1. Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University, Suwon 440-746, R. O. Korea
2. Department of Physics, Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University, Suwon 440-746, R. O. Korea
3. Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, R. O. Korea
4. Department of Physics, Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, R. O. Korea