1. School of Electrical and Computer Engineering, and Birck Nanotechnology Center, Purdue University, 465 Northwestern Avenue, West Lafayette, Indiana 47907, United States
2. Department of Electrical Engineering, University of Southern California, 3710 McClintock Avenue, Los Angeles, California 90089, United States
3. Department of Physics, Kyonggi University, Suwon, Gyeonggi-Do 443-760, Republic of Korea