1. Department of Materials Science and Engineering; Korea Advanced Institute of Science and Technology (KAIST); Daejeon 305-701 Korea
2. Creative Research Center for Graphene Electronics; Electronics and Telecommunications Research Institute (ETRI); Daejeon 305-700 Korea
3. Department of Electrical Engineering and Graphene Research Center; Korea Advanced Institute of Science and Technology (KAIST); Daejeon 305-701 Korea