1. Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, People's Republic of China
2. Key Laboratory of Radio Frequency and Micro-Nano Electronics of Jiangsu Province, Nanjing University of Posts and Telecommunications, Nanjing 210023, People's Republic of China
3. Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People's Republic of China