1. Department of Physics, Capital Normal University, Beijing 100048, People’s Republic of China
2. CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, People’s Republic of China
3. College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China
4. University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China