1. Creative Research Center for Graphene Electronics; Electronics and Telecommunications Research Institute (ETRI); 218 Gajeong-ro Yuseong-gu, Daejeon 305-700 Korea
2. Department of Advanced Device Technology; University of Science and Technology (UST); Daejeon 305-333 Korea
3. OLED Research Center, Electronics and Telecommunications Research Institute (ETRI); 218 Gajeong-ro Yuseong-gu, Daejeon 305-700 Korea
4. Department of Advanced Chemical Engineering; Mokwon University; Daejeon 302-729 Korea
5. Department of Electrical Engineering and Graphene Research Center; Korea Advanced Institute of Science and Technology (KAIST); Daejeon 305-701 Korea
6. Department of Physics; Columbia University; New York NY 10027 USA