Researches On Synthesis And Upconversion Luminescence Properties Of Rare-Earth Doped Fluoride Nano/Micron Crystals

In recent years,rare earth（RE）doped fluoride upconversion luminescent（UCL）materials have been widely used in optoelectronic devices and biomedical imaging due to their stable luminescence performance and high color-purity.It was found that the multi-color of RE doped fluoride UCL materials is beneficial to promoting the renewal of products in the field of display lighting.However,the application of RE doped UC materials is restricted by low UCL efficiency,thus the enhancement of UCL intensity is the focus of this paper.In addition,in terms of biological imaging,near-infrared window has unique advantages over visible light.More and more researchers are committed to developing upconversion nanocrystals（UCNPs）with second near-infrared（NIR-II）response and emitting high-intensity first near-infrared（NIR-I）emission,so as to reduce the damage to biological tissues,avoid the interference of spontaneous fluorescence of biological tissues,and improve the performance of deep biological imaging.Given this,in this paper,RE-doped fluoride nano/micron crystals were studied from the following aspects:（1）In this study,a range of Yb³⁺/Tm³⁺/Ho³⁺triple-doped NaYF₄ microcrystals（MCs）were successfully synthesized by hydrothermal method.The morphology of NaYF₄:20%Yb³⁺,1%Ho³⁺MCs with different doping amounts of Na OH and citric acid were studied using scanning electron microscope,and the distribution of elements of this doping system was characterized by energy dispersive spectrometer.UCL spectra of the MCs using a fluorescence spectrometer with 980 nm excitation were researched,and the results demonstrated that the tunable multi-color of the MCs can be achieved by varying the doping amount of different RE ions.The UC mechanism of the MCs were systematically described.In addition,Li⁺doped the series MCs improved their UCL efficiency.By investigating the UCL the MCs of the Li⁺-doped system,the emission intensities of blue（450 nm,475 nm）,green（541 nm）,and red（648 nm）light of the series MCs were increased by 2.75,2.10,2.65,and 2.87 times,which is of great significance for its application in light-emitting diodes and solar cells.（2）Due to the Tm³⁺807 nm NIR-I（700-1000 nm）emission excited by the NIR-II（1000-1700 nm）has potential applications in biomedicine,thus it urgently needs to be further developed.In this work,a range of Na Er F₄:Yb@NaYF₄:Yb@NaYF₄:Yb,Tm@NaYF₄multilayer core-shell UCNPs were successfully synthesized by co-precipitation method.The strongest UC emissions can be obtained by changing the concentration of Yb³⁺in the core and the first shell,and the proposed UC process was discussed at length.The analysis shows that the high-intensity NIR-I emission（807 nm）from Tm³⁺and visible light from Er³⁺were achieved through the energy migration among Yb³⁺and the energy back transfer from Yb³⁺to Er³⁺under 1532 nm excitation.Besides,comparing to bilayer UCNPs,the multilayer core-shell UCNPs display superior optical performance.The high-intensity NIR-I emission at 807 nm(Tm³⁺:³H₄→³H₆)under 1532 nm NIR-II excitation demonstrates huge advantages in bioimaging.