1. Department of Physics, National University of Singapore, 117542 Singapore, Singapore
2. Centre for Advanced 2D Materials, National University of Singapore, 117546 Singapore, Singapore
3. Institute for Advanced Study, Tsinghua University, 100084 Beijing, China
4. Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China
5. Institute of Materials Research & Engineering, A*STAR (Agency for Science, Technology and Research), 138634 Singapore, Singapore
6. Department of Chemistry, National University of Singapore, 117543 Singapore, Singapore