Image Contrast in Atomic Resolution STEM

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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.