Synthesis Of Rare Earth Doped Upconversion Luminescent Materials And Application Of Anticounterfeiting Technology

Rare earth luminescent materials have been the research hotspot in the field of fluorescence anti-counterfeit thanks to their good up-conversion luminescence properties.In this thesis,NaGdF₄:Yb,Er,Nd@NaGdF₄@Na YF₄:Yb,Tm and NaGdF₄:Yb³⁺/Eu³⁺/Li⁺were synthesized by hydrothermal method and solvothermal method respectively.The X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,fluorescence lifetime analysis,fluorescence spectrum analysis and other methods were selected to test and characterize the samples.The crystal size,composition,micro-area morphology and luminescence properties were analyzed.Select the crystal with the best luminous intensity and effect to make anti-counterfeiting ink,which has been successfully applied in the anti-counterfeiting security field.The primary coverage is as follows:（1）NaGdF₄:Yb³⁺/Eu³⁺crystals with different Eu³⁺doping concentrations were synthesized by hydrothermal method.The experimental results showed that the XRD diffraction patterns of NaGdF₄:Yb³⁺/Eu³⁺were consistent with the diffraction peaks of the standard hexagonal phase NaGdF₄,and the lattice parameters of the sample were similar to those of the standard hexagonal phase NaGdF₄.Under 980 nm laser excitation,the crystal mainly has six emission bands at 400-700 nm,which are 407 nm,476 nm,540 nm,583 nm,615 nm and 653 nm respectively.The green emission at 540 nm is the strongest,followed by the red emission peak at 654 nm and the blue emission peak at 476 nm.The main emission color of the sample is green.（2）NaGdF₄:Yb³⁺/Eu³⁺/Li⁺crystals with different Li⁺doping concentrations were synthesized by hydrothermal method.XRD and SEM results show that the co-doped Yb³⁺/Eu³⁺/Li⁺had little influence on the crystal size,morphology and phase structure,and high purity hexagonal phase NaGdF₄ had been synthesized.From the up-conversion emission spectrum,it can be seen that the green emission intensity of 15 mol%Li⁺doped NaGdF₄:Yb³⁺/Eu³⁺crystal is 6 times that of the undoped Li⁺sample.The fluorescence lifetime of NaGdF₄:Yb/Eu/Li was measured,and the fluorescence lifetime of ⁵D₁ level was1.4 times that of undoped.Finally,NaGdF₄:0.2Yb/0.02Eu/0.15Li with uniform morphology and strong luminous intensity was made into anti-counterfeiting ink,and fluorescent anti-counterfeiting patterns were printed on paper,glass,plastic and other materials through screen printing technology.Pumped by 980 nm laser,the anti-counterfeit pattern emits bright green light.The anti-counterfeit patterns have high resolution,are easy to recognize,and are less affected by the environment.The NaGdF₄:0.2Yb/0.02Eu/0.15Li has great application potential in anti-counterfeiting security.（3）NaGdF₄:Yb,Er,Nd@NaGdF₄@Na YF₄:Yb,Tm upconversion nanoparticles with different Tm³⁺doping concentration and different shell thickness were synthesized by solvothermal method.The XRD pattern of the sample showed that the hexagonal phase NaGdF₄ with high crystallinity and high purity was synthesized.The TEM image shows that the sample has uniform distribution,good monodispersity,clear lattice stripes,and the particle size increases significantly after coating the shell.Under the excitation of 980 nm,the luminous intensity of NaGdF₄:Yb,Er,Nd@NaGdF₄ increases with the core-shell ratio increased.Samples doped with different concentrations of Tm³⁺were prepared.With the increase of Tm³⁺concentration from 0.25 mol%to 0.5 mol%,the fluorescence intensity of the samples gradually increased,reaching the maximum at 0.5 mol%.When the concentration of Tm³⁺is greater than 0.5 mol%,the luminous intensity begins to decrease.The sample can be excited at 980 nm and 808 nm.The sample fluoresces blue when excited at 980 nm.Under 808 nm excitation,the main luminous color of the sample is red.The influence of 980 nm laser with different power on the upconversion luminescence properties of samples was inspected,and the up-conversion luminescence process of Yb/Tm co-doped is a three-photon process.By mixing nano-particle powder and metal ink to make luminous ink,and printing the word"XUPT",we can see that the anti-counterfeiting pattern is clear and can be reduced,and the luminous color is different under 980nm and 808nm laser,which realizes dual-mode anti-counterfeiting and improves the anti-counterfeiting security level.