1. School of Electronic Science and Engineering, Nanjing University, Nanjing 210023, China
2. Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
3. Department of Physics/Centre for Materials Science and Nanotechnology, University of Oslo, P.O. Box 1048, Blindern, N-0316 Oslo, Norway
4. Shenzhen Research Institute of Shandong University, Shenzhen 518000, China