1. Department of Physics, Sungkyunkwan University, Suwon 440-746, Republic of Korea
2. Department of Mechanical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea
3. School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea
4. Graphene Center, Samsung Advanced Institute of Technology, Yongin 446-712, Republic of Korea
5. SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, Republic of Korea