1. NUS Graduate School for Integrative Science and Engineering, National University of Singapore, Singapore 117542
2. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542
3. National University of Singapore Chongqing Research Institute, Chongqing 401123, China
4. Centre for Advanced 2D Materials (CA2DM), National University of Singapore, 6 Science Drive 2, Singapore 117546
5. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543