1. Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, P.R. China
2. 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
3. Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China
4. Department of Physics, Tohoku University, Sendai 980-8578, Japan