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. Beijing Graphene Institute (BGI) Beijing 100095 P. R. China
4. 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