1. Department of Electronics Engineering, Catholic University of Daegu, Hayang 712-702, South Korea
2. Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, 110 8th Street, Troy, New York 12180-3590, USA
3. Department of Applied Physics and Department of Bionanotechnology, Hanyang University, Ansan, Kyunggi-Do 426-791, South Korea