1. Department of Physics, National Sun Yat-sen University, Kaohsiung 80424, Taiwan
2. Department of Microtechnology and Nanoscience, Chalmers University of Technology, Gothenburg 41296, Sweden
3. Research Institute for Electronic Science, Hokkaido University, Hokkaido 001-0020, Japan
4. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
5. Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
6. Department of Chemical System Engineering, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan