Preparation And Spectroscopic Characterization Of Self-activated Vanadate And Tungstate Phosphors

A novel self-activated green emitting phosphor of Na₂YMg₂（VO₄）₃ was prepared by sol-gel method.The obtained samples were pure crystalline phases with garnet structure confirmed by Rietveld refinement of X-ray power diffraction pattern.It can be seen that particles with a diameter ranging from 0.3 to 1 μm show irregular shapes and have a tendency to be agglomerating.The band-gap energy of Na₂YMg₂（VO₄）₃ is estimated to be about 2.95 e V by UV-visible absorption spectra.The host can be efficiently excited by NUV light in 200⁴00 nm range and it exhibits a broad-band emission in the regions from 400 to 700 nm with the highest emission intensity at 521 nm.This broad band can be ascribed to the charge transfer of V-O within VO3-4groups.The experimental results indicated that the phosphor is a promising green phosphor applied in white LEDs.We also studied the crystal structure and photoluminescence properties of Na2Y1-x Mg2（VO4）3:x Eu³⁺ phosphors using sol-gel method.The formation of Na₂YMg₂（VO₄）₃:Eu³⁺ single phase without changing the lattice structure has been confirmed by XRD,indicating Eu³⁺ occupy Y³⁺ sites.Upon NUV light excitation at 340 nm,the strongest red emission line（5D0→7F2）of Eu³⁺ originating from the non-inversion symmetry sites in Na₂YMg₂（VO₄）₃ doped with Eu³⁺ was observed.A broad excitation band ranging from 200 to 400 nm owing to the charge transfer from VO3-4 groups was recorded when the 5D0→7F2 emission from Eu³⁺ was monitored,which suggest an efficient energy transfer from VO3-4 to Eu³⁺.Moreover,intensities and the decay curves of VO3-4 emission under 340 nm excitation depending on the doping concentration of Eu³⁺ were studied to investigate the VO3-4 to Eu³⁺ energy transfer process.The color-tunable emission from green to reddish-orange as a function of Eu³⁺ concentration was realized,which suggested potential application in UV-based white LEDs.Eu³⁺ doped Ca₈La₂（VO₄）₆O₂ red phosphors were prepared by sol-gel method.Ca₈La₂（VO₄）₆O₂:Eu³⁺ single phase was formatted indicating Eu³⁺ occupy La³⁺ sites without changing the lattice structure.Upon 355 nm laser excitation,red emission（5D0→7FJ（J=0,1,2,3,4））of Eu³⁺ was observed in Ca₈La₂（VO₄）₆O₂:Eu³⁺.Intensities and decay curves of Eu³⁺ emission under 305 nm depending on the doping concentration of Eu³⁺ was studied.Interestingly,emission intensity from 5D0→7F0 transition was stronger than 5D0→7F2 transition when the concentration of Eu³⁺ increased to 40% under 305 nm excitation.The CIE chromaticity coordinate（0.641,0.359）of Ca₈La₂（VO₄）₆O₂:60%Eu³⁺ is close to National Television Standard Committee standard value（0.670,0.330）of red phosphors,which indicates a potential application as an excellent red emitting phosphor.Yb³⁺ and Na⁺ codoped Sr₂CaWO₆ phosphors with different doping concentration were successfully synthesized via a solid-state reaction method.Upon ultraviolet（UV）light excitation,an intense near-infrared emission of Yb³⁺（2F5/2→2F7/2）around 1007 nm was observed in Sr₂CaWO₆:Yb³⁺,Na⁺ phosphors.A broad absorption band ranging from 250 to 350 nm was recorded when the Yb³⁺ emission around 1007 nm was monitored,indicating an efficient energy transfer from host to Yb³⁺.Intensities of host emission and Yb³⁺ emission depending on Yb³⁺ concentration were investigated.Decay curves of host emission and Yb³⁺ emission were measured upon the excitation of a 266 nm pulsed laser.Decay time of host emission at 1007 nm was remarkably reduced as an introducing of Yb³⁺,revealing an efficient energy transfer between host and Yb³⁺.Cooperative energy transfer process（CCT）is responsible for converting each UV photo into two NIR photos.The calculated quantum efficiency can reach an estimated maximum value of 190%,which enable a promising application of high efficiency silicon-based solar cells.