1. Department of Material Science & Engineering, Graphene Research Center (GRC), KAIST Institute for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea
2. Korea Institute of Industrial Technology, 711 Hosan-dong, Dalseo-gu, Daegu 704-203, Korea