1. Department of Physics, Ewha Womans University, Seoul 120-750, Republic of Korea
2. Electronic Materials Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea
3. Department of Nanomaterials Science and
Technology, University of Science and Technology, Daejeon, 305-333, Republic of Korea
4. Department of Materials
Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 151-742, Republic of Korea