Synthesis,Properties And Energy Transfer Of Rare Earth Fluoride Multicolor Phosphors

Gd³⁺-based fluorides are the most effective multifunctional luminescent materials due to their low phonon energy,high ionization degree,high resistance coefficient,and they can realize multicolor tunable luminescence,so they have potential applications in anti-counterfeiting,biological labels,MRI,solar cells,laser technology and FED,LED full color display,etc.In this dissertation,taking BaGdF5 and Ba₂GdF₇ as host,Eu³⁺,Tb³⁺,Yb³⁺,Er³⁺,Dy³⁺,Ho³⁺,Tm³⁺ doped BaGdF5/Ba₂GdF₇ multifunctional nanophosphors with multicolor tunable luminescence and paramagnetism have been successfully synthesized.The main research contents are as follows.Eu³⁺/Tb³⁺ doped BaGdF5 nanophosphors（NPs）were synthesized by a simple one-step hydrothermal method.The results showed that the BaGdF5:Eu³⁺ and BaGdF5:Tb³⁺ nanophosphors emit orange-red and green light,respectively.In BaGdF5:Eu³⁺,Tb³⁺ nanophosphors,Gd³⁺ ions transfer the energy to the Eu³⁺ and Tb³⁺ ions,Tb³⁺ ions transfer the energy to the Eu³⁺ ions and the transfer mechanism is the quadrupole–quadrupole interaction.At the same time,Ba GdF5 and BaGdF5:Eu³⁺/Tb³⁺ nanophosphors all have paramagnetic properties.Yb³⁺,Er³⁺ and Eu³⁺ co-doped BaGdF5 nanophosphors were successfully prepared by hydrothermal method using trisodium citrate（Cit3-）as surfactant.The up-conversion emission intensities of Cit3--modified BaGdF5: Yb³⁺,Er³⁺/Eu³⁺ samples were enhanced obviously.Under the ultraviolet 274 nm excitation and 980 nm NIR radiation,BaGdF5:Yb³⁺,Er³⁺/Eu³⁺ nanophosphors emit orange light and green light,respectively,which realize dual-mode luminescence and tunable colors.Moreover,the paramagnetic properties of BaGdF5 and BaGd0.78F5:0.2Yb³⁺,0.02Er³⁺ nanophosphors were studied.Under 274 nm excitation,in Yb³⁺,Er³⁺ and Eu³⁺ co-doped BaGdF5 system,Gd³⁺ ions transfer the energy to the Eu³⁺ ions.However,under 980 nm laser excitation,in BaGdF5:Yb³⁺,Er³⁺/Eu³⁺nanophosphors,Yb³⁺ ions transfer the energy to the Eu³⁺ ions and Er³⁺ ions,respectively,and the mechanism of up-conversion emissions involves two-photon absorption.Ba₂GdF₇:Eu³⁺/Dy³⁺,Ba₂GdF₇:Yb³⁺/Ho³⁺ and Ba₂GdF₇:Yb³⁺/Tm³⁺ nanophosphors were successfully synthesized by hydrothermal method.The Ba₂GdF₇:Eu³⁺/Dy³⁺ phosphors can be efficiently excited by 274 nm and emit yellow,blue,bluish green and white light.The Gd³⁺ ions firstly adsorb the energy and then transfer the energy to the Eu³⁺ and Dy³⁺ ions,respectively.The Ba₂GdF₇:0.2Yb³⁺,0.02Ho³⁺ phosphors emit green light under 980 nm laser and the mechanism of up-conversion emissions is determined to be two-photon absorption.Under the excitation of ultraviolet 355 nm and near-infrared 980 nm,Yb³⁺ and Tm³⁺ ions co-doped Ba₂GdF₇ phosphors exhibit bright blue dual-mode emission.Yb³⁺ ions transfer the energy to the Tm³⁺ ions in the up-conversion luminescence of Ba₂GdF₇:Yb³⁺,Tm³⁺.The mechanism of the up-conversion emissions is determined to be three-photon absorption.Moreover,the up-conversion luminescence intensities of Ba₂GdF₇:Yb³⁺/Ho³⁺ and Ba₂GdF₇:Yb³⁺/Tm³⁺ nanophosphors are greatly enhanced after the introduction of sensitizer Yb³⁺ ions and CTAB surface modification.Meanwhile,the paramagnetic properties of Ba₂GdF₇,Ba₂GdF₇:Eu³⁺,Dy³⁺,Ba₂GdF₇:Yb³⁺,Ho³⁺ and Ba₂GdF₇:Yb³⁺,Tm³⁺ nanophosphors were studied.