1. Department of Materials Science and Engineering, KAIST, Daejeon, 305-701, Korea
2. Electronics and Telecommunications Research Institute (ETRI), Daejeon 305-700, Korea
3. Department of Chemistry, Hanyang University, Seoul, 133-701, Korea
4. Korea Research Institute of Standards and Science (KRISS), Daejeon, 305-340, Korea
5. Department of Electrical Engineering, KAIST, Daejeon, 305-701, Korea
6. Department of Mechanical Engineering and the Texas Materials Institute, University of Texas at Austin, Texas, 78712-0292, United States
7. Department of Advanced Device Technology, University of Science and Technology (UST), Daejeon, 305-333, Korea