1. Key Laboratory of Artificial Micro, and Nano-Structures of Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan 430072, P. R. China
2. Hubei Yangtze Memory Laboratories, Wuhan 430205, P. R. China
3. Henan Province Engineering Research Center of Smart Micro-Nano Sensing Technology and Application, School of Physics and Electronics, Henan University, Kaifeng 475004, P. R. China
4. The Institute of Technological Sciences, Wuhan University, Wuhan 430072, P. R. China