1. Sungkyunkwan Advanced Institute of Nanotechnology, Department of Energy Science, Department of Physics, BK21 Physics Division, Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University, Suwon 440-746, South Korea, and Frontier Research Laboratory, Samsung Advanced Institute of Technology, Yong-In, Kyeongki-Do, 449-712 Korea