1. School of Electronic and Information Engineering, Changshu Institute of Technology, Changshu 215500, China
2. Department of Physics, Harbin Institute of Technology, Harbin 150001, China
3. Laboratory of Advanced Nano Materials and Devices, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China
4. College of Physics, Soochow University, Suzhou 215006, China