1. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542
2. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
3. Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546
4. WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
5. JST, PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
6. Materials Science and Engineering, Rutgers University, 607 Taylor Road, Piscataway, New Jersey 08854, United States