1. Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, P.R. China
2. School of Materials Science and Engineering, Peking University, Beijing 100871, P.R. China
3. Electron Microscopy Laboratory and International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, P.R. China
4. Beijing Graphene Institute (BGI), Beijing 100095, P.R. China
5. Collaborative Innovation Center of Quantum Matter, Beijing 100871, P.R. China