1. Department of Physics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
2. Center of Crystal Research, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
3. Department of Physics, National Chung Hsing University, Taichung 402, Taiwan
4. Institute of Nanoscience, National Chung Hsing University, Taichung 402, Taiwan