1. Yunnan Joint International Laboratory of Optoelectronic Materials and Devices, Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming Yunnan 650093, Yunnan, China
2. Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, College of Chemistry and Chemical Engineering, Northwest Normal University, 967 Anning East Road, Lanzhou 730070, P. R. China
3. Department of Modern Physics, Lanzhou University, Lanzhou 730000, People's Republic of China