1. Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore
2. Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, 117546 Singapore
3. Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, 117575 Singapore
4. School of Physical & Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 639798 Singapore
5. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551, Singapore