1. National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China
2. School of Electric Science and Engineer, Nanjing University, Nanjing 210093, China
3. Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
4. Jiangsu Provincial Key Laboratory of Photonic and Electronic Materials Sciences and Technology, Nanjing University, Nanjing 210093, China
5. Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, China