1. College of Physics and Information Science and Key Laboratory of Low-dimensional Quantum Structures and Quantum Control of the Ministry of Education, Hunan Normal University, Changsha 410081, Hunan, China
2. School of Materials Science and Engineering, Key Laboratory of Low-dimensional Materials and Application Technology (Ministry of Education), Xiangtan University, Xiangtan 411105, China
3. Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hong Kong, China