Research On Multicolor Luminescence Properties Of Core-shell Structured Phosphors With Gd³⁺ Ions As Host Ingredient

Due to their following merits:low phonon energy(<400 cm^-1),low nonradiative scattering rate,high refractive index,good physical and chemical stability,low toxicity and excellent optical transparency,the rare earth?RE?ions doped fluoride phosphors have been looked as one of the most promising luminescent materials,and are widely used as up-?or down-?conversion luminescent materials.The fluoride phosphors exhibit distinct application in numerous fields,such as LEDs,biological labeling,temperature sensing,three-dimensional display,solar cells,and so on.Recently,a lot of research have been focused on their multicolor luminescent properties.Obviously,the traditional way to realize multicolor luminescence is via synthesizing multiple RE³⁺-doped phosphors,especially tri-doped systems,such as BaY₂Si₃O₁₀:Ce³⁺,Tb³⁺,Eu³⁺,etc.However,for these tri-doped fluorescence systems,there exist two fundamental dilemmas:1.cross-relaxation phenomena between/among different co-doped RE³⁺ions can lead to fluorescence quenching of activators;2.because of the narrow and characteristic absorption bands of different RE³⁺ions,a common energy donor is lacking for sensitizing a broad range of RE³⁺activators due to mismatched resonant frequencies.In another word,excitation of many different doped RE³⁺ions by a single-wavelength light is still an arduous task.Up to now,many works have reported that Gd³⁺ions can effectively serve as an active bridge of energy transfer from sensitizers to activators through long-range energy migration?EM?procedure,and they also have matched energy levels with many other RE³⁺ions.Hence,it is a desired choice to obtain preferable multicolor luminescence by constructing core-shell structured?CSS?phosphors with Gd³⁺ions as host ingredient of the core and shell regions,which is also the main topic of this paper.In this work,?-NaGd F₄ or orthorhombic GdF₃ was used as host matrix to construct CSS phosphors.The prepared CSS phosphors exhibit superior multi-color luminescence performance via EM procedure of Gd³⁺ions.For the phosphors,X-ray diffraction?XRD?,transmission electron microscope?TEM?,including high resolution TEM?HRTEM?,Energy dispersive X-ray spectrometer?EDS?were used to analyze and characterize crystal structures,micromorphology,grain size and chemical properties.Besides,fourier transform infrared?FTIR?spectroscopy,ultraviolet-visible?UV-Vis?absorption spectra and photoluminescence?PL?spectra were tested to characterize their optical properties.The main content is depicted as follows:1.The ways to enhance luminescence of phosphors were considered,including annealing treatment with high temperature?some samples were coated with SiO₂shell layers before annealing treatment?,forming solid-solution structure and coordinating particles of the phosphors with PMA ligands.The influence of different annealing temperature and the coated SiO₂ shell layers on the morphology,dispersibility,emission intensity and luminous color of the phosphors were investigated.The luminescence performance was improved by constructing solid-solution phosphor due to the modified crystal field environment.On coordinated with PMA ligands,excitation peak of the phosphor around 250nm was broadened,and its emission intensity was enhanced with the aid of“antenna”effect of the PMA ligands.2.The CSS phosphor GdF₃:Ce³⁺,Dy³⁺@GdF₃:Eu³⁺was prepared with the solvothermal method.For Ce³⁺and Eu³⁺ions were respectively doped into the core and shell regions,the prepared CSS phosphor exhibit excellent multicolor luminescence performance due to electron transfer quenching effect avoided.3.?-NaGdF₄:Eu³⁺@?-NaGdF₄:Ce³⁺,Dy³⁺CSS phosphor was synthesized by hydrothermal method,with the core regions pre-annealed at 600 ^oC.In this CSS phosphor,Eu³⁺and Dy³⁺ions selected as model activators were respectively doped into core and shell regions,which respectively emit red light and yellow and blue lights.Due to the separation of Ce³⁺and Eu³⁺ions into the core and shell regions,the phenomenon of electron transfer quenching between them can be effectively avoided.After annealing treatment of the core region at 600 ^oC,the shape and site of the formed robust O^2--Eu³⁺charge transfer band?CTB,around 253 nm?almost coincide with those of the 4f-5d transition of Ce³⁺ions.Hence,under excitation of 253 nm,the two transitions constitute the dual-excitation channels.As a result,the CSS phosphor exhibits excellent multicolor luminescence performance.4.The orthorhombic GdF₃:Eu³⁺@PMA@GdF₃:Ce³⁺,Dy³⁺CSS hybrid nanophosphor was also synthesized by hydrothermal method.The core particals of the phosphor was coated with SiO₂ shell layer beforehand and then annealed in air at 800^oC.After annealing treatment,the SiO₂ shell layer was etched off,and pyromellitic acid?PMA?was added onto surfaces of the particles to form interlayer of the CSS phosphor,followed by formation of the shell layer via epitaxial growth.For the CSS phosphor,the coated SiO₂ shell layer can effectively avoid aggregation and sintering during thermal annealing process.Because of the so-called antenna effect of its organic ligands,the inserted PMA can effectively overcome adverse effect resulting from interface defects between the annealed core and as-prepared?AP?shell layer regions.The finally-formed CSS nanophosphor exhibits significantly-enhanced multicolor luminescence via multi-channel excitation and EM procedure.All in all,the CSS phosphors prepared in this work exhibit superior multicolor luminescence because the adverse effect resulting from interface defects can be effectively suppressed via dual?or multi?-channel excitation and EM procedure of Gd³⁺ions,and have potential application value in many fields.It is well known that up-conversion?UC?CSS nanophosphors with property of significantly-enhanced multicolor luminescence have great potential application value with respect to bioimaging,and so on.Therefore,it has always been one of the key issues that need to be addressed to synthesize UC CSS nanophosphors with desired properties.Obviously,the experimental scheme stated above can be readily applied for synthesizing these UC CSS nanophosphors,which make the prepared scheme in this work exhibits potential usefulness in UC luminescence.