1. School of Physics, Northwest University, Xi’an 710069, P.R. China
2. School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710062, P.R. China
3. School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, P.R. China
4. School of Materials Science and Engineering, Tianjin University, Tianjin 300072, P.R. China