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. State Key Laboratory of Silicon Materials and Center of Electron Microscopy; School of Materials Science and Engineering; Zhejiang University; Hangzhou 310027 P. R. China
3. Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics; Peking University; Beijing 100871 P. R. China
4. Academy for Advanced Interdisciplinary Studies; Peking University; Beijing 100871 P. R. China