1. Nano-Mechanical Systems Research Division, Korea Institute of Machinery & Materials (KIMM), 171 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea
2. Department of Nano-Mechatronics, University of Science and Technology, 217 Gajungro, Yuseong-gu, Daejeon 305-350,
Republic of Korea