1. Department of Physics, National Tsing-Hua University, Hsinchu 30013, Taiwan
2. Institute of Nanoengineering
and Microsystems, National Tsing-Hua University, Hsinchu 30013, Taiwan
3. Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
4. CREST, Japan Science and Technology Agency (JST), 1-1-1 Tennodai, Tsukuba,
Ibaraki 305-8571, Japan