1. Nanoscale Materials Center, National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, Photocatalytic Materials Center, National Institute for Materials Science (NIMS), Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan, and Department of Electronics Engineering, Korea University, 5-1 Anam-Dong, Seongbuk-Gu, Seoul 136-701, Korea