| Upconversion luminescence(UCL)materials have unique properties that render them received continuous attentions in a wide range of applications,including biomedical labeling,optical devices,solar-state lasers,and IR capture of solar cell.Choosing the appropriate host and activator are the most efficiency way to improve the properties of UCL material.Titanate compound oxide,ALnTiO4(A = Li,Na,K;Ln =rare earth),have attracted attentions owing to their typical layered structure and excellent chemical and physical stability.In this paper,NaYTiO4 upconversion phosphors doped with different concentrations of Er3+,Yb3+ and Ho3+ ions were successfully synthesized by the solid-state reaction method.The UCL properties of the phosphors are evaluated systematically under 1550 nm and 980 nm excitation,major conclusions described as follows:1.NaYTiO4:Er3+ phosphors were successfully synthesized by the solid state reaction method at 1100 ℃,and the UCL properties of the phosphor excited at 1550 nm are investigated.The phosphors show bright UCL,emission peaks locate at 530,550,and 670 nm,which attributed to the 2H11/2/4S3/2 →4I15/2 and 4F9/2 → 4115/2 transitions of Er3+ ion,respectively.Power dependence of UCL intensity proves that the observed UCL is a three-photon absorption process.Doping concentration of Er3+ ions have significant effect on the luminescence properties of the phosphors.Under low doping conditions,the phosphors generate strong red and weak green emissions,which is associated with the efficient population process of 4F9/2 red emission level via 4I9/2→4I11/2 non-radiative relaxation followed by a 4I11/2 → 4F9/2 transition.As the Er3+ ions concentration further increase,the green emission gradually dominate the UCL process,which is attributed to the energy transfer process of Er3+ ion pairs described as 4I13/2(Er3+)+ 4I9/2(Er3+)→ 4I15/2(Er3+)+ 2H11/2(Er3+).These results indicate that Er3+ ion can be used as excellent sensitizer for co-doping UCL materials under 1550 nm excitation.2.NaYTiO4:Yb3+,Er3+ phosphors were prepared by the solid-state reaction method.UCL properties of the phosphors are studied under the 1550 and 980 nm excitations.The observed emissions of the phosphors are similar,locate at 510-580 nm(red emission)and 635-698 nm(green emission),which correspond to the 2H11/2/4S3/2→4I15/2 and 4F9/2 →4I15/2 transitions of Er3+ ions,respectively.Power dependence of UCL intensity indicates that the UCL are two-and three-absorption processes under the 980 and 1550 nm excitations,respectively.Under 1550 nm excitation,the optimum doping concentration of Yb3+ ion is 12 mol%,compared with properties of the phosphors excited at 980 nm,emission color of the samples move red to green areas with the increment of Yb3+ concentration,it is due to the efficient 4I9/2 → 4I11/2 multi-phonon relaxation and Er3+ → Yb3+ → Er3+ energy transfer process.The related UCL mechanisms and energy transfer processes are detail discussed in the text.3.NaYTiO4:Ho3+,Er3+ phosphors were prepared by the solid-state reaction method.The UCL properties of the phosphors excited at 1550 and 980 nm excitations are studied.The phosphors show bright and high-purity red UCL.Appropriate increase of Er3+ doping concentration can effectively improve the luminescence efficiency of the phosphors.A small amount of Ho3+ ions can significantly weaken the green emission whereas enhance the red emission.However,the excessive doping of Ho3+ ions(>0.8 mol%)will lead to concentration quenching.The best component of the phosphors are determined according to the UCL spectra and the transition mechanism of Er3+-Ho3+system in NaYTiO4 are analyzed systematically in the text. |
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