1. College of Physics; Chongqing University; Chongqing 401331 P. R. China
2. Chongqing Institute of Green and Intelligent Technology; Chinese Academy of Sciences; Chongqing 400714 P. R. China
3. University of Chinese Academy of Sciences; Beijing 100044 P. R. China
4. Department of Physics; University of Michigan; Ann Arbor MI 48109 USA
5. Analytical and Testing Center of Chongqing University; Chongqing 401331 P. R. China
6. Beijing Key Laboratory of Microstructure and Property of Advanced Materials; Beijing University of Technology; Beijing 100024 P. R. China