1. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore
2. School of Electronics and Information, Northwestern Polytechnical University, Xi’an 710072, China
3. Department of Physics, Southeast University, Nanjing 210096, China
4. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore
5. Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117583, Singapore