1. Department of Physics, University of Seoul, Seoul 130-743, Korea
2. Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea
3. Department of Physics and Graphene Research Institute, Sejong University, Seoul 143-747, Korea
4. Department of Materials Science and Engineering and Department of Physics, University of Texas at Dallas, Richardson, TX 75080, U.S.A.
5. WCU Multiscale Engineering Division, School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, Korea