1. School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, P. R. China
2. School of Physics and Electronic Engineering, Mudanjiang Normal University, Mudanjiang 157011, P. R. China
3. Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China
4. Department of Physics, Liaoning University, Shenyang 110036, P. R. China