1. National Laboratory of Solid State Microstructures, and Jiangsu Provincial Laboratory for NanoTechnology Nanjing University, Nanjing 210093, P. R. China, Department of Chemistry and Center of Nanoscience and Nanotechnology, National Chung-Hsing University, Taichung 402, Taiwan, Republic of China, and Chemistry Department and Center for Surface Analysis and Research, Hong Kong Baptist University, Hong Kong, P. R. China