1. Centre
for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117542
2. National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario K1A 0R6, Canada
3. Department
of Physics, University of Ontario Institute of Technology, Oshawa, Ontario L1H 7K4, Canada
4. Department
of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551