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. Academy for Advanced Interdisciplinary Studies Peking University Beijing 100871 P. R. China
3. Beijing Graphene Institute Beijing 100095 P. R. China
4. LNM, Institute of Mechanics Chinese Academy of Sciences Beijing 100190 P. R. China
5. School of Engineering Sciences University of Chinese Academy of Sciences Beijing 100049 P. R. China
6. School of Physics and Astronomy University of Manchester Manchester M13 9PL UK