1. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542
2. Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546
3. Center of Physics and Department of Physics, University of Minho, PT-4710-057, Braga, Portugal
4. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543