1 Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures, Jiangsu Key Laboratory for Nano Technology, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China.
2 Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures, Jiangsu Key Laboratory for Nano Technology, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China. Electronic address: feng_jianyong@yeah.net.
3 Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures, Jiangsu Key Laboratory for Nano Technology, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China. Electronic address: zsli@nju.edu.cn.
4 Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures, Jiangsu Key Laboratory for Nano Technology, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China; School of Physics, Nanjing University, Nanjing 210093, China.