1. Graphene Research Institute, Sejong University, Seoul 143-747, Korea
2. Department of Physics, Sejong University, Seoul 143-747, Korea
3. Department of Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Suwon 443-270, Korea
4. Energy Semiconductor Research Center, Advanced Institutes of Convergence Technology, Seoul National University, Suwon 443-270, Korea
5. WCU Hybrid Materials Program, Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea
6. Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea