Enhanced red afterglow in Sr₂SnO₄:Sm³⁺ by co-doping Na⁺/K⁺

Red afterglow phosphors, Sr₂SnO₄:Sm³⁺ co-activated with alkali ions (K⁺, Na⁺) were prepared via solid-state reaction. The phase identification and photoluminescence were characterized and analyzed by X-ray diffraction (XRD), photoluminescence spectroscope, afterglow measurement and thermal luminescence spectroscope. XRD results confirmed an orthorhombic structure for all the samples prepared in the experiment, showing the doping ions had negligible influence on the crystal structure. The photoluminescent spectra of the Sm-doped phosphors revealed a group of reddish orange emission lines originating from ⁴G_5/2 → ⁶H_J transition of Sm³⁺. The optimal doping concentration of Sm³⁺ was determined as 1 mol% and concentration quenching was observed afterwards. For the alkaline ion co-doped samples, Na⁺ and K⁺ ions were found profitable to improve both the photoluminescence and the afterglow behavior. Thermal simulated luminescence study indicated that the persistent afterglow of Sr₂SnO₄:Sm³⁺ phosphor and alkali ions (K⁺, Na⁺) co-doped Sm³⁺:Sr₂SnO₄ phosphors were generated by the recombination of the electrons released from the electron traps with the holes in the valence band. The co-doping of alkaline ions (K⁺, Na⁺) increased not only the initial trap concentration, but also increased the depth of electron traps, which can be regarded as the main reasons for the enhanced afterglow. Based on such results, possible mechanisms for the afterglow of Sr₂SnO₄:Sm³⁺ and alkaline ions (K⁺, Na⁺) co-doped phosphors were proposed.