1. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
2. Institute of Nanoengineering and Microsystems, National Tsing Hua University, Hsinchu 30013, Taiwan
3. Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
4. Research Centre for Applied Sciences, Academia Sinica, Nankang, Taipei 11529, Taiwan