1. Multi-Functional Nano/Bio Electronics Lab, Kyung Hee University, Yongin 446-701, South Korea
2. Display Convergence Research Center, Korea Electronics Technology Institute, Seongnam 463-816, South Korea
3. Department of Electrical and Computer Engineering, Waterloo Institute for Nanotechnology (WIN), University of Waterloo, Waterloo, Ontario N2L 3G1, Canada