1. Center for Nanochemistry, Beijing Science and Engineering Center for Nanocarbons, Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China
2. Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, P. R. China
3. Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 7610001, Israel
4. Electron Microscopy Laboratory, School of Physics, Peking University, Beijing 100871, P. R. China
5. International Center for Quantum Materials, Peking University, Beijing 100871, P. R. China
6. Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, P. R. China