1. Research Division, National Synchrotron Radiation Research Center, Hsinchu, Taiwan 30076, Republic of China
2. Department of Physics, Tamkang University, Tamsui, Taiwan 25137, Republic of China
3. Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30010, Republic of China
4. Department of Materials Science and Engineering and ⊥Center for Nanotechnology, Materials Science, and Microsystems National Tsing Hua University, Hsinchu, Taiwan 30013, Republic of China