1. Department of Physics, Chung Yuan Christian University, Taoyuan 320314, Taiwan
2. Research Center for Semiconductor Materials and Advanced Optics, Chung Yuan Christian University, Taoyuan 320314, Taiwan
3. Physics Division, National Atomic Research Institute, Taoyuan 325207, Taiwan
4. Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu 300093, Taiwan
5. Department of Chemistry, Chung Yuan Christian University, Taoyuan 320314, Taiwan