1. School of Electronic Science and Applied Physics and Anhui Provincial Key Laboratory of Advanced Functional Materials and Devices; Hefei University of Technology; Hefei Anhui 230009 P. R. China
2. Department of Physics and Materials Science; City University of Hong Kong; Kowloon Hong Kong SAR P. R. China
3. School of Materials Science and Engineering; Hefei University of Technology; Hefei Anhui 230009 P. R. China