1. Department of Electronics, Nankai University, Tianjin 300071, China
2. Tianjin Key Laboratory of Photo-Electronic Thin Film Devices and Technology, Nankai University, Tianjin 300071, China
3. School of Material Science and Engineering, Tianjin University, Tianjin 300072, China
4. Department of Physics, Tianjin University, Tianjin 300072, China
5. School of Physics, The University of Sydney, Sydney, NSW 2006, Australia