1. School of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Korea
2. Department of Physics, 2 Science Drive 3, National University of Singapore, Singapore 117542, Singapore
3. Graphene Research Centre, 6 Science Drive 2, National University of Singapore, Singapore 117546, Singapore
4. NanoCore, 4 Engineering Drive 3, National University of Singapore, Singapore 117576, Singapore
5. Department of Mechanical and Aerospace Engineering, University of Central Florida, Orlando, Florida 32816-2450, United States
6. NUS Graduate School for Integrative Sciences and Engineering (NGS), Centre for Life Sciences (CeLS), 28 Medical Drive, Singapore 117456, Singapore