1. Institute of Photoelectronic Thin Film Devices and Technology, Nankai University; Tianjin 300071 P.R. China
2. Institute of New Energy Technology, Jinan University; Guangzhou, 510632 P.R. China
3. College of Physics, Nankai University; Tianjin 300071 P. R. China
4. Institute of Physics, Chinese Academy of Sciences; Beijing, 100190 P. R. China