1. Thin Film Materials Laboratory, Korea Research Institute of Chemical Technology, Yuseong Post Office Box 107, Daejeon 305-600, Korea, Department of Physics and Center for Nanotubes and Nanostructured Composites (CNNC), Sungkyunkwan University, Suwon 440-746, Korea, and Department of Advanced Materials Chemistry, Korea University, Jochiwan, Chungnam 339-700, Korea