1. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore
2. Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Singapore
3. Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore
4. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore