1. Department
of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore
2. Graphene
Research Centre, National University of Singapore, 6 Science
Drive 2, Singapore 117546, Singapore
3. Center
of Physics and Department of Physics, University of Minho, PT-4710-057, Braga, Portugal
4. School
of Materials Science and Engineering, Nanyang Technological University, N4.1 Nanyang Avenue, Singapore 639798, Singapore
5. Department
of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore