1. College of Optoelectronics Technology, Chengdu University of Information Technology, Chengdu 610225, China
2. School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China
3. State Key Laboratory of Electrical Insulation and Power Equipment & School of Electrical Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China
4. Department of Physics, College of Science, Shihezi University, Xinjiang 832003, China
5. School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, P. R. China