1. Graphene Research Center, Samsung Advanced Institute of Technology, Yongin 446-712, Republic of Korea
2. School of Chemical Engineering and Bioengineering, University of Ulsan, Ulsan 680-749, Republic of Korea
3. Analytical Research Division, Korea Basic Science Institute, Seoul 136-713, Republic of Korea
4. BK21 Physics Division, WCU Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea
5. Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea