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