1. UNIST Central Research Facilities (UCRF) and School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea
2. Creative Research Center for Graphene Electronics, Electronics and Telecommunications Research Institute (ETRI), Daejeon 305-700, Republic of Korea
3. Department of Chemistry, Seoul National University, Seoul 151-747, Republic of Korea