1. National Laboratory of Solid State Microstructures and Jiangsu Provincial Key Laboratory for Nanotechnology, School of Physics, Nanjing University, Nanjing 210093, People’s Republic of China
2. School of Electronics and Information, Hangzhou Dianzi University, Hangzhou, Zhejiang 310018, People’s Republic of China
3. Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparation Technology, School of Science, Tianjin University, Tianjin 300072, People’s Republic of China