1. Center for Nanochemistry Beijing Science and Engineering Centre for Nanocarbons Beijing National Laboratory for Molecular Science College of Chemistry and Molecular Engineering Peking University Beijing 100871 P. R. China
2. Beijing Graphene Institute Beijing 100095 P. R. China
3. Academy for Advanced Interdisciplinary Studies Peking University Beijing 100871 P. R. China
4. Songshan Lake Materials Laboratory Dongguan Guangdong 523808 P. R. China
5. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics Chinese Academy of Sciences Beijing 100190 P. R. China
6. School of Materials Science and Engineering Peking University Beijing 100871 P. R. China
7. Department of Engineering University of Cambridge Cambridge CB3 0FA UK
8. School of Physical Sciences University of Chinese Academy of Sciences Beijing 100049 P. R. China