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. School of Materials Science and Engineering, Nanyang Technological University, N4.1 Nanyang Avenue, Singapore 639798
4. State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
5. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543