1. Electronic Materials and Device Research Center, Korea Electronics Technology Institute, Gyeonggi-do 463-816, Korea
2. School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Korea
3. Department of Physics and Graphene Research Institute, Sejong University, Seoul 143-747, Korea
4. Department of Materials Science and Engineering, Korea University, Seoul 136-701, Korea
5. Department of Mechanical Engineering and the Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, United States