1. Key Laboratory of Photo-Electronic Thin Film Devices and Technology of Tianjin, Engineering Research Center of Thin Film Photo-Electronic Technology, Ministry of Education, Institute of Photo-Electronic Thin Film Devices and Technology of Nankai University, Tianjin 300350, P. R. China
2. School of Materials Science and Engineering, Yancheng Institute of Technology, Yancheng 224051, P. R. China