1. Institute of Optoelectronics & Nanomaterials, MIIT Key Laboratory of Advanced Display Materials and Devices, College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
2. Grünberg Research Centre, Nanjing University of Posts and Telecommunications, Nanjing 210003, China
3. School of Physics and Materials Science, Anhui University, Hefei 230601, P. R. China
4. Key Laboratory of Functional Materials and Applications of Fujian
Province, College of Materials Science and Engineering, Xiamen University of Technology, Xiamen 361024, P. R. China