Controllable Preparation And Upconversion Luminescence Properties Of NaErF₄ Core-Shell Materials

Different from traditional fluorescent markers（organic dyes and quantum dots）,rare earth doped upconversion luminescence materials have the advantages of near-infrared light excitation in the biological window region（650-1300 nm）,high optical stability,long luminescence life,low optical toxicity and narrow band emission,showing great application prospects in biological detection,photodynamic therapy,solar cells,sensors and other fields.Up to now,most upconversion luminescence materials adopt activator sensitizer co-doping system.However,due to the concentration quenching effect,the doping concentration of Er³⁺is usually limited to less than 3 mol%,resulting in the luminescence efficiency of upconversion materials cannot be further improved.This is also the key problem that restricts the application of upconversion luminescent materials.Based on the above problems,a series of upconversion luminescence materials based on NaErF₄were designed and prepared in this paper.By coating Si O₂and Na YF₄inert shells on the surface of NaErF₄,the effect of concentration quenching on upconversion luminescence intensity was effectively suppressed,and the upconversion enhancement mechanism was studied.The main research contents are as follows:（1）A series of NaErF₄upconversion materials with different morphologies and crystal types were prepared by simple hydrothermal method.The effects of hydrothermal reaction time and Er/F ratio on the morphology,size and crystal shape of NaErF₄were systematically studied.The results showed that when the molar ratio of Er:F was 1:4,pure cubic NaErF₄nanospheres with a diameter of about 100 nm were obtained.When the molar ratio of Er:F is 1:12,hexagonal NaErF₄hexagonal microsheets with side length and thichness of 1μm were obtained.At the same time,by comparing the luminescence properties of cubic phase and hexagonal phase NaErF₄crystal,it could be found that both of them showed characteristic emission peaks of Er³⁺under 980 nm excitation,and the upconversion luminescence of hexagonal NaErF₄crystal was stronger.（2）A series of NaErF₄@Si O₂core-shell upconversion micron crystals were synthesized using hexagonal NaErF₄crystals as cores and coated with Si O₂inert shells of different thickness by sol-gel method.The effect of Si O₂shell thickness on the upconversion luminescence intensity of NaErF₄@Si O₂core-shell samples was systematically studied.The results showed that the upconversion luminescence intensity of NaErF₄@Si O₂core-shell samples was significantly higher than that of NaErF₄bare core under 980 nm excitation,mainly because the coating of Si O₂shell can reduce the number of defect sites on the surface of NaErF₄particles and effectively inhibit the fluorescence quenching effect.Among them,when the amount of TEOS was 0.5 m L,the emission intensity of NaErF₄@Si O₂core-shell micron crystal is the strongest.（3）The NaErF₄@Na YF₄core-shell upconversion micrystals were successfully prepared using hexagonal NaErF₄as the core and coated with Na YF₄inert shell by hydrothermal method.The effect of Na YF₄shell thickness and Y/F ratio on the upconversion luminescence intensity of NaErF₄@Na YF₄micron crystal was systematically studied.The results showed that the upconversion luminescence intensity of NaErF₄@Na YF₄core-shell micron crystal was significantly better than that of NaErF₄bare core under 980 nm excitation,indicating the coating of Na YF₄shell on the surface of NaErF₄can effectively inhibited the fluorescence quenching effect.At the same time,by adjusting the ratio of Y/F in the shell,the luminescence color of NaErF₄@Na YF₄core-shell micron crystal transitioned from green to yellow,and the multicolor luminescence was realized.