1. Department of Applied Physics; Kyung Hee University; Yongin 17104 Korea
2. Multi-Functional Nano/Bio Electronics Laboratory; Kyung Hee University; Gyeonggi 17104 Korea
3. School of Electrical and Computer Engineering; Purdue University; West Lafayette IN 47907 USA
4. Carbon Convergence Materials Research Center; Korea Institute of Science and Technology; Wanju-gun 565-905 Republic of Korea