CaSr Al2SiO7Eu2+可調藍綠到黃綠的LED熒光粉的潛在應用

1、Appl.Phys.B92,195198(2008)DOI:10.1007/s00340-008-3089-0AppliedPhysicsBLasersandOpticsq.zhangj.wangum.zhangq.suuTunablebluishgreentoyellowishgreenCa2(1x)Sr2xAl2SiO7:Eu2+phosphorsforpotentialLEDapplicationMOELaboratoryofBioinorganicandSyntheticChemistry,StateKeyLaboratoryofOptoelectronicMaterialsandTe

2、chnologies,SchoolofChemistryandChemicalEngineering,SunYat-senUniversity,Guangzhou,Guangdong510275,P.R.ChinaReceived:13March2008/Revisedversion:26May2008Publishedonline:28June2008©Springer-Verlag2008ABSTRACTAseriesofsolidsolutionswithageneralformulaofCa2(1solidx)Sr2xAlstate2SiOreaction.7:Eu2+wer

3、esynthesizedbyahightem-peratureThestructure,diffusereectionspectra,photoluminescencespectra,color-coordinateparame-tersandlifetimesofphosphorswereinvestigated.XRDresultsshowthatCa2Al2SiOThesesolidsolutionphosphors7istotallymisciblewithSrshowabroadexcitation2Alband2SiOof7.350450nmthatmatcheswellwitht

4、heoutputlightsofnear-UVLEDsandtunableemissionfrombluishgreentoyellowishgreen.Theseopticalpropertiesoriginatefromthe4f74f65dtransitionofEu2+ions.Thecrystaleldstrengthwasconsid-eredtobetailedbycontrollingthehostcomposition,whichleadstotheshiftofabsorptionbandandemissionband,andthevaryingofcolorcoordin

5、ates.PACS78.55.-m;42.70.-a;61.05.C-1IntroductionGaN-basedwhitelight-emittingdiodes(LEDs)aregainingworldwideinterestasapotentialcandidateforasourceofwhitelightforgeneralillumination.Thelumi-nescenceefciencyhasalreadyexceededthatoftraditionalincandescentlampsandnowiscompetitivewiththatofuor-escentlamp

6、s13.Onewaytoproducethewhitelightis(YtocombinetheblueGaN3chip+withyellowphosphor1aGda)3(Al1bGab)5O12:Ce(YAG:Ce3+).SuchwhiteLEDshavealreadybeencommercialized.However,thecolorqualityofthesewhiteLEDshasnotbeenfullysatisfactoryduetothelimitedwhitehuetunability,colorrenderingindex,andcorrelatedcolortemper

7、ature46.Secondly,ablueLEDchipiscombinedwithagreen(530nm)andared-emitting(>600nm)phosphorinsteadofasingleyellow-emittingphosphor7.ThesetwophosphorsabsorbthebluelightfromtheInGaNchipandconvertitintogreenandredlight,andthenthewhitelightisgeneratedbycolormixing.Inadditiontothesetwomethods,whitelightc

8、anbeproducedbymix-ingblue,green,andredphosphorswithaUV-LED8.Colormixingofblue,redandgreeninsteadofonlyblueandyel-lowprovidesimprovedcolorrenderingindex,chromaticitystabilityandawiderangeofcorrelatedcolortemperatures.uuFax:E-mail:+86-20-84111038,suqiangE-mail:ceswjOpticaltransitionsofdivalenteuropium

9、havebeeninves-tigatedinmanycompositions.Theyaregenerallyduetoelec-tricdipoletransitionsof4f5dwithparityallowedsothattheyoccurwithhightransitionprobabilities.Thedecaytimesofthesetransitionsaremicroseconds9,10.Theemissionvariesfromlongwavelengthultraviolettoreddependingonthehostlattice.Covalence,theca

10、tionsizeandcrystaleldstrengthinuencetheemissioncolorofEu2+10.Rare-earthdopedmelilitesA2B3O7(A=Ca,Sr;B=Al,Si)havebeen2+extensively3+investigatedoverthelastfewdecades.EuandErionsdopedwithCabroadabsorptionbandsaidinginthemaking2Alof2SiOlaser7havediodepumpedlasermaterials11,12.Long-lastingphosphores-cen

11、cewasreportedinEu2+,Eu2+/Dy3+,Ce3+,Ce3+melilitesAA=Ca,Sr;B=Al,Si)1317./Mn2+dopedRecently,2B3O7(Eu3+dopedwithCa2Al2SiO7wereinvestigatedforapotentialcomponentusedinsemiconductor-basedLEDs18.Inthispaper,theinuenceofcompositiononthestructureproperties,luminescentcharacteristicsandcolorvariationsofCa2(1x

12、)Sr2xAl2SiO7:Eu2+phosphorsaresys-tematicallyinvestigated.2ExperimentalPowdersampleswiththegeneralformulaCa2(1x)Sr2xAl2SiO7:Eu2+(x=0,0.3,0.5,0.7,1)werepre-paredbytheconventionalsolid-statereactiontechnique.ThestartingmaterialswereCaCO3AnalyticalReagent(A.R.),SrCO3(A.R.),-Al2O3(99.99%),SiO2(A.R.)andEu

13、2O3(99.99%).AproperamountofHofthe3BOraw3(A.R.)servedasux.Thestoichiometricamountmaterialswasthor-oughlymixedbygrindinginanagatemortar.Themixturewaspre-redat800Cfor2h,thensinteredat1300Cfor2hunderareductiveatmosphere(Nat21500Cfor4hunderareductive+H2),andnallysinteredatmosphere(N2+H2)followingtheinter

14、mediategrinding.Thephasepu-rityoftheas-preparedphosphorswasinvestigatedbyusingX-raypowderdiffractionspectroscopy(XRD)withaRigakuD/max2200vpcdiffractometerwithCuK30mA.TheXRDpatternswerecollectedradiationat40kVandThediffuse2intherange10reection70.spectraofthesesamplesweremeasuredonaCary5000UV-Vis-NIRs

15、pectrophotometerequippedwithdoubleout-of-planeLittrowmonochromator,usingBaSO4asastandardreferenceinthemeasurements.Thephotoluminescence(PL),andphotoluminescenceex-196AppliedPhysicsBLasersandOpticscitation(PLE)spectraofCa2(1x)Sr2xAl2SiO7:Eu2+weremeasuredbyaFluorolog3-21spectrouorometer(JobinYvonInc/s

16、pecx)equippedwitha450WXelampanddoubleexcitationmonochromators.Theabovemeasurementswerecarriedoutatroomtemperature.ThedecaycurvesofCaweremeasured2(1byx)Sra2FLS920-combinedxAl2SiO7:Eu2+atroomtemperatureuo-rescencelifetimesteadystatespectrometerequippedwitha100WpulsedXeashlamp(EdinburghInstruments).3Re

17、sultsanddiscussionBothCaunitcell(space2Algroup2SiO7andSr2Al2SiO7haveatetrag-onalP421m).Thecrystalstructuresoftheas-obtainedphosphorsCa2(1x)Sr2xAlusing2SiO7:0.01Eu2+(x=0,0.3,0.5,0.7,1)wereexaminedtheX-raydiffraction.TheXRDpatternsofCa2Al2SiO7:Eu2+,CaSrAl2SiO7:Eu2+andSr2Al2SiO7:Eu2+matchwellwithJCPDS3

18、5-0755,JCPDS26-0327andJCPDS38-1333,re-spectively.Noimpurityphasesareobserved,indicatingthatallsamplesshowasinglephase,andeuropiumionshavenoinuenceonthecrystalstructureoftheCa2Al2SiO7-Sr2Al2SiO7system.TheXRDpatterns,variationsinlat-ticeconstantsandcellvolumeasafunctionofcomposi-tionalratioxinCa2(1x)S

19、rFig.1.Astheamount2xAlof2SiO7:0.01Eu2+areshowninSr2+ionsincreasesinCa2(1x)Sr2xAl2SiO7:0.01Eu2+,itisobviousthatthelatticeconstantaandcandcellvolumebecomelargerandlarger,andthatthepeakpositionofdiffractionpatternsshiftstowardlower2value,duetothesubstitutionofthesmallerCa2+ionbylargerSr2+ion.Thelinearr

20、elationbetweentheXRDpeakpositionandthecompositionindicatesthatCaistotallymisciblewith2Al2SiO7:Eu2+Sr2Al2SiO7:Eu2+fromtheviewpointFIGURE1XRDpatterns(a)andlatticeconstants(b)ofCa2(1x)Sr2x-Al2SiO7:Eu2+(x=0,0.3,0.5,0.7and1)phosphorsofcrystallography19,20.ItcanalsobeseenthatVegardslawholds.The+diffuseree

21、ctionspectraofCa2(1x)Sr2xAl2SiO7:yEu2(x=0,0.5,1;y=0,0.01)weremeasuredasshowninFig.2.TheabsorptionspectraF(R)wereobtainedfromthereectionspectrabyusingtheKubelkaMunkfunction21:F(R)=(1R)2/2R=K/S,(1)whereR,KandSarethereectivity,theabsorptioncoef-cientandthescatteringcoefcient,respectively.Forsamplesdope

22、dwithEu2+,therearetwoobviousabsorptionbandsinthewavelengthrangeof250300and350450nm,duetothe4f5dtransitionofEu2+ions.ForCa2Al2SiO7,CaSrAland2SiOshow7Sr2Al2SiO7hosts,thediffusereectionspectraaplatformofhighreectioninthewavelengthrangeof250800nmandthenstarttodecreasedramaticallyfrom250to200nm,attribute

23、dtothehostabsorptionband.ThehostabsorptionofCa2Al2SiO7,CaSrAl2SiO7andSr2Al2SiO7de-rivedfromFig.2areevaluatedtobearound5.74,5.73and5.72eV,respectively.Figure3showsthephotoluminescenceexcitationspec-traoftheCa2Al2SiO7:0.01Eu2+,CaSrAl2SiO7:0.01Eu2+andSr2Al2SiO7:0.01Eu2+phosphors.Theexcitationspectracon

24、-sistoftwobroadbandsforallthesamples.Therstoneislocatedat250300nm,theotherissituatedintherangeof350450nm.TheyarebothcausedbythetransitionsofEu2+fromthe4fgroundstatetothe5dexcitationstate.Moreover,theabsorptionbandofEu2+ionsinlongerwavelengthhasagradualblueshiftwiththeincreaseofSr2+concentration.Such

25、ablueshiftisrelatedtothecrystaleldstrength.Thecrystaleldstrengthcanberoughlydeterminedby:Dq1/R5,(2)whereDqisthecrystaleldstrength,andRisthebondlengthofMObetweenacentermetalionandligandions22.AsdiscussedintheXRDsection,Ca2Al2SiO7istotallymis-ciblewithSr2Al2SiO7.TheionicradiusofCa2+andSr2+ionscoordina

26、tedwitheightligandatoms(CN=8q)areFIGURE2Thediffusereectionspectra(a)andtheabsorptionspectra(b)ofundopedandEu2+-dopedCa2Al2SiO7,CaSrAl2SiO7andSr2Al2SiO7ZHANGetal.TunablebluishgreentoyellowishgreenCa2(1x)Sr2xAl2SiO7:Eu2+phosphorsforpotentialLEDapplication197FIGURE3TheexcitationspectraofCa2(1x)Sr2xAl2S

27、iO7:Eu2+withdif-ferentx(x=0,0.5and1):(a)493nmem=536nm,(b)em=507nm,(c)em=1.12and1.26Å,respectively23.Thus,+thebondlengthofMOincreasesastheamount2+ofSr2ionsincreasesinCa2(1x)Sr2xAl2SiO7:0.01Eu,whichissupportedbyXRDpatterns.ThelongeraveragedistanceofSrOthanthatofCaOmakesEu2+ionsexperienceaweakercr

28、ystaleld.ItisthereforereasonablethatthelowestexcitedlevelofEu2+ionsshiftstowardhigherenergy.Becauseofthebroadbandabsorp-tionintherangeof350450nm,Ca2(1x)Sr2xAl2SiO7:Eu2+canperfectlymatchwiththeradiativelightfromUV-LEDs(370410nm).Usually,theemissionwavelengthofphosphorscanbetunedbycompositiontailoring

29、.Inthisinvestigation,aregularblueshiftisobservedwhenCa2+ispartiallyre-placedbySr2+inCa2Al2SiO7.Figure4showsthenor-malizedemissionspectraofCaunder2(1400x)Sr2nmxAl2SiOexcitation.7:0.01Eu2+(x=0,0.3,0.5,0.7and1)TheemissionspectraofCa2(1x)Sr2xAl2SiOcharacteristic7:0.01Eu2+exhibitawellknownEu2+emission.Th

30、eemissionbandisduetothe4f65d4f7transitionofEu2+ions.ItisclearinFig.4thattheemissionpeakfrom537nm(x=0)shiftsto493nm(x=1)with+increasingx.TheStokesshiftofCa2Al2SiO7:0.01Eu2,CaSrAl2SiO7:0.01Eu2+andSr2Al2SiO7:0.011Eu2+arecalculatedtobeabout4500,3710and3700cm,respectively.Therefore,bothweakercrystaleldst

31、rengthandsmallerStokesshiftcontributetotheblueshiftoftheemissionastheincreasingamountofSrionsinCa2(1x)Sr2+CIE2xAlchromaticity2SiO7:0.01Eu2+.ThediagramofCa2(1x)Sr2xAl2SiO7:0.01Eu(x=0,0.3,0.5,0.7and1)under400nmexcita-tionisshowninFig.5.ItcanbeseenthatthecolorofCatobluish2(1x)Srgreen2xAl2SiOwith7:0.01E

32、u2+variesfromyellowishgreenincreasingx.Inordertoavoidthesuperimpositionofimages,itiswellknownthatthelifetimeofphosphorsappliedindis-playsdevicesshould6besuitable10.Fluorescencede-caycurvesforthe4f5d4f7transitionofEu2+ionsinCa2(1x)Sr2xAl2SiO7:0.01Eu2+(x=0,0.3,0.5,0.7and1)weremeasured.Allthedecaycurve

33、scanbewellttedbyFIGURE4Theemissionspectra(ex0.3,0.5,0.7and1).Theinsetshowsthe=dependence400nm)withofpeakdifferentwavelengthx(x=on0,thecompositionparameterx(x=0,0.3,0.5,0.7and1)FIGURE+2+5CIEchromaticityofEu2iondopedwithCa2(1x)Sr2xAl2SiO7:Eu(x=0,0.3,0.5,0.7and1)(µs)Ca2(1x)Sr2xAl2SiO7:0.01Eu2+x=0x

34、=0.3x=0.5x=0.7x=1(µs)0.500.540.560.640.66TABLE1ThedecaytimesofCa2(1x)Sr2xAl2SiO7:Eu2+(x=0,0.3,0.5,0.7and1)phosphorsasingle-exponentialequation:I(t)=I0exp(t/),wheretisthetimeandIandI0aretheluminescenceintensityattandinitialtime,respectively.isthedecaytimeforthesamples.ThelifetimesarelistedinTabl

35、e1.Itcanbeseenthattheygraduallyincreasefrom0.50µsinCa+analogueto0.66µsinSranaloguewiththeincreasingSr2content,whichmaybeduetothedecreaseinnon-radiativetransitionratescausedbyphononcoupling22.Theseresultsarereasonablefortheallowedtransition4f65d4f7ofEu2+andthelifetimesareshortenoughfordispl

36、ays24.198AppliedPhysicsBLasersandOptics4ConclusionsAseriesofsolidsolutionswithageneralformulaofCa2(1x)Sr2xAl2SiO7:Eu2+weresynthesizedbyhightem-peraturesolidstatereaction.Withtheincreaseofx,theemissionbandsareblueshiftedduetothedecreaseincrys-taleldsplitting.Becauseofthebroadbandabsorptioninthe350450

37、nmrange,perfectlymatchingwiththeemissionwavelengthofUV-LEDs(350410nm),itisapotentialpho-toluminescentmaterialfornear-UV-LEDs.ACKNOWLEDGEMENTSThisworkwassupportedbytheNa-tionalNatureScienceFoundationofChina(20501023),theNational“863”ProjectFoundationofChina(2004AA001340),theNatureScienceFounda-tionof

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