1. National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Collaborative Innovation Center of Advanced Microstructures, and Jiangsu Key Laboratory of Artificial Functional Materials, Nanjing University, Nanjing 210093, P. R. China
2. Haian Institute of New Technology, Nanjing University, Haian 226600, P. R. China
3. School of Physics and Microelectronics, Zhengzhou University, Zhengzhou, Henan 450001, P. R. China
4. SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of the Ministry of Education, Southeast University, Nanjing 210096, P. R. China