1. Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, Lausanne-1015, Switzerland.
2. Department of Chemistry and Center of Nanoscience and Nanotechnology, National Chung Hsing University, Taichung, Taiwan 402, ROC.
3. Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu, Taiwan 300, ROC.