Study On The Fluorescence Characteristics Of Mo⁵⁺ Co-doped NaYF₄:RE³⁺ Micro-crystals

Rare earth doped upconversion luminescent materials have unique optical properties including narrow spectral lines,rich spectral bands,long life and high stability,which make them suitable for biomarkers,security,3D multi-color displays and optical devices etc.Due to the relatively small absorption cross section of rare earth dopants,the quenching effect caused by surface defects of nanomaterials and the cross-relaxation between rare earth ions,which limits the practical application.Therefore,improving the luminscence efficiency of upconversion is of great significance for promoting the fundamental research and application of rare earth doped micro/nano materials.It is well known that luminescence intensity of rare earth ions depends mainly on the probability of electron transition,which is significantly affected by the crystal field.The upconversion emission can be enhanced by regulating the crystal field around the rare earth ions.By affecting the coordination environment of rare earth ions in the matrix,the fluorescence emission,crystal phase and morphology also have been influenced.Since Mo5+is smaller than Y3+,and the valence of Mo5+ is higher than Y3+,some good properties are produced when Mo5+ ions enters the NaYF4 lattice.But the effect of Mo5+ co-doping on fluorescence emission is rarely reported.In order to improve the efficiency of rare earth doped upconversion luminescent materials,Mo5+ ions have been doped into NaYF4 crystals in current study.In addition,the parameters of Judd-Ofelt theory were used to quantitatively analyze the effects of concentration change of Mo5+ co-doping on the local environment around Eu3+ in the NaYF4 lattice.Finally,a temperature probe based on Eu3+emission intensity ratio of different energy levels is constructed,and the sensitivity of Eu3+ temperature probe is improved by Mo5+ co-doping.The main work of current thesis is as follows:1.A group of different concentrations of Mo5+ co-doped β-NaYF4:Yb3+/Er3+,NaYF4:Eu3+ microcrystals were synthesized by hydrothermal method.In terms of sample morphology,as the Mo5+ doping concentration increases,the hexagonal disk gradually grows into a hexagonal prism,which is related to the re-adsorption of surface citrate caused by Mo5+ doping.In terms of crystal structure,since the Mo5+ ion radius is smaller than Y3+,when the Mo5+ ion doping concentration is low,it is mainly doped in the substitution form,causing the host lattice to shrink;as the Mo5+ doping concentratiol is further increased,a part of Mos+ enters the interstitial sites,which causes the lattice to expand.Finally,in terms of fluorescence emission,enhanced fluorescence emission because of Mo5+ ion co-doping increases the asymmetry of the surrounding environment of the rare earth ions in the crystal lattice.And the red-green ratio of the output light increases due to the Na+ vacancies and the participation of Mo5+ in the energy transfer process.2.The parameters of Judd-Ofelt theoretical were used to quantitatively analyze the effects of different concentrations of Mo5+ co-doping on the local environment around Eu3+of NaYF4 microcrystal.The value of Ω2 increases with the increase of Mo5+ content.Higher Ω2 value indicates that Eu3+ in NaYF4 is located in the low symmetry site of the lattice without reversal center,higher asymmetry results in fluorescence emission.Ω4 value depends on the viscosity and dielectric constant of the host,causing a long-range effect.3.The temperature probe based on Eu3+ emission intensity ratio was constructed.The influence of temperature before and after Mo5+ doping on the layout of Eu3+ two Stark level ions was measured and analyzed,and the sensitivity of Eu3+ temperature probe was improved.The requirement for the formation of such a temperature probe were analyzed.