1. Department of Physics and Astronomy; Seoul National University; Seoul 151-747 Korea
2. Department of Chemistry and the Smalley Institute for Nanoscale Science and Technology; Rice University; Houston TX 77005 USA
3. School of Material Science and Engineering, Gwangju Institute of Science and Technology; Gwangju 500-712 Korea
4. Department of Applied Physics; Kyung Hee University; Yongin-si Gyeonggi-do 446-701 Korea