1. Department of Physics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, ROC
2. Department of Materials Science and Engineering, City University of Hong Kong, Kowloon 999077, Hong Kong S.A.R., P. R. China
3. Department of Materials Science and Engineering and Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu 30010, Taiwan, ROC
4. National Nano Device Laboratories (NDL), Hsinchu 30078, Taiwan, ROC
5. School of Materials Science and Engineering, State Key Laboratory of Advanced Processing and Recycling of Non-Ferrous Metals in Gansu Province, Lanzhou University of Technology, Lanzhou City 730050, Gansu Province, P. R. China