1. School of Physics, Northwest University, Xi'an 710069, P. R. China
2. School of Materials Science and Engineering, Shaanxi Normal University, Xi'an 710119, P. R. China
3. College of Sciences, China Jiliang University, Hangzhou 310018, China
4. School of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore
5. Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR 999077, China