1. Department of Electrical Engineering, University of California, Los Angeles, California 90095, United States
2. Center for Electron Microscopy and State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
3. School of Physics, Beijing Institute of Technology, Beijing 100081, China
4. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China