1. State Key Laboratory of Electronic Thin Films and Integrated Devices School of Optoelectronic Science and Engineering University of Electronic Science and Technology of China (UESTC) Chengdu 610054 P. R. China
2. Chongqing Key Laboratory of Materials Surface & Interface Science School of Materials Science and Engineering Chongqing University of Arts and Sciences Chongqing 402160 P. R. China