1. School of Physics, Southeast University, Nanjing 211189, China
2. College of Electronic and Information Engineering, Changshu Institute of Technology, Changshu 215500, China
3. Department of Physics, Nanjing University, Nanjing 210093, China
4. National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210008, China
5. School of Materials Science and Enigneering, Southeast University, Nanjing 211189, China
6. Jiangsu Key Laboratory for Nano Technology, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China