1. School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, P. R. China
2. School of Chemistry and Chemical Engineering, Yulin University, Yulin 719000, P. R. China
3. College of Physics and Electronic Information, Yan’an University, Yan’an 716000, P. R. China
4. School of Environmental Science and Engineering, Chang’an University, Xi’an 710064, P. R. China