1. Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701, Republic of Korea, and Bio-Nano Technology Research Center, Korea Research Institute of Bioscience and Biotechnology, 52 Oeun-dong, Yuseong-gu, Daejeon, 305-806, Republic of Korea