1. Beijing Graphene Institute (BGI) Beijing 100095 P. R. China
2. Center for Nanochemistry (CNC) College of Chemistry and Molecular Engineering Peking University Beijing 100871 P. R. China
3. Center of Materials Science and Optoelectronics Engineering University of Chinese Academy of Sciences Beijing 100049 P. R. China
4. Research and Development Center for Semiconductor Lighting Technology Institute of Semiconductors Chinese Academy of Sciences Beijing 100083 P. R. China
5. Academy for Advanced Interdisciplinary Studies Peking University Beijing 100871 P. R. China
6. Department of Physics and Astronomy University of Manchester Manchester M13 9PL UK
7. Electron Microscopy Laboratory and International Center for Quantum Materials School of Physics Peking University Beijing 100871 P. R. China
8. Collaborative Innovation Center of Quantum Matter Beijing 100871 P. R. China
9. Department of Materials Science and Engineering National University of Singapore Singapore 117575 Singapore