1. Collaborative Innovation Center of Advanced Microstructures National Laboratory of Solid State Microstructures School of Physics Nanjing University Nanjing 210093 P. R. China
2. School of Electrical Engineering Shanghai DianJi University Shanghai 201306 P. R. China
3. Kuang Yaming Honors School and Institute for Brain Sciences Nanjing University Nanjing 210023 P. R. China
4. Department of Materials Science and Engineering Jiangsu Key Laboratory for Artificial Functional Materials and Collaborative Innovation Center of Advanced Microstructures Nanjing University Nanjing 210093 P. R. China