1. Department of Physics, Southeast University, Nanjing 211189, China
2. Graphene Research and Characterization Center, Taizhou Sunano New Energy Co., Ltd., Taizhou 225300, China
3. State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
4. National Laboratory of Solid State Microstructures, Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China
5. National Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing 210093, China