1. Department of Physics and Astronomy, The University of Toledo, 2801W. Bancroft Street, Mail Stop 111, Toledo, Ohio 43606, USA
2. Department of Nanomechatronics, Korea University of Science and Technology, Daejeon 305-350, South Korea
3. Nanomechanical Research Division, Korea Institute of Machinery and Materials, Daejeon 305-343, South Korea