1. State Key Laboratory of Electronic Thin Films and Integrated Devices; University of Electronic Science and Technology of China; Chengdu 610054 P. R. China
2. School of Applied and Chemical Engineering; Xichang College; Xichang 615053 P. R. China
3. Faculty of Science; Zhejiang University; Hangzhou 310012 P. R. China
4. School of Physics; University of Electronic Science and Technology of China; Chengdu 610054 P. R. China
5. MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage School of Chemistry and Chemical Engineering; Harbin Institute of Technology; Harbin 150001 P. R. China