Study On Preparation And Luminescence Mechanism Of Eu And Sr Co-doped MCaO·nAl₂O₃ Phosphors

Rare earth doped phosphors have a broad future in lighting and display applications,such as light-emitting diodes?LEDs?and Field emission displays?FEDs?.Preparing,designing and improving the luminescence effiency of new materials is becoming a research hotspot.LEDsoccupies an important place in the solid lighting market due to itspractical advantages,such as high brightness,high efficiency,low energy,long life and small size.Field emission displays?FEDs?have been considered as one of the most promising technologies in flat panel displays because of many advantages,such asthin panels,self-emission,distortion-free image,wide viewing angle,low weight,quick response and low power consumption.In general,Eu²⁺ and Eu³⁺are selected as luminescence centers of the phosphors for LEDs and FEDs.It is because that Eu²⁺exhibits many advantages,such ashigh effiency,high color purity and tuning emissions in different crystals and Eu³⁺hasthe stable and high-efficiency red emissions due to forced electric dipole transition.Especially,the strategy of mixed valence europium activated phosphor under a single wavelength excitation,has attracted more attention dueto overcoming many drawbacks of conventional means of white light generation for LEDs,such as low device efficiency and colour rendering index.Onthe other hand,to further improve the luminescence effiecny of phosphors,many efforts by modifying the microstructure of materials with special structure have been a research hotpot.Especially,many researchs have been concentrated on the optical properties of lanthanide doped mCaO·nAl₂O₃ aluminate with good stability and envirioment friendly.12CaO·7Al₂O₃(C₁₂A₇)is a unique material that possesses nanocaged structure,Ca²⁺ of nanocaged framework can be substituted by Eu³⁺and Eu²⁺,the encaged O2-ions can be replaced by other anions,such as OH-?H-?e-.Deformation of the cages can be induced through different encaged anions in C₁₂A₇ and the deformation can induce the symmetry of Eu³⁺and Eu²⁺ in nanocaged frameworks,and thus the radiative transition rates modified.In the meantime,OH-could accelerate the nonradiative process of the emission centres around it because of its high frequency vibration.Thus the luminescence of Eu³⁺ and Eu²⁺ can be adjusted by tuning the radiative and nonradiative transition rates through the sorts and amounts of encged anions in C₁₂A₇.In addition,CaO·Al₂O₃?CaAl₂O₄?is an oxide with good stability and envirioment friendly,which can overcome the toxic action of sulfide-based compounds due to its instability.Moreover,CaAl₂O₄ can be a good host because CaAl₂O₄ belongs to monoclinic phase and its symmetry is low.In this work,we have studiedthe luminescence properties of C₁₂A₇:Eu²⁺,Eu³⁺ and CaAl₂O₄:Eu²⁺ through annealing in differernt atomosphere?H₂,air,CO?and cation/anions substitution.Then we have explored their applications in the LEDs and FEDs.Single phase C₁₂A₇: Eu³⁺ red phosphorswere prepared by a self-propagating combustion.Both the Eu²⁺ blue and Eu³⁺ red emissions are simultaneously generated,under a single wavelength excitation at 255 nm through annealing in H₂ atmosphere.Varying the annealing duration,we have modulated radiative and nonradiative channels of two emission centres based on the control of the encaged anions in C₁₂A₇.Under asingle wavelength excitation,the emissions of mixed valence euripum actived phosphorshave been tuned by modulating radiative and nonradiative channels instead of the substitution of cations.The CIE chromaticity coordinates are tuned from?0.630,0.352?to?0.201,0.086?.Europium doped Ca_11.94-xSr_xAl₁₄O₃₃ phosphors were prepared by a self-propagating combustion method followed by thermal annealing in H₂ atmosphere.Based on the discussion of the absorption spectra,the photoluminescence spectra and FTIR transmittance spectraof the phosphors,Eu³⁺ doing supressed the generation of encaged H-and OH-,but Sr²⁺ dopingenhanced the reduction of Eu³⁺ to Eu²⁺ in Ca_11.94-xSr_xAl14O₃₃:Eu_0.06 phosphors.The obtained higher thermal activation energy of the Sr doped sample can be attributed to the more rigid framework structure after Sr doping.It can be found that the enhanced effect of light-induced electrons by Sr doping was hidden by the addition of Eu³⁺.Finally,the emission colors were tuned from red to purple upon 254 nm excitation and from pink to blue under electron beam excitation through tuning radiative transition rates,nonradiative transition rates and the ratio of luminescence centers in Ca_11.94-xSr_xAl14O₃₃:Eu_0.06 after Sr²⁺ doping.Eu and Sr co-doped Ca_11.94-xSr_xAl₁₄O₃₃:Euphosphors were prepared by combustion in combination with the subsequent annealing in carbon embedding and CO atmosphere.We found that Sr²⁺ doping induced more electrons to sustain the stability of phase in CO atmosphere,which is different from that of enhancing the reduction of Eu³⁺ to Eu²⁺ in H₂ atomosphere.We did not accurately study how the radiative transiton rates and the ratio of Eu²⁺/Eu³⁺ influencethe luminescence propertiesbecausenew phase produced due to the instability of Ca_11.94-xSr_xAl₁₄O₃₃:Euannealed in CO atmosphere.Instead of that,we found CaO·Al₂O₃:Eu²⁺ may have high efficient luminescence performance.Pure phaseCa_0.995Al₂O₄:Eu_0.005 phosphors were prepared by a combustion in combination with the subsequent annealing in CO atmosphere.We found that the photoluminescence of the annealed sample at highertemperature showed stronger intensity.Thiscan be explained by the decreased nonradiative transition rates of the annealed sample due to less surface defects induced by high volume-to-surface-area ratio after the annealing.Meanwhile,the cathodoluminescence of the Ca_0.995Al₂O₄:Eu_0.005 phosphor annealed at lower tempreatureexhibited stronger intensity.This is becausethat the electron beam with a constantan excite more luminescent centers of the sample with smaller particle sizes and more uniform size distribution.To understand how the different hosts influence the reaction of Eu³⁺ to Eu²⁺ and the emission of the luminescent center,single-phased Ca_0.98-ySr_yEu_0.02Al₂O₄ blue phosphor has been prepared by a combustion method in combination with the subsequent annealing in H₂ atmosphere.The comparative study on the optical properties of Ca_0.98-ySr_yEu_0.02Al₂O₄ and Ca_11.94-xSr_xAl₁₄O₃₃:Eu phosphors suggests that the reduction of Eu³⁺ to Eu²⁺ was enhanced.Meanwhile,the cathodoluminescence intensity increases after Sr²⁺ doping and the highest of the Sr doping concentration is 10%.In contrast,the cathodoluminescence intensity of Ca_0.98-ySr_yEu_0.02Al₂O₄ is higher than the commercial phosphor BAM:Eu²⁺,which is because low symmetry,deeper penetrate distance,smaller and uniform sizes and lower nonradiative transition rates of CaAl₂O₄.Under continuous low voltage electron-beam bombardment,Ca_0.98-ySr_yEu_0.02Al₂O₄ have displayed excellent degradation resistance.Thus,Ca_0.98-ySr_yEu_0.02Al₂O₄ phoshormay have potential applications in FEDs.