1. National Laboratory of Solid State Microstructures, School of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China.
2. Department of Electrical and Computer Engineering, University of Massachusetts, Amherst, MA, USA.
3. School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China.
4. National Institute for Materials Science, 1-1 Namiki Tsukuba, Ibaraki 305-0044, Japan.