1. Institute of Funemotiondamental and Frontier Sciences University of Electronic Science and Technology of China Chengdu 610054 P. R. China
2. College of Chemistry and Chemical Engineering Henan University of Technology Zhengzhou 450001 P. R. China
3. Department of Electronic and Electrical Engineering University College London Torrington Place London WC1E 7JE UK
4. State Key Laboratory of Electronic Thin Films and Integrated Devices University of Electronic Science and Technology of China Chengdu 610054 P. R. China