1. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
2. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551
3. Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Singapore 138634
4. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 3 Science Drive 3, Singapore 117546