1. Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, Xiangtan University, Hunan 411105, China.
2. Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan.
3. Institute of Physics, Academia Sinica, Taipei 11529, Taiwan.
4. Department of Materials Science and Engineering, National Chung Hsing University, Taichung 40227, Taiwan.
5. Institute of Electronics Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan.
6. Department of Physics, National Cheng Kung University, Tainan, Taiwan.
7. Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan.