1. Department of Materials Science and Engineering, Korea Advanced Institute of Science & Technology (KAIST), Daejeon 305-701, Republic of Korea
2. Graphene Research Center and KI for NanoCentury, Korea Advanced Institute of Science & Technology, Daejeon 305-701, Republic of Korea
3. Department
of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea