1. Department of Materials Science and Engineering and Program for Integrated Molecular Systems (PIMS), Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Korea, Eco-Nano Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Republic of Korea, and Korea Basic Science Institute, Daejeon 350-333, Republic of Korea