[1] Korea Univ, Dept Mat Sci & Engn, Seoul 136701, South Korea
[2] Korea Univ, KU KIST Grad Sch Converging Sci & Technol, Seoul 136713, South Korea
[3] Pohang Univ Sci & Technol, PCTP, Pohang 790784, South Korea
[4] Pohang Univ Sci & Technol, Dept Phys, Pohang 790784, South Korea
[5] Pohang Univ Sci & Technol, Basic Sci Res Inst, Pohang 790784, South Korea
[6] NIST, Ctr Nanoscale Sci & Technol, Gaithersburg, MD 20899 USA