1. Department of Physics; National University of Singapore; 2 Science Drive 3 Singapore 117542 Singapore
2. Center for Advanced 2D Materials and Graphene Research Center; National University of Singapore; 6 Science Drive 2 Singapore 117546 Singapore
3. Institute of Materials Research and Engineering; A*STAR (Agency for Science, Technology and Research); 2 Fusionopolis Way. Innovis, #08-03 Singapore 138634 Singapore