1. Department of Electronics Engineering , Catholic University of Daegu , Hayang 38430 , 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