Near infrared fluorescence and energy transfer in Ce/Nd Co-doped CaxSr1−xS

Novel near infrared (NIR) phosphors CaxSr1−xS:Ce3+,Nd3+ were synthesized by a solid state reaction. The NIR emission was realized through an efficient absorption by the allowed 4f–5d transition of Ce3+ and efficient energy transfer to Nd3+ via well-matched energy levels. Ce3+ and Nd3+ content in CaS/SrS was optimized. It was found that CaS:Ce3+,Nd3+ gave much stronger NIR emission than that of SrS:Ce3+,Nd3+. Further studies on CaxSr1−xS:Ce3+,Nd3+ indicated that both visible emission of Ce3+ and NIR emission of Nd3+ were observably affected by Ca/Sr ratio. The energy transfer efficiency, which can be estimated from fluorescence lifetime of Ce3+, increased from 52% to 74% for the CaxSr1−xS:Ce3+,Nd3+ (x=0 to 1) series, accompanied with a shift of maximal emission wavelength of Ce3+ from 482 to 505 nm. The results showed that overlap between emission spectrum of Ce3+ and excitation spectrum of Nd3+ plays an important role in the energy transfer efficiency, and Ce3+ emitting in green or blue-greenish region sensitized the Nd3+ NIR fluorescence emission more efficiently than that in blue region.