1. Department of Materials Science and Engineering, I-Shou University, Kaohsiung 840, Taiwan
2. Department of Physics and Astronomy, University of New Mexico, Albuquerque, New Mexico 87131, United States
3. Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan
4. Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 81148, Taiwan