1. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
2. Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan
3. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
4. Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
5. Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan