1. School of Physics and Nano Systems Institute-National Core Research Center, Seoul National University, Seoul, 151-747, Korea, Department of Chemistry, Renmin University of China, Beijing, PR China, A. F. Ioffe Physical-Technical Institute, Russian Academy of Sciences, St. Petersburg 194021, Russia, and Institute of Material Science, Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), University of Tsukuba, Tsukuba, Ibaraki 350-8573, Japan