1. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore.
2. NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, 28 Medical Drive, Singapore 117456, Singapore.
3. Centre for Advanced 2D Materials (CA2DM), National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore.
4. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551, Singapore.
5. Department of Physics, Washington University in St. Louis, St. Louis, MO 63130, USA.
6. Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Singapore 138634, Singapore.
7. National Institute of Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan.
8. Yale-NUS College, 16 College Avenue West, Singapore 138527, Singapore.