1. Samsung-SKKU Graphene Center and School of Electronics and Electrical Engineering, Sungkyunkwan University, Suwon 440-746, Korea
2. SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Korea
3. Department of Electrical Engineering, Stanford University, Stanford, California 94305, United States
4. Samsung Advanced Institute of Technology, Samsung Electronics Co., Ltd., Yongin 446-712, Korea
5. Center for Human Interface Nanotechnology (HINT), Suwon 440-746, Korea