1. Department of Chemistry, Seoul National University, Gwanak-gu, Seoul 151-747, Korea
2. Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
3. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Maryland 20899
4. Department of Materials Science and Engineering, Seoul National University, Gwanak-gu, Seoul 151-744