1. Department of Physics and Research Institute for Natural Sciences , Hanyang University , Seoul 04763 , South Korea
2. Department of Applied Physics , Hanyang University , Ansan, Kyunggi-Do 15588 , South Korea
3. Department of Bionanotechnology , Hanyang University , Ansan, Kyunggi-Do 15588 , South Korea
4. Department of Electrical and Computer Engineering , MC 0407, University of California , 9500 Gilman Drive, La Jolla , San Diego, CA 92093-0407 , USA , E-mail: