1. Institute of Nanoscience and Department of Physics, National Chung Hsing University, Taichung 402, Taiwan
2. Department of Physics, University of North Sumatra, Medan, Sumatra Utara 20155, Indonesia
3. Department of Chemistry, Bahir Dar University, P.O. Box 79, Bahir Dar, Ethiopia
4. Department of Electronic Engineering, Feng Chia University, Taichung 40724, Taiwan