1. Department of Intelligent Precision Machine, Nanomechanisms Group, Korea Institute of Machinery & Materials, 171 Jang-dong, Yuseong-gu, Daejeon, 305-343, Korea, and Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea