Preparation And Luminescence Properties Of Rare Ions Activated Borate Phosphors

Rare-earth luminescence materials have always been a research hotspot in the field of luminescence.They have good color rendering properties,environmental protection and energy conservation,which have excellent performance that other luminescent materials have unmatched.Rare earth ions show an extremely rich energy level transition due to the structural characteristics of the 4f layer electron,which can absorb or emit light from ultraviolet to infrared region thereby forming diverse luminescent materials.In recent years,rare earth luminescent materials have been widely used in display imaging,ultraviolet disinfection,medical care,X-ray screens,white LED lighting and other fields,which effectively promote the replacement of the lighting display field.Therefore,researchers at home and abroad have been working hard to explore and develop new and efficient rare earth luminescent materials.Borate is excellent luminescent matrix materials due to its rich structures,good thermal stability,wide band gap,low calcining temperature,high optical damage threshold,and transparency in a wide spectral range.In this paper,rare earth ions(Eu³⁺,Eu²⁺,Ce³⁺,Tb³⁺,Dy³⁺)activated borate luminescent materials were prepared via solid-state method or hydrothermal method.X-ray powder diffraction,scanning electron microscopy,transmission electron microscopy,Fourier transform infrared spectroscopy,excitation and emission spectra,decay lifetime,quantum efficiency,and CIE chromaticity coordinates were used to characterize the structure,morphology,and luminescence properties of the samples.In addition,luminous mechanisms were discussed.The main works are as follows:In chapter 3,YK₃B₆O₁₂:RE(RE=Eu³⁺,Tb³⁺,Ce³⁺)phosphors were prepared by the solid-state synthesis.The single crystal-phases of the samples were confirmed by XRD measurements and Rietveld structural refinements.SEM,TEM,and EDS measurements were applied to analyze the morphologies and element composition of the phosphors.The photoluminescence excitation spectra,emission spectra,and decay curves were investigated.Under the near-UV light excitation,YK₃B₆O₁₂:Eu³⁺,YK₃B₆O₁₂:Tb³⁺,and YK₃B₆O₁₂:Ce³⁺phosphors exhibit the characteristic emission transitions from Eu³⁺?⁵D₀?⁷F₂ transitions,red color?,Tb³⁺?⁵D₄?⁷F₅ transitions,green color?,and Ce³⁺?5d?4f transitions,blue color?,respectively.The luminescence internal quantum efficiency,optimal doping level and thermal stability were measured.The result in this work could be useful for the further applications of this new host as a potential phosphor.In chapter 4,Ca₂B₂O₅:RE(RE=Eu³⁺,Tb³⁺,Dy³⁺)nanofibers were synthesized by the hydrothermal reaction method.The crystal structure characterization was conducted by XRD and Rietveld structural refinement.The surface properties of the nanofibers were investigated by the measurements such as SEM,TEM,and EDS.The results show that the prepared samples show fibrious shape with a diameter of about 100 nm and a length of 1-5?m.The luminescence properties such as photoluminescence excitation and emission spectra,decay lifetime,color coordinates,and the absolute internal quantum efficiency were reported.Ca₂B₂O₅:Eu³⁺nanofibers show the red luminescence with CIE coordinates of?x=0.41,y=0.51?and the lifetime of 0.63 ms.Ca₂B₂O₅:Tb³⁺nanofibers show the green luminescence with CIE coordinates of?x=0.29,y=0.53?and the lifetime of 2.13 ms.Ca₂B₂O₅:Dy³⁺nanofibers present a single phase white-color with the fluorescence decay of 3.05 ms.Upon near-UV excitation,the absolute quantum efficiency is measured to be 65,35,and 37%for Eu³⁺,Tb³⁺,Dy³⁺-doped Ca₂B₂O₅ nanofibers,respectively.It is suggested that the nanofibers could be an efficient phosphor for lighting and display.In chapter 5,Mg₃?BO₃?F₃:Eu²⁺phosphors were synthesized by solid-state reaction.The crystal structure and luminescent properties were investigated.The phosphor presents a broad emission band in blue range assigned to 4f⁶5d?4f⁷(⁸S_7/2)transitions of Eu²⁺.The sharp line emission from 4f⁷(⁶P_7/2)?4f⁷(⁸S_7/2)transitions were observed even at room temperature.In addition,4f⁷(⁶P_7/2)?4f⁷(⁸S_7/2)transitions present extremely strong intensity compared with 4f⁶5d?4f⁷(⁸S_7/2)transitions.The decay properties and the thermal activation energy??E?were reported.It shows high activation energy of 0.71 eV on temperature quenching.The maximum absolute quantum efficiency was measured to be 83%under UV light.The phosphor could deserve further investigations because of its peculiar spectral characteristics and excellent application performances.This paper systematically studied the preparation process,structural characteristics and luminescence properties of rare earth ions(Eu³⁺,Eu²⁺,Ce³⁺,Tb³⁺,Dy³⁺)activated borate luminescent materials.The luminescent properties of Eu³⁺,Tb³⁺,and Ce³⁺doped YK₃B₆O₁₂were firstly studied.This series of phosphors can emit high-purity red,green,and blue light under near-UV light,which can be used as three-color phosphors for white LEDs.The luminescent properties of Ca₂B₂O₅:RE(RE=Eu³⁺,Tb³⁺,Dy³⁺)were investigated for the first time.Ca₂B₂O₅:Dy³⁺could emit white light directly under near-UV excitation and is a new single-phase white light emitting material.Besides,the effective emission in blue light region of Mg₃?BO₃?F₃:Eu²⁺was conducted firstly.Its excellent luminescence properties are related to the structural characteristics of the matrix and high concentrations of F^-ions.The research content of this paper further promotes the development and application of phosphors for white LEDs.