1. Department of Physics, National Cheng Kung University, Tainan 701, Taiwan
2. Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan
3. Material and Chemical Research Laboratories, Nanotechnology Research Center, Industrial Technology Research Institute, Hsinchu 310, Taiwan
4. Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan
5. Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
6. Center for Quantum Frontiers of Research & Technology (QFort), National Cheng Kung University, Tainan 701, Taiwan