun (FEG) (scanning) transmission electron microscope (S)TEM which is integrated with two CEOS aberration correctors for both the TEM image-forming and the STEM probe-forming lenses. The performance and utility of this newly commission hardware has been reviewed with a particular focus on operation at an acceleration voltage of 80 kV, thus bringing the primary electron beam voltage below the knock-on threshold for carbon materials and opening up a range of possibilities for the study of carbon-based nanostructures in the aberration-corrected electron microscope. The ability of the microscope to obtain both atomic TEM images and high-quality electron diffraction patterns from carbon nanotubes was demonstrated. The chiral structure of a double-walled carbon nanotube was determined from its diffraction pattern. The aberration corrected TEM imaging technique facilitates a unique approach to accurate determination of single-walled carbon nanotube diameters. On the other hand, the probe-corrected high angle annular dark field (HAADF) STEM imaging performance allows for the detection of single gold atoms at 80 kV and was used to study the graphite interlayer spacing in a multi-walled carbon nanotube.     

Incidence of element distribution on the ultrasonic field of segmented annular arrays

Segmented annular arrays are sometimes used for 3D ultrasonic imaging. However, owing to their geometrical complexity, the acoustic field generated by this type of aperture has not been adequately described. In this work, a method based on the array factor approach is used to describe the field radiated by sector annular arrays. This approach allows one to analyse the influence on the field of several aperture parameters, such as the number of elements per annulus, size and spatial distribution of elements, etc. In addition, strategies to reduce grating lobes are presented.     

Low voltage STEM imaging of multi-walled carbon nanotubes

We present high magnification STEM images of multi-walled carbon nanotubes recorded with a 5 keV electron beam using a Helios Dual Beam microscope and a dedicated multi-segment transmission (STEM) detector. Images of carbon nanotubes recorded with bright-field (BF), annular dark-field (ADF) and high angle annular dark-field (HAADF) signals all show high contrast features, with internal structures 1-2 nm in width clearly revealed in the STEM images. Thicker regions of the nanotubes appear to show an unusual contrast reversal when comparing ADF and HAADF images. An understanding of the image contrast, and its dependence on thickness, is obtained by computing simulations of the ADF and HAADF images using Monte-Carlo software taking into account electron scattering in the nanotube.     

A Macroscopically Relevant 3D‐Metrology Approach for Nanocatalysis Research

A 3D nanometrological approach, which considers as an unbiased validation criterion the quantitative match between values of properties determined by macroscopic characterization techniques and those determined from the nanoscopic results, is developed to unveil the details of complex nanocatalysts. This approach takes into account both the peculiar characteristics of this type of materials and the large influence of noise in the tilt series. It combines, in an optimized way, the latest experimental developments in high angle annular dark field scanning transmission electron microscopy mode (HAADF‐STEM) tomography, such as batch tomography, image denoising by undecimated wavelet transforms, improved reconstructions by total variation minimization and a more efficient, user‐independent, segmentation scheme. To illustrate the use of this novel approach, the 3D structural characterization of a model nanocatalyst comprising gold nanoparticles dispersed on the surface of CeO₂ nanocubes is performed, and the obtained results used to compute the values of different macroscopic chemical and textural properties. Comparison with values obtained by macroscopic characterization techniques match very closely those obtained by 3D nanometrology. Importantly, the new approach described in this work also illustrates a pipeline for nearly fully automated HAADF‐STEM tomography studies, guaranteeing reliable correlations between nanoscopic and macroscopic properties.     

TEM characterization of chemically synthesized copper–gold nanoparticles

Dodecanethiol-capped Cu–Au nanoparticles, synthesized via a successive two-phase (water/toluene) and galvanic-exchange procedure, were characterized using transmission electron microscopy (TEM). The size range of the particles is around 1–7 nm. Electron-induced morphological evolution was observed under high resolution (HR) TEM. Cuboctahedral morphology was found to be thermodynamically stable. Electron-induced aggregation of two particles was also observed. Chemical ordering of cuboctahedral particles was studied by atomic-resolution high angle annular dark field (HAADF) imaging in scanning TEM (STEM) mode and energy dispersive X-ray (EDX) element mapping using a silicon drift detector (SDD). The particles were found to be Cu–Au mixed, and to be stable in air. Surface plasmon resonance (SPR), which is dependent on local structure and morphology, was investigated by electron energy loss spectroscopy (EELS).     

The study of Al3(Mn,Pd) crystal and Al–Mn–Pd decagonal quasicrystal by spherical aberration-corrected scanning transmission microscopy and atomic-resolution energy dispersive X-ray spectroscopy

An Al₃Mn-type Al₃(Mn, Pd) crystal and an Al–Mn–Pd decagonal quasicrystal (DQC) in an Al₇₀Mn₂₀Pd₁₀ alloy are studied using a spherical aberration (Cs)-corrected scanning transmission electron microscope (STEM) with high-angle annular dark-field (HAADF) and annular bright-field (ABF) techniques, together with atomic-resolution energy dispersive X-ray spectroscopy (EDS). Mn and Pd atomic positions in the Al₃(Mn, Pd) structure projected along the b-axis (pseudo-tenfold rotational axis) are represented by separate bright dots in observed HAADF-STEM images. Besides, Al as well as Mn and Pd atomic positions are represented as dark dots in ABF-STEM images. Most Mn and Pd atomic positions in the Al₃(Mn, Pd) structure can be observed on atomic-resolution EDS maps. On the basis of the good correlation between the STEM images and the EDS maps, and also considering the structure of the Al₃(Mn, Pd) crystal, which was determined by X-ray diffraction using a single crystal, observed HAADF and ABF-STEM images of the Al–Mn–Pd DQC have been interpreted. Pd and Mn atomic positions in the Al–Mn–Pd DQC can be detected on the observed EDS maps. It can be seen that Pd is enriched around the centre of the columnar clusters, having a decagonal section with 2 nm in diameter. It can therefore be concluded that Pd plays an important role in the stabilization of the decagonal clusters, which form the Al–Mn–Pd DQC structure.     

Single atom doping in 2D layered MoS2 from a periodic table perspective

Molybdenum Disulfide (MoS₂) is a well-known transition metal dichalcogenide with a hexagonal structure arrangement analogous to graphene. Two dimensional (2D) MoS₂ has attracted wide attention in various applications such as energy storage, catalysis, sensing, energy conversion and optoelectronics due to its unique properties including tunable bandgap, substantial carrier mobility, outstanding mechanical strength and dangling-bond free basal surface. Moreover, MoS₂ has shown an excellent capability to be a host for foreign atoms which tune its physicochemical properties. Herein, currently known structural changes in the MoS₂ crystals introduced by various single atom dopants coming from all over the chemical table of elements are reviewed. Accompanying electrical, optical and magnetic properties of such structures are discussed in detail. Potential applications of the doped MoS₂ are introduced briefly as well. The review concentrates on the recent state-of-the-art results obtained mostly by the high resolution scanning transmission electron microscopy (STEM), such as high angle annular dark field (HAADF) imaging as well as scanning probe microscopy (SPM) such as scanning tunneling microscopy (STM). These techniques have been used to decipher dopant positions and other sub-atomic structural changes introduced to the MoS₂ structure by isolated dopants.     

可实现偏振无关单向传输的二维硅基环形孔光子晶体

基于光子晶体来构筑偏振无关光二极管在光电集成领域具有重大的应用价值.首先提出了一种环形孔光子晶体,能带结构显示其对横电及横磁模式同时展现出显著的方向带隙.以此构建了三角形状的环形孔光子晶体,利用时域有限差分法计算其透过谱及场分布图,发现该结构能实现偏振无关单向传输特性,然而正向透过率太低(约20%).进一步引入尺寸较小的三角形状的环形孔光子晶体构成光子晶体异质结结构,有效地提高了偏振无关单向传输性能,正向透过率增大了一倍.通过界面结构的调整,正向透过率进一步增大,优化后的环形孔光子晶体异质结结构能同时对类横电及类横磁模式入射光实现单向传输,且正向透过率达到了44%.     

Identification of ionic aggregates in PVDF-g-PSSA membrane by tapping mode AFM and HADDF STEM

Previous literature has shown that poly(vinylidene fluoride)-graft-poly(styrene sulfonated acid) (PVDF-g-PSSA) exhibits a lower methanol permeability than commercial Nafion and so is better suited to use as a proton exchange membrane (PEM) in direct methanol fuel cells (DMFCs). A number of studies have suggested that the microstructures of ionic aggregates explain their lower methanol permeability, but few direct morphological observations have been reported. In this study, the use of a tapping mode atomic force microscope (AFM) and a high-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) has identified the phase separation of PVDF and sulfonated PS, and ionic sulfonic aggregates, 3-5 nm, in sulfonated PS regions. An experiment to elucidate the microstructural changes in the membrane with and without methanol immersion shows that PVDF-g-PSSA has ionic aggregates with a more stable microstructure than Nafion.     

同轴放电环形增益区谐振腔镜面的场分布

 对几类环形增益区谐振腔进行了比较，得出利用谐振腔两镜之一的全工作面输出的新型环形增益区谐振腔是适合的谐振腔。提出了一种计算全工作面输出的同轴放电环形增益区谐振腔镜面场分布的方法，利用几何方法对腔镜镜面进行划分,得出了基于菲涅尔-基尔霍夫衍射积分定理的积分区域，并对该区域进行了有限元划分，应用计算机编程计算出了同轴放电环形增益区谐振腔镜面处的光场分布和相位分布。近场分布在环形区中央部分近似地呈高斯分布，且沿径向出现高阶模场分布特点，沿角向无差别。     

环形狭缝腔阵列光学特性的研究

设计了一种六角密排的二维环形纳米腔阵列结构, 利用时域有限差分算法对该结构的光学特性进行了探究. 仿真结果表明, 在线性偏振光入射时, 环形腔内可以形成多重圆柱形表面等离激元谐振, 谐振波长的个数和大小与环形腔的结构参数相关. 根据透、反射光谱, 电场矢量的模式分布及截面电荷密度的分布, 谐振波长处形成圆柱形表面等离激元, 谐振波长处入射光能量大部分在环形腔内损耗, 此时反射率为极小值, 环形腔内的电场增强效应为极大值(光强增强可达1065倍). 谐振波长与环形腔的结构参数(狭缝内径、狭缝外径、膜厚、环境介质折射率、金属的材质)相关, 通过调节结构参数, 谐振波长在350–2000 nm范围内可调. 通过对比相同结构参数的单个环形腔和环形腔阵列的仿真结果, 周期排布对环形腔内的圆柱形表面等离激元吸收峰位置影响不明显. 该结构反射光谱对入射光电矢量偏振方向不敏感. 谐振波长的可调控性对于表面拉曼增强和表面等离激元共振传感器的设计与优化具有指导性意义, 且应用于折射率传感器时灵敏度可达1850 nm/RIU.     

High-resolution electron microscopy for heterogeneous catalysis research

Heterogeneous catalysts are the most important catalysts in industrial reactions. Nanocatalysts, with size ranging from hundreds of nanometers to the atomic scale, possess activities that are closely connected to their structural characteristics such as particle size, surface morphology, and three-dimensional topography. Recently, the development of advanced analytical transmission electron microscopy(TEM) techniques, especially quantitative high-angle annular darkfield(HAADF) imaging and high-energy resolution spectroscopy analysis in scanning transmission electron microscopy(STEM) at the atomic scale, strengthens the power of(S)TEM in analyzing the structural/chemical information of heterogeneous catalysts. Three-dimensional reconstruction from two-dimensional projected images and the real-time recording of structural evolution during catalytic reactions using in-situ(S)TEM methods further broaden the scope of(S)TEM observation. The atomic-scale structural information obtained from high-resolution(S)TEM has proven to be of significance for better understanding and designing of new catalysts with enhanced performance.     

HAADF imaging of the omega (ω) phase in a gum metal-related alloy

Nanosized precipitates have been observed in a Nb-lean gum metal-related alloy, Ti–20.0Nb–0.6Ta–1.7Zr–1.1O?at.% (Ti–31.9Nb–2.0Ta–2.7Zr–0.3O?wt.%) using probe-corrected high-resolution scanning transmission electron microscopy with a high-angle annular dark-field detector (HAADF). This characterization yields three distinct atomic motifs and STEM multislice simulations are semi-quantitatively used to verify that each motif can be attributed to the widely observed “athermal” omega phase. However, the presence of chemical ordering cannot be unambiguously ruled out in this system. Data presented here, demonstrate the complexity of interpreting HAADF images of multiphase, multicomponent alloys when complementary experimental data are unavailable.     

环形电子束积分图像诊断技术

 采用图像诊断方法对高能环形电子束形状及空间尺寸进行了研究,以高能脉冲环形电子束轰击高Z靶材料产生脉冲X射线,X射线经过X射线增感屏转换为可见光,用单次图像采集系统获取可见光的积分图像。为满足诊断所需的空间分辨和系统灵敏度,通过理论计算确立了靶的材料、厚度及X射线增感屏的型号和厚度等参数。根据测试环境,设计了系统的现场安装结构,系统基本满足测试要求。分析从实验中获取的图像,可知环形电子束的内径为36.5 mm,环厚为1 mm,环形不均匀,水平方向电子束强。     

有限引导磁场下相对论环形电子注色散特性的研究

采用等效媒质处理方法来研究有限引导磁场下沿纵向运动的相对论环形电子注.首先建立运动坐标系以电子注纵向速度匀速运动,在运动坐标系中电子注可以被考虑成静止的磁化等离子体,再通过四维空间的洛伦兹变换得到电子注在静止的实验室坐标系下可以被等效为双各向异性媒质,其不仅具有张量形式的电导率和磁导率,还具有手征特性.在此基础上同时考虑了由于电子注表面波动所引起的表面电流密度.采用该方法研究了有限引导磁场下圆柱波导中沿纵向运动的相对论环形电子注,推导出该模型的色散方程,并进行了数值计算.计算结果表明该研究方法能够得到更准 关键词： 相对论环形电子注 磁化等离子体 色散特性     

The structure of an Al–Rh–Cu decagonal quasicrystal studied by spherical aberration (Cs)-corrected scanning transmission electron microscopy

The structure of an Al–Rh–Cu decagonal quasicrystal formed with two quasiperiodic planes along the periodic axis in an Al₆₃Rh_18.5Cu_18.5 alloy has been studied by spherical aberration (Cs)-corrected high-angle annular detector dark-field (HAADF)- and annular bright-field (ABF)-scanning transmission electron microscopy (STEM). Heavy atoms of Rh and mixed sites (MSs) of Al and Cu atoms projected along the periodic axis can be clearly represented as separate bright dots in observed HAADF-STEM images, and consequently arrangements of Rh atoms and MSs on the two quasiperiodic planes can be directly determined from those of bright dots in the observed HAADF-STEM image. The Rh atoms are arranged in pentagonal tiling formed with pentagonal and star-shaped pentagonal tiles with an edge-length of 0.76 nm, and also MSs with a pentagonal arrangement are located in the pentagonal tiles with definite orientations. The star-shaped pentagonal tiles in the pentagonal tiling are arranged in τ²(τ: golden ratio)-inflated pentagonal tiling with a bond-length of 2 nm. From arrangements of Rh atoms placed in pentagonal tilings with a bond-length of 2 nm, which are generated by the projection of a five-dimensional hyper-cubic lattice, occupation domains in the perpendicular space are derived. Al atoms as well as Rh atoms and MSs are represented as dark dots in an observed ABF-STEM image, and arrangements of Al atoms in well-symmetric regions are discussed.     

Non-approximate method for designing annular field of two-mirror concentric system

Annular field aberrations of a three-reflection concentric system,which are composed of two spherical mirrors,are analyzed.An annular field with a high level of aberration correction exists near the position where the principal ray is perpendicular to the object-image plane.Aberrations are determined by the object height and aperture angle.In this letter,the general expression of the system aberration is derived using the geometric method,and the non-approximate design method is proposed to calculate the radii of the annular fields that have minimum aberrations under different aperture angles.The closer to 0.5（the ratio of the radius of convex mirror to the radius of concave mirror）is,the smaller the system aberration is.The examples analyzed by LABVIEW indicate that the annular field designed by the proposed method has the smallest aberration in a given system.