1. School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea
2. Creative Research
Center for Graphene Electronics, Electronics and Telecommunications Research Institute (ETRI), Daejeon 305-700, Korea
3. UNIST Central Research Facilities (UCRF) and School of Natural Science, UNIST 50 UNIST Road, Ulsan 689-798, Korea