1. Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures and College of Engineering and Applied Sciences, Nanjing University, 22 Hankou Road, Nanjing 210093, China
2. School of Electronic Science and Engineering, Nanjing University, 22 Hankou Road, Nanjing 210093, China
3. Jiangsu Key Laboratory for Nano Technology, Nanjing University, 22 Hankou Road, Nanjing 210093, China