1. Key Laboratory for Micro/Nano Optoelectronic Devices of Ministry of Education & Hunan Provincial Key Laboratory of Low-Dimensional Structural Physics and Devices, School of Physics and Electronics, Hunan University, Changsha 410082, People’s Republic of China
2. School of Microelectronics & School of Electronics and Information, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China
3. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551, Singapore
4. Centre for Advanced 2D Materials (CA2DM) and Graphene Research Centre (GRC), National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore