1. Contribution from the Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea, Department of Physics, Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University, Suwon 440-746, Republic of Korea, and Department of Physics and Division of Nano Science, Ewha Womans University, Seoul 120-750, Republic of Korea