Juanxia Wu1, Liming Xie1,2,3. 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.