1. Division of Quantum Phases and Devices, School of Physics, Konkuk University, Seoul 143-701, South Korea
2. School of Electrical Engineering, Korea University, Seoul 136-701, South Korea
3. Department of Nano Applied Engineering, Kangwon National University, Kangwon-do 200-701, South Korea
4. Department of Physics and Graphene Research Institute, Sejong University, Seoul 143-747, South Korea