1. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan
2. Institute of Physics and Freiburg Institute for Advanced Studies, Albert-Ludwigs-Universität, Freiburg, Germany
3. Department of Engineering and Systems Science, National Tsing Hua University, Hsinchu, Taiwan
4. Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan