1. Display Device & Processing Lab and Analytical Engineering Center, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, Korea, and Department of Physics, Department of Nanoscience and Nanotechnology, and Center for Nanotubes and Nanostructured Composites, Sungkyunkwan Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, Korea