Photoluminescence Tuning For 4f-5d Transition Of Divalent Europium Rare-earth-ion

Rare-earth luminescent materials play important roles in solid-state lighting and display applications.With the increasing demands of light qualities and diverse needs of people,the design and development of rare-earth luminescent materials become very important in basic and applied researches.Among which,the development of high performance rare-earth luminescent materials such as high efficient luminescent materials,narrow-band emitting materials and full-spectral white-light emitting materials remain big challenges.In this dissertation,we have focused on Eu²⁺ion with typical 4f-5d transition.Under the guidelines of crystal structure-optical properties relationships,we have carefully selected and designed crystal structures,thus several novel Eu²⁺activated luminescent materials including narrow-band blue emitting MBe₂(PO₄)₂:Eu²⁺(M=Ca,Sr),ultra-broadband white light emitting Lu₂Sr Al₄SiO₁₂:Eu²⁺,red emitting Sr₃Ln Al₂O_7.5:Eu²⁺(Ln=Y,Lu)phosphors and Mg₂Al₄Si₅O₁₈:Eu²⁺glass ceramic with admirable luminescent performances were obtained.Their potential applications for solid-state lighting and displays are also investigated in detail.The main achievements are shown as follows:(1)Narrow-band blue emitting MBe₂(PO₄)₂:Eu²⁺(M=Ca,Sr)phosphors with high efficiency and high thermal stable emission properties were designed,much attentions have been paid to uncover the behind principles responsible for achieving narrow-band emission.It reveals the rigid and condense connected[Be O₄]and[PO₄]tetrahedral network is the decisive factor for achieving narrow-band emission,due to it can greatly suppress structural relaxation.Benefiting from the fascinating narrow-band emission(FWHM=26-27 nm),high quantum efficiency of 63.4%,and high color purity of 99.1%,MBe₂(PO₄)₂:Eu²⁺(M=Ca,Sr)could apply in wide color gamut display.(2)Super-broad band emission in Lu₂Sr Al₄SiO₁₂:Eu²⁺phosphor with FWHM=245 nm(7260 cm^-1)at room temperature and FWHM=290 nm(8000 cm^-1)at 11 K was synthesized by high-temperature solid-state reaction method.Transient-and steady-state spectroscopy reveal that the broad emission consists of three strongly overlapped emission bands with centered emision peaks at 470 nm,520 nm and 7000 nm.It reveals that 470 nm and 520emission are due to 5d?4f transition of Eu²⁺that locates at Lu³⁺and Sr²⁺crystallographic site,respectively,while emission band at 700 nm with broad FWHM=210 nm is originated from Eu²⁺-Eu³⁺IVCT emission.Due to super-broad emission characteristics,a full-spectrum W-LED device with high color rendering index(R_a=85.7)and low correlated color temperature(CCT=3373 K)was obtained by only using Lu₂Sr Al₄SiO₁₂:Eu²⁺as color convertor,implying the phosphor holds great potential for full-spectral LED lighting.(3)Blue light excitable red-emitting Sr₃Ln Al₂O_7.5:Eu²⁺(Ln=Y,Lu)phosphor were designed and prepared.Eu²⁺site occupancies were systematically investigated,results show at least three different crystallographic sites are occupied by Eu²⁺,and the red emission originates from Eu²⁺ions locate at the octahedrally coordinated Ln³⁺site.By using Eu³⁺as structural probe,we further identified a single crystallographic site with site symmetry of C₁or C₂is occupied by Eu³⁺,and it may selectively doped at Sr2²⁺site and serves as a charge compensator for Eu²⁺?Ln³⁺substituent.The WLED fabricated by Sr₃Ln Al₂O_7.5:Eu²⁺shows a low CCT and High CRI,implying that Sr₃Ln Al₂O_7.5:Eu²⁺could apply in warm white LED lighting.(4)Mg₂Al₄Si₅O₁₈:Eu²⁺glass ceramic phosphor with highly efficient and thermal stable red emission was designed with the help of MgO-Al₂O₃-SiO₂phase diagram.DFT calculation and EXAFS confirm that Eu²⁺insets at the vacant channel of Mg₂Al₄Si₅O₁₈crystal with six-fold coordinated oxygen,which could responsible for the fascinating red emission at 620nm.The as-prepared glass ceramic shows many desired properties,such as high internal/external quantum efficiencies of 94.5%/70.6%,and emission intensity at 423 K can remain 78%of that at 77 K,excellent physicochemical stability and low cost.Moreover,the laser driven red-emitting device fabricated by coupling 445 nm blue laser diode(LD)with Mg₂Al₄Si₅O₁₈:Eu²⁺shows a high luminous flux and luminous efficiency of 274 lm and 54 lm W^-1,respectively.All which suggests that the Mg₂Al₄Si₅O₁₈:Eu²⁺glass ceramic phosphor is hopeful applied in high-power LD driven warm lighting or LD driven displays.