Image Contrast in Atomic Resolution STEM

技术支持：该项目旨在获得原子分辨率扫描透射电子显微镜（STEM）中化学对比度的定量理解，最终目标是全面了解固体中的缺陷和界面。STEM中的高角度环形暗场（HAADF或Z对比度）成像已被公认为提供具有化学灵敏度的原子结构图像的技术。在这些图像中的原子序数对比度的定量理解将允许分析杂质或掺杂剂原子附近的界面和缺陷与原子级的空间分辨率。尽管在过去十年中在STEM Z对比成像的理论中已经取得了显著的进展，但是在由理论做出的图像对比度的预测与实验图像的预测之间存在显著的不匹配。虽然这种差异不会影响完美晶体图像的解释，但它排除了对缺陷和界面进行定量的原子级化学分析。在这个项目中，理论和实验之间的分歧的物理起源将在一系列系统的实验中进行研究，旨在区分仪器，环境和样品效应以及与高能电子散射物理学相关的贡献。特别是，温度（声子散射），样品厚度，加速电压，收敛角，非弹性散射，样品表面层，点缺陷，和原子位移对图像对比度的影响将被检查。实验结果将与墨尔本大学的科学家合作，与模拟结果进行比较。非技术性：该项目解决了材料科学领域的基础研究问题，具有高度的技术相关性，预计将提供原子尺度的结构和化学的新科学知识，这对现代材料科学和纳米技术的进步至关重要。该项目将有助于研究生的跨学科和国际培训。通过与澳大利亚的理论家合作，该计划支持的学生将了解理论的最新发展，弥合应用和基础材料研究之间的差距，学习广泛的科学方法，并接受国际合作的培训。该项目还为本科生提供为期两到三个月的独立实习。在这个项目中获得的结果将被用来开发基于网络的电子显微镜数据库（EMdb）的主题，定量分析原子分辨率成像中的化学对比度的教学模块。

Technical: This project aims to gain a quantitative understanding of the chemical contrast in atomic resolution scanning transmission electron microscopy (STEM) with the ultimate objective of achieving a comprehensive knowledge of defects and interfaces in solids. High-angle annular dark-field (HAADF or Z-contrast) imaging in STEM has been recognized as a technique that provides atomic structure images with chemical sensitivity. A quantitative understanding of the atomic number contrast in these images would allow for analysis of impurity or dopant atoms near interfaces and defects with atomic-level spatial resolution. Despite the significant progress that has been made over the last decade in the theory of STEM Z-contrast imaging a significant mismatch exists between predictions of image contrast made by theory and that of experimental images. While this discrepancy does not affect the interpretation of images of perfect crystals, it precludes a quantitative, atomic-scale chemical analysis of defects and interfaces. In this project, the physical origins of the disagreement between theory and experiments will be investigated in a systematic series of experiments designed to distinguish between contributions from instrumental, environmental and sample effects and those related to the scattering physics of high-energy electrons. In particular, the influence of temperature (phonon scattering), sample thickness, acceleration voltage, convergence angle, inelastic scattering, sample surface layers, point defects, and atom displacements on the image contrast will be examined. The experimental findings will be compared with simulation results, in collaboration with scientists at the University of Melbourne.Non-technical: The project addresses basic research issues in a topical area of materials science with high technological relevance, and is expected to provide new scientific knowledge of structure and chemistry at the atomic scale, which is critical for advances in modern materials science and nanotechnology. The project will contribute to the interdisciplinary and international training of graduate students. Through collaboration with theorists in Australia, the students supported by the program will learn about the latest developments in theory, bridge the gap between applied and fundamental materials research, learn a broad range of scientific methods, and become trained in international collaboration. The project also offers two- to three-month-long, self-contained internships for undergraduate students. Results obtained in this project will be used to develop teaching modules for a web-based Electron Microscopy Database (EMdb) on the topic of quantitatively analyzing chemical contrast in atomic resolution imaging.