1. Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan
2. Nano Science and Technology Program, Taiwan International Graduate Program, Academia Sinica and National Tsing Hua University, Taiwan
3. Institute of Physics, Academia Sinica, Taipei 115, Taiwan
4. National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan
5. Department of Physics, National Taiwan University, Taipei 106-17, Taiwan