1. Department
of Physics, National University of Singapore, 2 Science Drive 3, 117542 Singapore
2. Center
for Advanced 2D Materials and Graphene Research Center, National University of Singapore, 6 Science Drive 2, 117546 Singapore
3. Research
School of Engineering, College of Engineering and Computer Science, The Australian National University, Canberra, Australian Capital Territory 2601, Australia
4. Institute
of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, #08-03, 138634 Singapore