Study On Upconversion Luminescence Properties Of Alkali Metal Co-doped Alkaline Earth Scandium Nanocrystals

Rare earth doped luminescent materials have attracted extensive attention due to their unique luminescent properties.However,the luminescence intensity of up-conversion luminescent materials is generally low,which limits their application in production and life.According to relevant reports,doped alkali metal ions（Li⁺,Na⁺,K⁺） can improve the emission intensity by occupying the gap,grain boundary or substitution position.This is because the doped alkali metal ions will increase the crystallinity,reduce the defect state,reduce the quenching center,charge compensation,and prolong the life.In substitutions,for example,some of the base ions are used to compensate the charge,while the rest of the ions produce oxygen vacancies.These oxygen vacancies,also known as defect centers,act as sensitizers in many cases to enhance the emission intensity of the activated ions;In addition,the doping of alkali metal ions can also increase the emission intensity by causing the asymmetry of crystal field.In this paper,MgSc₂O₄,CaSc₂O₄ and SrSc₂O₄,the alkali metal co-doped rare earth and scandium based oxide materials are taken as the research object to study the UCL characteristics,luminescence mechanism and temperature sensing characteristics of the materials.The specific content and results of this paper are as follows:（1）A series of MgSc₂O₄:y%Er³⁺/x%Yb³⁺（x=0,5,10,15,y=0.5,1,2,4） nanocrystals were prepared by hydrothermal method.X-ray diffraction patterns（XRD） and transmission electron microscopy（TEM） showed that the MgSc₂O₄:Er³⁺/Yb³⁺ co-doped with alkali metal ions prepared were spherical nanocrystals basically consistent with the standard card.Due to the maximum asymmetry caused by K⁺in the crystal field,the K⁺co-doped MgSc₂O₄:Er³⁺/Yb³⁺up-conversion luminescence nanoparticles have the strongest up-conversion fluorescence emission intensity,and with the increasing of the amount of K⁺ions,the emission intensity of the nanocrystals will also increases.By changing the doping concentration of Yb³⁺ and Er³⁺,the optimal luminescence intensity of K⁺ion co-doped Mg SC₂O₄:1%Er³⁺/5%Yb³⁺ nanocrystals was obtained.According to the curve of up-conversion fluorescence intensity and excitation power,the red and green up-conversion fluorescence of Er³⁺ in the K⁺ion co-doped MgSC₂O₄:1% Er³⁺/5%Yb³⁺ nanocrystals are two-photon absorption processes.（2）A series of alkaline earth scandium MSc₂O₄:1%Er³⁺/5%Yb³⁺（M=Mg,Ca,Sr,Ba）nanocrystals were prepared by hydrothermal method.The structure and morphology of the nanocrystals were analyzed by XRD and TEM.The results were analyzed by upconversion luminescence（UCL）,photoluminescence（PL）,raman spectroscopy,diffuse reflection（DR）and fourier transform infrared（FTIR）spectroscopy,and finally the best luminescence was obtained by CaSc₂O₄:1%Er³⁺/5%Yb³⁺.Then a series of alkali metals（Li⁺,Na⁺,K⁺）co-doped alkaline earth Scandiate MSc₂O₄:1%Er³⁺/5%Yb³⁺（M=Mg,Ca,Sr,Ba）were synthesized by hydrothermal method.The alkali metal co-doped alkaline earth scandium SrSc₂O₄:1%Er³⁺/5%Yb³⁺and CaSc₂O₄:1%Er³⁺/5%Yb³⁺were analyzed.XRD and TEM show that the prepared alkali metal ion co-doped SrSc₂O₄:1%Er³⁺/5%Yb³⁺and CaSc₂O₄:1%Er³⁺/5%Yb³⁺ are basically consistent with the standard card nanorods.Through spectral analysis,it was found that Li⁺doped SrSc₂O₄:1%Er³⁺/5%Yb³⁺and CaSc₂O₄:1%Er³⁺/5%Yb³⁺had better luminescence,and Li⁺doped SrSc₂O₄:1%Er³⁺/5%Yb³⁺nanocrystals had the best luminescence intensity.According to the curve of fluorescence intensity and excitation power,the green and red light of Er³⁺in Li⁺co-doped SrSc₂O₄:1%Er³⁺/5%Yb³⁺and CaSc₂O₄:1%Er³⁺/5%Yb³⁺nanocrystals are normal two-photon absorption processes.In addition,the temperature characteristics of CaSc₂O₄:1%Er³⁺/5%Yb³⁺ and SrSc₂O₄:1%Er³⁺/5%Yb³⁺were explored.The relationship between the green up-conversion luminescence（500-600 nm） and the temperature（300-600 K）in the excitation range of 980 nm was measured.The greater the absolute sensitivity（S_A）,the higher the sensitivity,indicating the stronger the temperature measurement ability of the material.It was found that the absolute sensitivity（S_A）increased with the increase of temperature.At 600 K,the maximum absolute sensitivity（S_A）of CaSc₂O₄ and SrSc₂O₄was about 0.0041k^-1 and 0.0036k^-1,respectively.With the increase of temperature,the relative sensitivity（S_R）decreased gradually.At 600 K,the minimum values of the relative sensitivity（S_R）of CaSc₂O₄ and SrSc₂O₄ were about 0.0032k^-1 and 0.0031k^-1,respectively.The results show that SrSc₂O₄:1%Er³⁺/5%Yb³⁺ and CaSc₂O₄:1%Er³⁺/5%Yb³⁺ can be applied to high temperature measurement.