1. Department of Materials and Optoelectronic Science, Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung, 804, Taiwan, Department of Materials Science and Engineering, I-Shou University, 84041 Kaohsiung, Taiwan, and Department of Polymer-Nano Science and Technology, Chonbuk National University, Jeonju 561-756, Korea