1. Electronic Materials Research Laboratory, Key Laboratory of Ministry of Education & Shaanxi Engineering Research Center of Advanced Energy Materials and Devices, School of Electronic Science and Engineering, Xi'an Jiaotong University, Xi'an 710049, P. R. China
2. Institute of Thin Films, Sensors and Imaging, University of the West of Scotland, SUPA Paisley PA1 2BE, Scotland, UK
3. School of Material Science and Engineering, Zhengzhou University, Henan 450001, China