1. Department of PhysicsNational Chung Hsing University Taichung 40227 Taiwan
2. Department of Electrical Engineering and Institute of Electronic EngineeringNational Tsing Hua University Hsinchu 30071 Taiwan
3. Department of ElectrophysicsNational Chiao Tung University Hsinchu 30010 Taiwan
4. Department of ChemistryGraduate School of Science and EngineeringSaitama University Saitama 338‐8570 Japan
5. WPI Center for Materials Nanoarchitechtonics (WPI‐MANA)National Institute for Materials Science (NIMS) Tsukuba Ibaraki 305‐0044 Japan