1. Department of Physics and Astronomy and Department of Nano Science and Technology, Seoul National University, Seoul 151-747, Republic of Korea
2. Department of Materials Chemistry and Engineering, Konkuk University, Seoul 143-701, Republic of Korea
3. Department of Chemistry Education, Seoul National University, Seoul 151-742, Republic of Korea
4. Department of Physics, Incheon National University, Incheon 406-772, Republic of Korea