1. Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences Beijing 100400 P. R. China
2. Department of Applied Physics College of Science Nanjing University of Aeronautics and Astronautics Nanjing 211106 P. R. China
3. Institute of Condensed Matter and Material Physics Peking University Beijing 100871 P. R. China
4. CAS Key Laboratory of Standardization and Measurement for Nanotechnology CAS Center of Excellence for Nanoscience National Center for Nanoscience and Technology Beijing 100190 P. R. China