1. Graphene Research Institute, Sejong University, Gwangjin-gu, Seoul 143-747, South Korea
2. Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, Gwangjin-gu, Seoul 143-747, South Korea
3. Hybrid Materials Research Center, Sejong University, Gwangjin-gu, Seoul 143-747, South Korea
4. Department of Chemistry, Sejong University, Gwangjin-gu, Seoul 143-747, South Korea
5. Department of Physics, Sejong University, Gwangjin-gu, Seoul 143-747, South Korea
6. Materials and Life Science Research Division, Korea Institute of Science and Technology, Hwarang-ro 14-gil, Seoul 136-791, South Korea