1. Department of Physics and Institute of Nanoscience, National Chung Hsing University, Taichung, 402, Taiwan
2. Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Sumatera Utara, Medan 20155, Sumatera Utara, Indonesia
3. Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Sumatera Utara, Medan 20155, Sumatera Utara, Indonesia
4. Department of Electronic Engineering, Feng Chia University, Taichung, 40724, Taiwan