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. State Key Laboratory for Mesoscopic Physics, Frontiers Science Center for Nano-optoelectronics, School of Physics, Peking University, Beijing 100871, P. R. China
3. Institute of Interdisciplinary Physical Sciences, School of Science, Nanjing University of Science and Technology, Nanjing 210094, P. R. China
4. Academy for Advanced Interdisciplinary Studies, Collaborative Innovation Center of Quantum Matter, Peking University, Beijing 100871, P. R. China
5. Beijing Graphene Institute (BGI), Beijing 100095, P. R. China
6. Department of Electronics and Nanoengineering, Aalto University, Espoo 02150, Finland
7. Center for the Physics of Low-Dimensional Materials, School of Physics and Electronics, Henan University, Kaifeng 475004, P. R. China