1. Center for Nanochemistry (CNC), Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China
2. Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China
3. Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China