[1] Purdue Univ, Sch Elect & Comp Engn, W Lafayette, IN 47907 USA
[2] Purdue Univ, Birck Nanotechnol Ctr, W Lafayette, IN 47907 USA
[3] Kyonggi Univ, Dept Phys, Suwon 443760, Gyeonggi Do, South Korea
[4] Univ Illinois, Dept Mat Sci & Engn, Urbana, IL 61801 USA