题目：Advanced understanding of materials structure and dynamics at the atomic scale with new tools in scanning transmission electron microscopy

报告人：Dr. Xiahan Sang

时间：6月15日（周五）上午9:00-11:30

地点：李薰楼249室

Advanced understanding of materials structure and dynamics at the atomic scale with new tools in scanning transmission electron microscopy

Dr. Xiahan Sang, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory

Understanding the atomic scale mechanisms that govern materials structure, growth and transformation is essential to develop new methods for controlled synthesis and design of materials for advanced functional applications. Here, I will introduce two examples where novel scanning transmission electron microscopy (STEM) technique can greatly advance our understanding of materials behavior at atomic scale. First, I will show how to remove deleterious drift distortion that is prevalent in STEM images using the revolving STEM (RevSTEM) method that was developed to measure sample drift without any a priori knowledge of the crystal structure directly from a series of fast frames acquired with 90°scan direction change between consecutive frames. The resulting drift-distortion-corrected RevSTEM images thus represent the true projected atomic structure and enable accurate real space crystallography measurements in STEM. Combined with quantitative intensity analysis, several distortion mechanisms in complex perovskite oxide were directly observed and then confirmed by DFT calculation. Second, using in situ STEM, combined with density functional theory (DFT) and molecular dynamics (MD), the atomistic dynamics of reconstruction and growth in 2D dichalcogenides and MXenes are directly observed. Edge evolution of nanopores in Mo_1-xW_xSe₂ monolayers was investigated via atomic-resolution in situ STEM and these edges are structurally transformed to theoretically predicted metastable atomic configurations by thermal and chemical driving forces. The coupling of predictive modeling and in situ STEM imaging in changing chemical environments demonstrated here provides a pathway for the controlled atomic scale manipulation of matter for the directed synthesis of edge configurations in Mo_1-xW_xSe₂ to achieve desired functionality. Also, we obtain direct insight into the homoepitaxial Frank-van der Merwe atomic layer growth mechanism and demonstrate how the process can be exploited to obtain new transition metal carbides (TMC) phases that is synthesized on surfaces of Ti₃C₂ MXene substrates with the substrate being the source material. This work could lead to the development of bottom-up synthesis methods, such as CVD and MBE, for controllable synthesis of larger-scale and higher quality single-layer TMC.
The combination of quantitative STEM, in situ microscopy, theory, and simulation as presented in this talk will provide valuable information that can be used to guide the synthesis and tailor the functional properties of materials.

报告人介绍:

桑夏晗博士，2005年毕业于武汉大学动力与机械学院金属材料工程系，2008年获中国科学院金属研究所硕士学位，2012年获美国匹兹堡大学机械与材料工程系博士学位。之后在美国北卡州立大学从事博士后工作，现在美国橡树岭国家实验室纳米中心从事博士后研究工作。他发展了定量电子显微学方法包括通过会聚束电子衍射精确测量电荷密度，用旋转STEM技术来完全去除样品漂移的畸变以达到皮米级的测量精度。近期主要工作集中在原位电镜，包括加热，气相和液相。报告人发表SCI论文 50余篇，授权专利1项。