1. Samsung-SKKU Graphene Center; Sungkyunkwan Advanced Institute of Nanotechnology (SAINT) and School of Electronics and Electrical Engineering; Sungkyunkwan University; Suwon 16419 Korea
2. Department of Physics; Pusan National University; Busan 46241 Korea
3. Center for Integrated Nanostructure Physics; Institute for Basic Science (IBS); Department of Energy Science; Sungkyunkwan University; Suwon 16419 Korea
4. Department of Mechanical Engineering; Sungkyunkwan University; Suwon 16419 Korea
5. Department of Material Science and Engineering; Yonsei University; Seoul 03722 Korea
6. Department of Mechanical Engineering; Columbia University; New York NY 10027 USA
7. ICT Components and Materials Technology Research Division; Electronics and Telecommunications Research Institute; Daejeon 34129 Korea