1. Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan
2. Center of Atomic Initiative for New Materials, National Taiwan University, Taipei 106, Taiwan
3. Department of Physics, National Taiwan University, Taipei 106, Taiwan
4. Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan
5. Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu 300, Taiwan