1. National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, People's Republic of China
2. College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, People's Republic of China
3. School of Physics, Nanjing University, Nanjing 210093, People's Republic of China