1. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore
2. Centre
for Advanced 2D Materials, National University of Singapore, Block S14,
Level 6, 6 Science Drive 2, Singapore 117546, Singapore
3. Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
4. Institute of Materials Research and Engineering, Agency for Science Technology and Research, 3, Research
Link, Singapore 117602, Singapore