1. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
2. Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
3. Material and Chemical Research Laboratories and Nanotechnology Research Center, Industrial Technology Research Institute, Hsinchu 31040, Taiwan
4. Department of Applied Physics, Tunghai University, Taichung 40704, Taiwan