1. 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
2. School of Nanoscience and Technology University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. Department of Materials Science and Engineering College of Engineering Peking University Beijing 100871 P. R. China
4. Department of Physics Southern University of Science and Technology Shenzhen 518055 P. R. China