1. Department of Nanotechnology, Hanyang University, Seoul 133-791, Korea, Division of Advanced Materials Science and Engineering, Hanyang University, Seoul 133-791, Korea, Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), San-31 Hyoja-dong, Pohang 790-784, Korea, and Department of Physics, Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University, Suwon 440-746, Korea