1. Key
Laboratory of Standardization and Measurement for Nanotechnology of
Chinese Academy of Sciences, National Center for Nanoscience and Technology, Beijing 100190, PR China
2. Department of Physics and Beijing Key Laboratory of Optoelectronic Functional Materials & Micro-nano Devices, Renmin University of China, Beijing 100872, PR China
3. School
of Materials Science and Engineering, Shaanxi Normal University, Xi’an, 710062, PR China