Abstract ：
　　Two great advances in materials research are the control of nanoscale interfaces and surfaces and harness of emergent properties from heavily doped and complex crystals. Examples include all sorts of nanostructures, oxide and semiconductor hetero-structures, superconductors and ferro- and piezo-electric crystals. However, precise determination of atomic and electronic structures at nanoscale surfaces and interfaces and dopant atoms remain as a major challenge in materials research. Here I will report on the progresses in electron optics and detector that enable the study of local structure. For examples, I will report on atomic resolution strain mapping, high resolution EELS for probing local electronic structure and our efforts on in-situ electron microscopy for studying chemical reactions.
　　Particular emphasis will be atomic resolution strain mapping based on Z-contrast images acquired with aberration corrected scanning transmission electron microscopy (STEM) using a high angle annular dark field detector (HAADF) [1]. By directly measuring atomic column positions, strain can be measured with reliability, resolution and precision at unprecedented level for interfacial strain and point defects characterization. One application is the determination of atomic vacancies in the InAs/GaSb strained layer superlattice [2]. We show that cation and anion vacancies in the InAs/GaSb SLS give rise to local lattice relaxations, especially the nearest atoms, which can be detected using a statistical method and confirmed by simulations. The second is a study of Sb substitution for As in an MBE grown InAs/InAsSb strained layer superlattice, which we show is accompanied by significant strain fluctuations. Strain analysis based on atomic column positions reveals asymmetrical transitions in the strain profile across the SLS interfaces. The averaged strain profile is quantitatively fitted to the segregation model, which yields a distribution of Sb in agreement with the scanning tunneling microscopy result. The subtraction of the calculated strain reveals an increase in strain fluctuations with the Sb concentration, as well as isolated regions with large strain deviations extending over ~ 1 nm, which suggest the presence of point defects [3].

[1] J. M. Zuo, A. B. Shah, H. Kim, Y. F. Meng, W. P. Gao, and J. L. Rouviere, Ultramicroscopy 136, 50 (2014). [2] Honggyu Kim, Yifei Meng, Ji-Hwan Kwon, Jean-Luc Rouviere, and Jian Min Zuo, IUCrJ 5 (1), 67 (2018). [3] Honggyu Kim, Yifei Meng, John F. Klem, Samuel D. Hawkins, Jin K. Kim, and Jian-Min Zuo, Journal of Applied Physics 123 (16), 161521 (2017).