1. Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30049, Taiwan
2. Faculty of Basic Sciences, Can Tho University of Medicine and Pharmacy, 179 Nguyen Van Cu Street, Can Tho, Vietnam
3. Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan
4. Institute of Physics, National Chiao Tung University, Hsinchu 300, Taiwan