1. SNU Graphene Center & Department of Chemistry, Seoul National University, Seoul 151-744, Republic of Korea
2. Department of Mechanical Engineering, Columbia University, New York, New York 10027, United States
3. SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, Republic of Korea
4. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, United States