1. Faculty of Physics and Electronic Science, Hubei University, Wuhan 430062, P. R. China
2. Shenzhen Engineering Center for the Fabrication of Two-Dimensional Atomic Crystals, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, P. R. China
3. Department of Physics, Department of Materials Science and Engineering, and Department of Biomedical Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong 999077, P. R. China
4. Department of Applied Physics, East China Jiaotong University, Nanchang 330013, P. R. China