1. Department of Engineering and System Sciences, National Tsing Hua University, Hsinchu 30013, Taiwan; Nano Science and Technology Program, Taiwan International Graduate Program, Academia Sinica, Nankang, Taipei 11529, Taiwan; and Institute of Physics, Academia Sinica, Nankang, Taipei 11529, Taiwan
2. Department of Engineering and System Sciences, National Tsing Hua University, Hsinchu 30013, Taiwan
3. Institute of Physics, Academia Sinica, Nankang, Taipei 11529, Taiwan