1. Department of Physics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
2. Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
3. National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
4. Institute of Physics, Academia Sinica, Taipei 11529, Taiwan