Synthesis And Luminescence Properties Of NaLuF₄:Yb³⁺,Ce³⁺,Eu³⁺Upconversion Nanocrystals

Since the discovery of upconversion luminescence,lanthanide upconversion materials have gotten more and more attention because of their excellent optical properties.Nowadays,lanthanide upconversion materials have been applied to many fields included biological imaging,bioinstrumentation,photo-catalysis and energy sources.However,numerous scientists continuously strive to find better lanthanide upconversion materials with excellent optical properties,adjustable morphology and size,excellent solubility and biocompatibility.NaYF₄:Yb³⁺,Er³⁺was the most intensively used lanthanide upconversion materials,but little research of other lanthanide upconversion materials was to be found.NaLuF₄ have similar crystal structure with NaYF₄,so NaLuF₄ is also suitable to be the matrix compound of lanthanide upconversion materials due to low lattice phonon energies.Yb³⁺ is the most frequently used sensitizer of lanthanide upconversion materials.In addition,other two rare earth ion,Eu³⁺ and Ce³⁺,can be the doping ion of lanthanide upconversion materials.So,the research of the lanthanide upconversion materials NaLuF₄:Yb³⁺,Ce³⁺,Eu³⁺ have certain scientific significance.In this paper,we synthetized the lanthanide upconversion materials NaLuF₄:Yb³⁺,Eu³⁺ with NaLuF₄ in hexagonal phase as the host,with Yb³⁺ as the sensitizer and Eu³⁺as the activator,via a solvothermal processing.Then,the nanoparticles were tested and characterized by TEM,XRD and upconversion luminescence spectrum.In addition,we explained the mechanism of upconversion luminescence of NaLuF₄:Yb³⁺,Eu³⁺.The results illustrate NaLuF₄:Yb³⁺,Eu³⁺nanoparticles had low size distribution,regular morphology and good optical properties.Therefore,the nanoparticles have potentiality to be a good luminescence material in the field of biological imaging,bioinstrumentation.Then,we synthetized a series of NaLuF₄:Yb³⁺,Ce³⁺,Eu³⁺ nanoparticles with different doping concentration of Ce³⁺ to draw it into the nanoparticles.the nanoparticles were tested and characterized by TEM,XRD and upconversion luminescence spectrum.Compared with the result of nanoparticles without Ce³⁺,we found that the intensity of the upconversion luminescence of nanoparticles doped with a small quantity of Ce³⁺ enhanced.However,with the doping concentration of Ce³⁺increasing,the intensity of upconversion luminescence reduced.Therefore,we make a hypothesis that the generation of Yb³⁺-dimers was obstructed in existence of a small quantity of Ce³⁺.With the doping concentration of Ce³⁺ increasing,Ce³⁺ will occupy the position of Yb³⁺ so that the distance of two Yb³⁺ ions will increase.Therefore,the generation of Yb³⁺-dimers will be obstructed,and the intensity of upconversion luminescence will reduceIn order to enhance the water-solubility of the nanoparticles,we did the surface modification to the NaLuF₄:Yb³⁺,Ce³⁺,Eu³⁺ by using PAA to displace oleic acid.Then,the nanoparticles,surrounded by PAA,were tested and characterized by TEM,XRD and upconversion luminescence spectrum.Compared with the results of nanoparticles without being modified,we found that size of the nanoparticles increased form 15 nm to 150-200 nm,and the intensity of upconversion luminescence reduced.However,the NaLuF₄:Yb³⁺,Ce³⁺,Eu³⁺ nanoparticles also can to be a good luminescence material in the field of biological imaging,bioinstrumentation.The study of the mechanism of upconversion luminescence NaLuF₄:Yb³⁺,Ce³⁺,Eu³⁺ nanoparticles contributes to the development of the mechanism of lanthanide upconversion materials.Besides,The research of NaLuF₄:Yb³⁺,Ce³⁺,Eu³⁺nanoparticles has a guiding significance for the development of the fields of biological imaging and bioinstrumentation.