1. School of Physics, Nanjing University, Nanjing 210093, China
2. National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
3. Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, China
4. Jiangsu Provincial Key Laboratory for Nanotechnology, Nanjing University, Nanjing 210093, China