1. WPI Center for Materials Nanoarchitechtonics (WPI-MANA), National Institute for Materials Science (NIMS); Tsukuba Ibaraki 305-0044 Japan
2. Department of Physics; National Chung-Hsing University; Taichung 40227 Taiwan
3. Department of Electrophysics; National Chiao Tung University; Hsinchu 30010 Taiwan
4. Department of Chemistry; Graduate School of Science and Engineering, Saitama University; Saitama 338-8570 Japan