1. Department of Physics, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon 999077, Hong Kong
2. National
Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. Department
of Materials Science and Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong
4. Key Laboratory of Advanced Micro/Nano Functional Materials,
School of Physics and Electronic Engineering, Xinyang Normal University, Xinyang 464000, China
5. Materials
Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron
Rd., Berkeley, California 94720, United States