Growth And Properties Of Red And Near-Infrared Cs₄Eu_1-yI_6-xBr_x:ySm Crystals

In recent years,with the rapid development of photoelectric conversion devices,some red-sensitive photoelectric conversion devices have appeared,such as Avalanche photodiode（APD）,which has a high quantum efficiency of about 80%in red-near-infrared band.To take advantage of their high quantum efficiency,researchers are focusing on novel luminescent materials with long wavelength infrared emission,such as CsBa₂I₅:Eu²⁺,Sm²⁺crystals.Its excellent performance proves that Eu²⁺-Sm²⁺co-doped system is an effective way to obtain excellent red near-infrared scintillation crystals.In this paper,Cs₄EuBr₆ and Cs₄EuI₆ crystals are selected as matrix crystals and Sm²⁺is mixed to prepare red-NIR scintillation crystals.Eu²⁺matrix can transfer more energy to Sm²⁺.Cs₄EuBr_6-xI_x:y%Sm²⁺crystals and Cs₄EuI₆:y%Sm²⁺crystals with different concentrations of Sm²⁺have been successfully grown by crucible descent method.This paper carried out a series of experiments to study their growth and performance,and discussed their energy transfer and luminescence principle.The specific research results are summarized as follows:（1）0.5 inch Cs₄EuBr_6-xI_x:y%Sm²⁺crystals and Cs₄EuI₆:y%Sm²⁺crystal mixed with different concentrations of Sm²⁺were grown by crucible descent method.Large transparent crystals were successfully grown by adjusting the growth conditions.The XRD pattern is almost identical with the calculated standard pattern,indicating that Sm²⁺successfully replaced the lattice of Eu²⁺,and Sm²⁺is proved to be divalent by XPS pattern.（2）The fluorescence spectra of Cs₄EuBr_6-xI_x:y%Sm²⁺crystal and Cs₄EuI₆:y%Sm²⁺crystal showed Eu²⁺and Sm²⁺emission,and Sm²⁺emission was located at825 nm and 844 nm,respectively.The Eu²⁺emission intensity decreases with the increase of Sm²⁺concentration,and the overlap of Eu²⁺and Sm²⁺excitation spectra proves the existence of energy transfer between Eu²⁺-Sm²⁺.They show high transmittance after 800 nm,so they do not affect the red-near infrared emission of Sm²⁺.Because Cs₄EuI₆ crystal has more deep electron traps and the trap depth is deeper,the energy transfer efficiency between Cs₄EuI₆:y%Sm²⁺crystal Eu²⁺-Sm²⁺is lower,and the emission intensity of Sm²⁺is lower than that of Cs₄EuBr_6-xI_x:y%Sm²⁺crystal.Under 410 nm laser excitation,the Sm²⁺emission intensity of Cs₄EuBr_5.96I_0.04:2%Sm²⁺crystal is the highest,and the Sm²⁺emission ratio reaches 94.2%.Cs₄EuI₆:3%Sm²⁺crystals have the highest Sm²⁺luminescence intensity,and the Sm²⁺luminescence ratio is only 80.66%.（3）Deliquescence experiments on Cs₄EuBr_6-xI_x:y%Sm²⁺crystals and Cs₄EuI₆:y%Sm²⁺crystals showed that both of them had deliquescence.The increase of Sm²⁺concentration would slightly increase deliquescence,and Cs₄EuI6:y%Sm²⁺crystal deliquescence is more serious.（4）There are radiative energy transfer and non-radiative energy transfer between Eu²⁺-Sm²⁺,while non-radiative energy transfer is more rapid and efficient.Therefore,in order to obtain more non-radiative energy,Cs₄EuBr₆ crystals and Cs₄EuI₆ crystals with Eu²⁺as the matrix are selected.Moreover,the average distance between Eu²⁺and Sm²⁺can be reduced by adjusting the Sm²⁺concentration to obtain more non-radiative transfer energy.This is also proved by the fact that the decay time of Eu²⁺decreases rapidly with the increase of Sm²⁺molar concentration.In addition,the increase of non-radiative energy transfer also makes the decay component of Sm²⁺（52.7 ns）in Cs₄EuBr_5.94I_0.06:3%Sm²⁺crystal higher（31.8%）,which is much higher than that of other Eu²⁺-Sm²⁺co-doped red-NIR scintillist crystals.