1. BK21 Physics Division, Department of Energy Science, and Center for Nanotubes and Nanostructured Composites, Sungkyunkwan Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, Korea
2. Department of Electrical Engineering and Computer Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
3. School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Korea