1. CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in 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. International College, University of Chinese Academy of Sciences, Beijing 100049, P. R. China