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. Department of Chemistry, Boston University, 590 Commonwealth Avenue, Boston, Massachusetts 02215, United States
4. Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Singapore