1. Pen-Tung Sah Institute of Micro-Nano Science and Technology; Xiamen University; Xiamen 361005 P.R. China
2. College of Materials; Xiamen University; Xiamen 361005 P.R. China
3. Synergetic Innovation Center for Quantum Effects and Application, Key Laboratory of Low-dimensional Quantum Structures and Quantum Control of Ministry of Education, College of Physics and Information Science; Hunan Normal University; Changsha 410081 P.R. China