1. Department of Engineering and System Science, National Tsing Hua University, Hsinchu 300, Taiwan
2. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan
3. Nano Science and Technology Program, Taiwan International Graduate Program, Institute of Physics, Academia Sinica, Taipei 115, Taiwan
4. Institute of Molecular and Cellular Biology, National Taiwan University, Taipei 106, Taiwan