1. National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China
2. National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China
3. Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China