YVO4 Bi3+,Eu3+一种包含红色光谱成分适于紫外LED的特殊黄色荧光材料

CombinatorialApproachtotheDevelopmentofaSingleMassYVO4:Bi3+,Eu3+PhosphorwithRedandGreenDualColorsforHigh

ColorRenderingWhiteLight-EmittingDiodes

LeiChen,†,‡Kuo-JuChen,§Chun-CheLin,†Cheng-IChu,†Shu-FenHu,|Min-HungLee,§andRu-ShiLiu*,†DepartmentofChemistry,NationalTaiwanUniVersity,Taipei106,Taiwan,SchoolofMaterialsScienceandEngineering,HefeiUniVersityofTechnology,Hefei230009,China,InstituteofElectro-optical

ScienceandTechnologyandDepartmentofPhysics,NationalTaiwanNormalUniVersity,

Taipei116,Taiwan

ReceiVedApril14,2010

Insteadofdevelopinganovelredphosphorindividually,thisworkproposestheproductionofwhitelightbycombininganear-ultraviolet/ultravioletdiodechipwithblueandspecialyellowphosphors:theyellowphosphorincludestheredandgreencomponentswithhighcolorsaturation.Theavailabilityofthisschemeisdemonstratedbypreparingawhitelight-emittingdiode(WLED)withcolorrenderingindex(Ra)upto90.3.Thedesiredsingle-massyellowphosphorissuccessfullyscreenedoutfromtheYVO4:Bi3+,Eu3+systembyusingacombinatorialchemistryapproach.Whentheemissioncolorandluminousefficiencyarebothconsidered,thebestcompositionforproducingwhitelightis(Y1-s-tBisEut)VO4with0.040ese0.050and0Introduction

Whitelight-emittingdiodes(WLED)haveattractedcon-siderableattentionowingtotheirlonglifetime,lowenergyconsumption,low-voltagepowerrequirement,environmentalfriendliness,andhighreliability.Theyhavegreatpotentialtoreplaceconventionalincandescentandfluorescentlamps.1-4TheprogressmadeinthefieldofLEDsissurprising.Forexample,theluminescenceefficiencyhasbeenimprovedfromnearly60lm/Win2005to150lm/Win20095(Nichia6reported249lm/WforawhiteLEDat20mAexperimen-tally).Theirrangeofapplicationshasbeenextendedfromvoltagesignalindicators,initially,toliquidcrystaldisplay(LCD)panelsbacklightformobilephonesandTVsets,automobilelights,trafficlights,streetlighting,andoutdoordecoration,amongothers,butobstaclestotheiruseareconsiderable.MostcommerciallyavailableWLEDsareproducedbycombiningabluediodechipwithY3Al5O12:Ce3+(YAG:Ce3+)phosphor,whichexhibitsyellowlumi-nescence.ThemaindisadvantagesofYAG-basedWLEDsaretheirpoorcolorrenderingandtheseverethermalquenchingoftheluminescence.Thecolorrenderingindex(Ra)oftheYAG-basedLEDisabout80,whichsufficesforgeneralillumination.4However,itisnotsuitableforcertain

*Towhomcorrespondenceshouldbeaddressed.E-mail:rsliu@ntu.edu.tw.†NationalTaiwanUniversity.‡HefeiUniversityofTechnology.§InstituteofElectro-opticalScienceandTechnology,NationalTaiwanNormalUniversity.|DepartmentofPhysics,NationalTaiwanNormalUniversity.

medicalapplicationsorarchitecturallighting(suchas,museums,libraries,andclothingorcosmeticsalesstores),becauseofthepoorperformanceintheredpartofthespectrum.4Accordingly,theproductionofwhitelightbycombinationofred,green,andbluephosphorswithanear-ultraviolet/ultraviolet(NUV/UV)GaInNdiodeishighlyfavored.LEDmanufacturersgloballywanttodeveloptheminviewofthehugepotentialmarketinhomelighting.Facedwiththistrend,thediodechipwithshort-wavelengthemissionfrom460nm(blue)through380-400nm(NUV)to325-350nm(deepultraviolet)andashortestextremeultravioletemissionof210nmwasabreakthroughquickly.7,8Correspondingly,newhighlyefficientred,green,andbluephosphorsmustbedevelopedrapidlysoastokeepupwiththeadvancesmadeindiodeemissionwavelength.

Acombinatorialchemistryapproach,characterizedbyparallelsynthesisandhighthrough-putscreening,haspotentialtoacceleratethedevelopmentofnewphosphorsforLEDs.9-11Evensomeintelligentalgorithms,suchasGenetic,MonteCarlotechniques,SimulatedAnnealing,andArtificialNeuralNetworksalgorithms,canbeusedtospeedupleaddiscoveryandoptimizationandimprovetheprob-abilityofthediscoveryofobjectivecompounds.12-17Recently,Sohnetalhasusedageneticalgorithm,whichisasystematicapproachbasedonaglobaloptimizationstrategythroughimitatingtheevolutionaryprocesswithelitism,selection,crossover,andmutationoperationsrandomly,incombinatorialapproachtooptimizenovelphosphorsforlight-emittingdiodes.9,1010.1021/cc100063x2010AmericanChemicalSociety

PublishedonWeb06/18/2010

588JournalofCombinatorialChemistry,2010Vol.12,No.4AlthoughsomegoodcandidatesforgreenandbluephosphorshavebeendevelopedforNUV/UVtricolorWLEDs,promisingredphosphorshavenotyetbeenfound.10,18Hence,thedevelopmentofagoodredphosphoriscriticaltoNUV/UVWLEDs.Alternatively,whitelightwithhighRacanbeproducedbymixingbluewithaspecialyellowthatcomprisesredandgreenwithhighcolorsaturation.AplantgrowthlamputilizingaNUVlight-emittingdiodechipandA3MgSi2O8:Eu2+,Mn2+(A)Ca,Sr,Ba)(AMS-EM)phosphorwithemissionspectraconsist-ingofblueandredbandshasbeenreported,whichisthroughtailoringthecationscompositionofAMS-EMinaccordancewiththechangeofenergytransferfromEu2+toMn2+inthehostlatticetoadjustlightqualityofthelamp.19,20Ontheotherside,stronggreenemissionfromBi3+wasobservedinYVO4hostunderNUVexcitation,21andseveralinvestiga-tionsofEu3+luminescencesensitizedbyBi3+inYVO4hosthavebeenpublishedincludingforthepurposeofsolidstatelightingapplications.21-25However,noworkhasyetex-ploitedmonochromaticgreenlightemittedfromBi3+andredlightemittedfromEu3+simultaneouslytoproducepolychromaticyellowlight.Inthisstudy,aspecialdual-colorphosphorofYVO4:Bi3+,Eu3+,whichisverysuitableforfabricatinghigh-RaNUV/UVWLEDsforcontainingaredcomponentfromEu3+emission,wasoptimizedbyusingacombinatorialchemistryapproach;andaWLEDhasbeenpreparedfromtheoptimizedphosphorandacommerciallyavailablebluephosphor,withRaofupto90.3.

ExperimentalSection

CombinatoriallibrariesweresynthesizedbyinkjettingprecursorsolutionsintomicroreactorsusingahomemadeinkjetdeliverysysteminthesequenceVfYfBifEufVfYfBifEu.Thedetailsoftheinkjetinstrumentcanbefoundinourearlierreport.26TheprecursorsolutionswerepreparedbydissolvingappropriateamountsofY(NO3)3·6(H2O)(99.9%),NH4VO3(98%),Eu(NO3)3·5H2O(99.99%),andBi(99.5%)indeionizedwater.NH4VO3solutionwaspreparedbyaddinganappropriateamountofNH4OHtodeionizedwaterandheatingto70-80°Ctoacceleratedissolution.TheBi3+solutionwaspreparedbyaddinganappropriateamountofnitricacidtodeionizedwaterbeforetheBiwasadded,andheatingto70-80°Ctoacceleratedissolution.Afterdryingat60-70°C,themicroreactorsthatcontainedprecursorswerepresinteredat600°Cfor3h,andthensinteredat1100°Cfor3htoyieldthefinalcombina-toriallibrary.

Theoverallluminescenceofthecombinatoriallibraryofsampleswascharacterizedwithaphotographymethodusingadigitalcamera.Theemissionspectrumofeachsampleinthelibrarywasobtainedusingacombinatorialscanningfluorescencecharacterizationsystem.Thedetailsoftheequipmentcanbefoundelsewhere,27whichmainlycom-prisesanHglamp,aportableopticalfiberspectrometer(OceanOptics,Inc.,modelSD2000),andanx-ystage.Thematerialslibrarywasfixedonthex-ystage.Emissionspectrafromthematerialsineachmicroreactorweremeasuredwhenthefiber-opticprobewasfocusedonthebottomofthe

Chenetal.

Figure1.Compositionmapofthe(Y1-s-tBisEut)VO4combinatoriallibraryanditsluminescentphotographunder365nmexcitation.

Figure2.Compositionmapofthe(Y1-s-tBisEut)VO4combinatoriallibraryanditsluminescentphotographunder254nmexcitation.

reactor.Alightshieldwasusedtopreventinterferencefromothersamples.

ResultsandDiscussion

CombinatorialExperimentsResults.Figure1showsthecompositionmapoftheY1-s-tVO4:Bi3+s,Eu3+tcombinatoriallibraryandluminescentphotographunder365nmexcitation.ThefirstcolumninFigure1,withoutBi3+co-doping(s)0),clearlyrevealsthatthefaintredemissionofEu3+increaseswithEu3+concentrationfromt)0tot)0.060,eventhoughtheemissionofEu3+isveryweak.Theothercolumns(s)0.005,0.015,0.025,0.040,and0.050,respectively)demonstratebrightluminescenceisobserved,andtheemissioncolorchangesfromgreentoorangeastheEu3+concentrationincreasesfromt)0.005tot)0.060.ThefirstrowinFigure1,withoutEu3+co-doping(t)0),clearlyshowsthatthestronggreenemissionofBi3+increaseswithBi3+concentrationfroms)0tos)0.050,andthisincreaseisnotobviouswhenBi3+contentexceedss)0.025.Theotherrows(t)0.005,0.015,0.030,0.045,and0.060,respectively)indicatethattheemissioncoloralmostdoesnotchange,butthebrightnessincreaseswithBi3+concentra-tionfroms)0.005tos)0.040,andthendeclinesasBi3+concentrationincreasesfurther.Whent)0.015-0.060,thebrightnessisgreatlyreducedats)0.050.ThesephenomenasuggestthatEu3+cannotemitefficientlywithoutBi3+co-dopingunder365nmexcitation,butthatexcessiveEu3+co-dopingseriouslyquenchesBi3+luminescence.

Figure2displaystheluminescentphotographofthe(Y1-s-tBisEut)VO4combinatoriallibraryunder254nmexcitation.AcomparisonwithFigure1clearlyrevealsasignificantdifferencebetween365and254nmexcitation,andthereby,thespecialroleofBi3+inluminescence.When

DevelopmentofaSingleMassYVO4:Bi3+,Eu3+PhosphorFigure3.Emissionspectraofsampleswiths)0-0.050andt)0inthe(Y1-s-tBisEut)VO4combinatoriallibraryunder365nmexcitation.

theBi3+contentiszero,strongemissionfromEu3+isobservedinFigure2.TheemissionincreaseswithEu3+concentrationfromt)0tot)0.045.EventhoughgreenemissionfromBi3+isalsoobservedat254nmexcitation,theredemissionfromEu3+dominateswhentheEu3+concentrationisnotlessthantheBi3+concentration.Whenadiagonallineisdrawnfromthetoplefttothebottomright,thebottomleftareaisbrighterthanthetoprightarea.TheYVO4hostcanabsorbanexcitationwavelengthof254nmandtransfertheenergyefficientlytotheEu3+activators.However,thisprocessisnotefficientwithexcitationat365nm.21,25Thesignificantdifferenceofluminescencebetween365and254nmexcitationrevealsthatco-dopingBi3+inYVO4:Eu3+increasesitssuitabilityforuseinNUVWLEDsbecausedopedBi3+ionspromotethered-shiftoftheabsorptionbandfromUVtoNUV.

Toanalyzespectralcompositionandluminousefficiency,theemissionspectraofeverysampleinthe(Y1-s-tBisEut)VO4combinatoriallibrarywereobtained.Figure3presentstheemissionspectraofthesamplesinthefirstrowwitht)0ands)0-0.050inthecombinatoriallibraryunder365nmexcitation,inwhichonlyonebroademissionbandwithpeakat545nmisobserved.Thebroademissionbandoriginatesfromthe3P13+f1S0transitionofBi.28,29TheemissionintensityincreaseswithBi3+con-centrationfroms)0tos)0.050,andnoconcentrationquenchingisobserved.TheinsetinFigure3presentsthevariationoftherelativeheightoftheBi3+emissionpeaksat545nmwithBi3+concentrations,wheretheheightofthe(Y0.95Bi0.05)VO4sampleat545nmissetas100%.First,theincreaseofBi3+luminescenceats)0to0.015isdeterminedtoberapid,thenbecomesslowats)0.015to0.025,andfinallytheincreaseremainsstablewithlowspeedats)0.025to0.050.Areasonableexplanationforthisphenom-enonisthatwhentheconcentrationofBi3+isfarbelowitscriticalconcentration,theintensityofBi3+emissionwillincreasequicklyastheBi3+concentrationincreases,andthenremainsstablewhentheconcentrationofBi3+isclosetoitscriticalconcentration.OncetheconcentrationofBi3+ishigherthanitscriticalconcentration,theintensityofBi3+emissionwilldecreasefortheenergytransferamongdifferentBi3+ions.

Figure4displaystheemissionspectraofthesamplesinthesecondrowwitht)0.005ands)0-0.050inthecombinatoriallibraryunder365nmexcitation.Theemission

JournalofCombinatorialChemistry,2010Vol.12,No.45

Figure4.Emissionspectraofthesampleswiths)0-0.050andt)0.005inthe(Y1-s-tBisEut)VO4combinatoriallibraryunder365nmexcitation.

Figure5.Emissionspectraofsampleswiths)0-0.050andt)0.015inthe(Y1-s-tBisEut)VO4combinatoriallibraryunder365nmexcitation.

linesat592,618,650,and702nmareattributedtothe5

D0f7FJ(J)1,2,3,4)transitionsofEu3+andthebroademissionbandwithapeakat545nmisassignedtothe3

P1f1S0transitionofBi3+.28-30TheemissionofEu3+stillincreaseswithBi3+concentrationfroms)0tos)0.050,indicatingthatBi3+providesenergyforEu3+toemitlight.Meanwhile,theemissionofBi3+alsoincreaseswithBi3+concentrationfroms)0tos)0.040.UnlikeinFigure3,thereisnoobviousincreaseinintensityofBi3+emissionwithconcentrationfroms)0.040tos)0.050,eventhoughnosignificantdecreasesisobserved.TheinsetofFigure4showsthevariationoftherelativeheightoftheEu3+emissionpeaksat618nmandtheBi3+emissionpeaksat545nmwithBi3+concentrations:theincreaseinEu3+andBi3+emissionsaresimilartoeachotherats)0to0.025,buttherateofincreaseofEu3+emissionexceedsthatofBi3+ats)0.025to0.050,andthereisalmostnoincreasewiths)0.040to0.050fortheheightoftheBi3+emissionpeak.Therefore,theenergytransferfromBi3+toEu3+doesnotinfluenceBi3+emissioniftheconcentrationofBi3+isnohigherthans)0.040andthatofEu3+islessthant)0.005;however,asBi3+concentrationfurtherincreasestos)0.050andEu3+concentrationismaintainedatt)0.005,themeandistanceofBi3+andEu3+willbeshortened,andthenEu3+cancaptureenergyfromBi3+efficiently.Accord-ingly,Bi3+emissionisdegradedevidently.Figure5showsthisphenomenonmoreclearly.

Figure5presentstheemissionspectraofthesamplesinthethirdrowwitht)0.015ands)0-0.050incombina-

590JournalofCombinatorialChemistry,2010Vol.12,No.4Figure6.Emissionspectraofsampleswiths)0.040andt)0-0.060inthe(Y1-s-tBisEut)VO4combinatoriallibraryunder365nmexcitation.

toriallibraryunder365nmexcitation:theconcentrationofBi3+equalsthatinFigure3andFigure4withs)0-0.050,butEu3+concentrationisincreasedtot)0.015.Figure5clearlyshowsthatbothBi3+andEu3+emissionsincreasewithBi3+concentrationfroms)0tos)0.040,buttheydecreasesignificantlyasBi3+concentrationincreasesfroms)0.040tos)0.050.ExcessiveEu3+co-dopinghasquenchedtheluminescenceofBi3+anditselfseriously.TheinsetinFigure5revealsthattheEu3+andBi3+emissionsaregreatestats)0.040.ComparingFigures3,4,and5,andinparticulartherelativeheightofBi3+emissionpeaksat545nmandEu3+emissionpeakat618nmpresentedasinsets,demonstratesthattheBi3+emissionisstrongestats)0.050inFigure3and4,buts)0.040inFigure5;theEu3+emissionisstrongestats)0.050inFigure4,buts)0.040inFigure5.TheseresultsdemonstratethatthetransferofenergyfromBi3+toEu3+dependsontheBi3+andEu3+interionicdistance.Thischaracteristicistypicalofresonanceenergytransfer.

Figure6presentstheemissionspectraofthesamplesinthefifthcolumnwiths)0.040andt)0-0.060inthecombinatoriallibraryunder365nmexcitation:theemissionofBi3+declinesgraduallyasEu3+contentincreasesfromt)0tot)0.060,furtherindicatingthatthetransferofenergyfromBi3+toEu3+toanextentthatincreasesgraduallywithEu3+concentration,becauseoftheshorteningofthedistancebetweenBi3+andEu3+.However,theemissionofEu3+increaseswithEu3+concentrationfromt)0tot)0.045andthendeclinesastheEu3+concentrationfurtherincreasesfromt)0.045tot)0.060.ThisfindingisclearlyrevealedbythevariationoftherelativeheightofBi3+andEu3+emissionpeakspresentedintheinsetinFigure6.Thisphenomenoniswell-knownasconcentrationquenchingofluminescence.

Theemissionspectraofothersamplesareomittedhere.However,therelativeluminescenceefficiencyofeverysampleinthecombinatoriallibraryobtainedbyintegratingtheemissionspectrumfrom400to700nmisshowninFigure7,fromwhichthecompositionrangeofsamplesthatdisplaysrelativelymoreefficientluminescenceis0.040ese0.050and0Besidesefficiency,anotherfactorthatdetermineswhetherphosphorscanbeappliedinLEDsiscolorquality.Thecolorcoordination(CIE(x,y)),correlatedcolortemperature(Tc),

Chenetal.

Figure7.Relativeemissionefficiencyofeverysampleinthe(Y1-s-tBisEut)VO4combinatoriallibrarydeterminedbyintegrationemissionspectrafrom400to700nmunder365nmexcitation.

andcolorrenderingindex(Ra)ofemissionspectra,ascorrespondingtoFigure6,ofthe(Y1-s-tBisEut)VO4(s)0.040;t)0-0.045)samplesobtainedbycalculatingwiththeCIE13software,aresummarizedinTable1.Thecorrelatedcolortemperaturedecreasesgraduallyfrom4530to1781KasEu3+concentrationincreasesfromt)0tot)0.045,indicatingthatco-dopingEu3+benefitstoobtainwarmlight.ThecolorrenderingindexincreaseswithEu3+con-centrationfromt)0tot)0.015andthendecreasesasEu3+increasesfromt)0.015to0.045.Thissuggeststhatco-dopingalittleEu3+ishelpfultoimprovecolorrendering;however,toomuchcanbeharmful.Figure8displaystheCIEcoordinatesvaryingwithEu3+concentrationfromt)0tot)0.030visually,wherethecolorchangesfromyellow-greentoorange.Differentiatingfromjudgingbyvisualinspection,theCIEdiagramdemonstratesthevariationofemissioncolorobjectivelywithoutvisualfunctioneffect.ThehighestRaof86.86isobtainedforthesampleof(Y0.945Bi0.040Eu0.015)VO4,whichhasbeenhigherthanthatofYAG-basedLEDsbutlowerthanthevalueof90-100fortheclassified1AgradeRa.However,thereisnobluecomponentintheemissionspectrumof(Y0.945Bi0.040-Eu0.015)VO4.Otherwise,theRavaluewouldbehigher.Figure8alsopresentstheCIEcoordinatesofcommerciallyavailablebluephosphorforNUVWLEDs,suchas(0.1576,0.0836)forSr3MgSi2O8:Eu2+(3128).Onthebasisoftheprinciplethatwhitelightcanbeproducedbymixingpolychromaticlights,wemakesurethatanidealWLEDwithhighRacanbetunedupbycoatingtheoptimizedyellowphosphorandotherbluephosphorontoaNUVdiodechipwithintheCIErangeof0.3857exe0.5224and0.4250eye0.5146,asthetrianglemarkedinFigure8.

ConventionalExperimentsResults.Toconfirmthefeasibilityoftheaboveresultsstudiedbycombinatorialchemistryapproach,somebulksamplesweresynthesizedwiththeconventionalsolidstatereactionmethodfromsourcesofY2O3(99.9%),NH4VO3(98%),Eu(NO3)3·5H2O(99.99%),andBi2O3(99.5%).Thesamesinteringtechniqueaswasemployedincombinatoriallibrarysynthesiswasused.Beforesintering,therawmaterialsinastoichiometricratiowerethoroughlyground.Thephotoluminescence(PL)ofthebulksampleswasmeasuredusingFluoroMax-3andFluo-roMax-pspectrometers.Thermalquenchingluminescencewasobtainedbyusingaheatingapparatus(THMS-600)andincombinationwiththespectrometer.Thecrystalstructure

DevelopmentofaSingleMassYVO4:Bi3+,Eu3+PhosphorJournalofCombinatorialChemistry,2010Vol.12,No.4591

Table1.ColorCoordination(CIE(x,y)),CorrelatedColorTemperature(Tc),andColorRenderingIndex(Ra)ofEmissionSpectraofthe(Y1-s-tBisEut)VO4(s)0.040;t)0-0.030)SamplesinCombinatorialLibraryunder365nmExcitation,AsCorrespondingtoFigure6

samples

(Y0.960Bi0.040)-VO4(15)

propertiescolorcoordinationx0.3857

y0.5146

(Y0.955Bi0.040Eu-0.005)

VO4(25)x0.4209

y0.5003(Y0.945Bi0.040Eu-0.015)

VO4(35)x0.4666

y0.4651(Y0.930Bi0.040Eu-0.030)

VO4(45)x0.5224

y0.4250(Y0.915Bi0.040Eu-0.045)

VO4(55)x0.5331

y0.3853(CIE(x,y))correlatedcolor45303887temperature(Tc)colorrendering56.98

68.35

index(Ra)

Figure8.CIEcordinatesofemissionspectraof(Y0.960Bi0.040)VO4(15),(Y0.955Bi0.040Eu0.005)VO4(25),(Y0.945Bi0.040Eu0.015)VO4(35),(Y0.935Bi0.040Eu0.025)VO4(45),andcommerciallyavailableSr3MgSi2O8:Eu2+(3128)phosphorsunder365nmexcitation.

Figure9.Experimental(crosses),calculated(solidline),anddifference(bottom)resultsofXRDrefinementof(Y0.955Bi0.040Eu0.005)VO4.Insetpresentsthethree-dimensionalstrucuture.

ofbulksampleswasidentifiedbyX-raydiffraction(XRD)analysisusinganX’PertPROadvancedautomaticdiffrac-tometerwithCuKRradiation,operatedat45kVand40mA.TheGSASprogram31wasusedtoperformtheRietveldrefinementstoobtaininformationonthecrystalstructureofYVO4:Bi3+,Eu3+.

YVO4hasthewell-knownzircontypeofcrystalstructure.Theexperimental,calculated,anddifferenceresultsoftheXRDrefinementof(Y0.955Bi0.040Eu0.005)VO4atroomtem-perature,obtainedbyusingtheGSASprogram,aredisplayedinFigure9.31Table2presentsthecorrespondingcrystal-lographicdata.(Y0.955Bi0.040Eu0.005)VO4crystallizesasatetragonalstructure(insetinFigure9)withaspacegroupofI41/amd(141)andlatticeconstantsofa)b)7.129(6)Å,c)6.301(6)Å,andV(cellvolume))320.318(7)Å3.Alloftheobservedpeakssatisfythereflectionconditionwith

29992109178186.86

82.04

71.63

Table2.CrystallographicDatafor(Y0.955Bi0.040Eu0.005)VO4

atomsXYZfractionUi(Å2)Y0.0000(0)0.750(0)0.125(0)0.9550.0148(8)V0.0000(0)0.250(0)0.375(0)1.0000.0137(7)O0.0000(0)0.433(3)0.202(4)1.0000.0069(1)Eu0.0000(0)0.750(0)0.125(0)0.0050.0148(8)Bi

0.0000(0)

0.750(0)

0.125(0)

0.040

0.0149(4)

SpaceGroup:I41/amd

cellparameter

reliabilityfactorsa)b)7.129(6)Å,󰀄2)1.569c)6.301(6)ÅRwp)5.17%R)󰀁)γ)90°

Rp)3.21%

cellvolume)320.318(7)Å3

reliabilityfactorsasfollows:󰀄2)1.569,Rp)5.17%,andRwp)3.21%.ThecellvolumeisalittlehigherthanthatoftheidealYVO4(JCPDScard16-0250,whoselatticecon-stantsarea)b)7.123Å,c)6.291Å,andV)319.19(Å3)crystallattice,becausetheradiioffreeBi3+andEu3+arelargerthanthatofY3+(Bi3+:0.96,Eu3+:0.95,andY3+:0.).Accordingly,Bi3+andEu3+shouldreplacetheidealsitesofY3+.

Figure10presentstheemissionspectraof(Y0.957-Bi0.040Eu0.003)VO4,(Y0.956Bi0.040Eu0.004)VO4,(Y0.955Bi0.040Eu0.005)-VO4(25),and(Y0.945Bi0.040Eu0.015)VO4(35)bulksamplesunder365nmexcitation.Forsamples25and35,Figure10clearlyrevealsthattheemissionintensityofEu3+increasesandtheBi3+emissiondeclinesastheEu3+concentrationincreasesfroms)0.005tos)0.015.Thisresultisconsistentwiththeinvestigationscitedabovethatutilizedacombinatorialchemistryapproach.TheinsetinFigure10showsthephotographofluminescencefromsamples25and35under365nmexcitation.Thecolorsareconsistentwith

Figure10.Emissionspectraof(Y0.957Bi0.040Eu0.003)VO4,(Y0.956Bi0.040Eu0.004)VO4,(Y0.955Bi0.040Eu0.005)VO4(25),and(Y0.945Bi0.040Eu0.015)VO4(35)under365nm.

592JournalofCombinatorialChemistry,2010Vol.12,No.4Figure11.Excitationspectraof(Y3+0.957Bi0.040Eu0.003)VO4and(Y0.956Bi0.040Eu0.004)VO4uponEuemissionat618nm.

thoseinthecombinatoriallibrary,asjudgedbyvisualinspection.AnotherveryinterestingfindingisthatbothBi3+andEu3+emissionintensitiesincreasewithEu3+contentfromt)0.003tot)0.004andthentot)0.005.ThisresultdemonstratesthatdopingtheproperamountofEu3+intoYVO4:Bi3+improvesitsgreenluminescenceefficiency.Atthesametime,thenecessaryredcomponentrequiredforhigh-Rawhitelightisalsoobtained.Therefore,thismethodisaperfectmeansofobtaininghigh-Rawhitelight,bothefficiencyandcolorrenderingindexRawereallimproved.Figure11presentsthenormalizedexcitationspectraofthebulksamplesof(Y0.957Bi0.040Eu0.003)VO4and(Y0.956Bi0.040Eu0.004)VO4uponEu3+emissionat618nm.Thisbroadexcitationbandwithapeakat341nmisattributedtothe1A1f1T1transitionofVO43-andthe1S0f3P1transitionofBi3+,asdiscussedinthefollowingparagraph.Theabsorptionlinesat394and466nmareattributedtothe7

F0f5L6and7F0f5D2transitionsofEu3+.ThepresenceofVO43-andBi3+absorptioninEu3+excitationspectraindicatesthatthetransferofenergyfromVO43-toEu3+andBi3+toEu3+occurssimultaneously;30however,theBi3+fEu3+energytransferismoreefficientunder365nmexcitationbecausetheYVO4hostcannotabsorbtheenergyof365nmexcitationefficiently,whichcanbeconcludedfromthesignificantdifferenceoftheluminescenceofthecombinatoriallibrarybetweenunder365and254nmexcitation.Here,boththeabsorptionbandintheregion250-385nmandtheabsorptionlinesofthe7F0f5L6and7

F0f5D2transitionsofEu3+increasewithEu3+concentrationfroms)0.003tos)0.004,furtherconfirmingthatdopingproperamountofEu3+intoYVO4:Bi3+increasesitslumi-nescenceefficiency.

Finally,apracticalwhiteLEDlamphasbeenpreparedfromanoptimal(Y0.956Bi0.040Eu0.004)VO4phosphorandacommerciallyavailablebluephosphorofSr3MgSiO8:Eu2+(3128)assistedbyEverlightElectronicsCo.Ltd.Figure12presentstheemissionspectrumofthepreparedLEDlamp,withaRaofupto90.3.Theinsetalsopresentsthephotographsof(Y0.956Bi0.040Eu0.004)VO4and3128phosphorsluminescencetakenunder365nmexcitation.

MechanismofLuminescenceandTheoreticalAnalysis.Tostudythemechanismofluminescenceandthepathsofenergytransferduringluminescence,thePLspectraoftheYVO4purehostandtheBi3+-dopedYVO4bulksampleswereexaminedandshowninFigure13(a)and(b),

Chenetal.

Figure12.Emissionspectrumofawarm-whiteLEDlamp,fabricatedbycoatingaNUVLEDchipwithcommerciallyavailableSr3MgSi2O8:Eu2+(3128)andself-optimized(Y0.956Bi0.040Eu0.004)VO4phosphors.

Figure13.EmissionandexcitationspectraoverlapofYVO4(a),YVO4:Bi3+(b),andYBO3:Eu3+(c).

respectively.ThefullenergylevelofEu3+rangedfrom250to560nmwasobtainedbymeasuringintheYBO3host,asshowninFigure13(c).ThereasonforselectingYBO3ashostisthattheabsorptionbandofYBO3(150-175nm)isfarhigherthantheO2--Eu3+chargetransfer(223nm).AlthoughnoBi3+andEu3+weredopedintoYVO4,astrongemissionbandwithapeakataround430nmwasobservedunder365nmexcitation,andastrongexcitationbandthatpeakedatapproximately325nmwasobservedbymonitoring430nmemission.Thesebandsareattributedtochargetransferbetweenthep6orbitalofO2-andthedorbitalofV5+withinthepolyanionofVO43-.ThegroundstateofVO43-withthep6configurationis1S0(t16,1A1),andtheexcitedstatesthatarisefromp5d(t15e)configurationwithTdspacegroupandsplitbythecrystalfieldare3T2,3T1,1T1,and1T2inorderofincreasingenergy,asshowninFigure14(a).32Therefore,theexcitationbandofYVO4inFigure13(a)isassignedtothespinallowed1A1f1T1transitionofVO43-.Theelectronsthatoccupythe1T1excitedstateareunstableandtendtorelaxtothelow3T2or3T1level.Whenanelectronreturnsfromanexcitedstatetothegroundstate,aphotonisemitted.Here,theemission-bandpeakedatabout430nmisattributedtothe3T2f1A1transitionfor∆J)1(whereJrepresentsthetotalangularmomentumoftheelectron).Hence,theexcitationandemissionprocessesinYVO4canbesummarizedasFigure14(a).

ThegroundstateofafreeBi3+withns2configurationis1

S0,andtheexcitedstatesthatarisefromthensnpconfig-urationare3P0,3P1,3P2,and1P1inorderofincreasing

DevelopmentofaSingleMassYVO4:Bi3+,Eu3+PhosphorFigure14.SchematicdiagramofVO43-,Bi3+,andEu3+energylevelsandexcitation,emission,andenergytransferinYVO3+3+4(a),YVO4:Bi(b),andYVO4:Bi3+,Eu(c).

energy.33-35Althoughtheelectrontransitionfromnstonsnpisallowedbytheparityselectionrule,bythespinselectionrule,onlythetransitionfrom1S0(or1P1)to1P1(or1S0)isallowedfor∆S)0(whereSdenotesthespinangularmomentumoftheelectron)ifnootherconfigurationisconsidered.However,the1S0f3P1transitionismadepartiallyallowedbythemixingofthesingletstateandthetripletstate.Therefore,theemissionbandwithapeakat545nminFigure13(b)isattributedto3P1f1S0.AcomparisonwithFigure13(a)revealsthattheemissionof3T2f1A1transitionofVO43-isabsentintheemissionspectrumofYVO4:Bi3+underthesameexcitationcondition.Areasonableexplana-tionisthattheabsorptionenergyofVO43-hasbeentransferredtoBi3+.However,anextraabsorptionbandoverYVO4excitationoccurstotheexcitationspectrumofYVO4:Bi3+,asshowninFigure13(b).Datta21attributedthisextrabandtoO-VandO-Bichargetransfer.ThetransferofchargefromO2-ofVO43-toBi3+properlyexplainsthetransferofenergyfromVO43-toBi3+,butthesignificantdifferenceofluminescenceoftheabovecombinatoriallibrarybetweenunder365nmandunder254nmexcitationindicateselectronsaredirectlyexcitedfromthe1S0groundstatetothe1P1excitedstatewhenexcitingwithlowenergy365nm.Ifexcitedwith254nm,electronsaremainlyexcitedfromthe1A1groundstatetothe1T1excitedstateofVO43-.Hence,theexcitationbandintheregion250-385nminFigure13(b)isattributedtothecommoncontributionofthehostsensitizationbandfromVO43-toBi3+viaO2-Bi3+chargetransferandthe1S0f3P1transitionofBi3+.Accordingly,theexcitationandemissionprocessesofYVO4:Bi3+lumines-cencecanbesummarizedasinFigure14(b).

TheconcentrationdependenceofEu3+luminescence,describedabove,demonstratesthatthetransferofenergyfromBi3+/VO43-toEu3+isbyresonance.Theresonantenergylevelsbetweentheemissionofthesensitizerandtheabsorptionoftheactivatormustmatchwelltosatisfythecriticalrequirementofanefficientenergytransfer,andtheprobabilityofenergytransferbetweenthesensitizerandtheactivatorisexpressed,accordingtoDexter,36as

PSA)

2π

|〈SA*|HSA|S*A〉|2h

∫gS(E)gA(E)dE

(1)

wherethematrixrepresentstheinteractionbetweentheinitialstate|S*A〉andfinalstate〈SA*|,HSAistheinteraction

JournalofCombinatorialChemistry,2010Vol.12,No.4593

Hamiltonian,SandArepresenttheemitterandtheabsorber,gs(E)andgA(E)arenormalizedopticallineshapefunctions,andtheintegralrepresentsthespectraloverlap,respectively.The3P1f1S0emissionofBi3+largelyoverlapsthe7F0f5D2and7F0f5D1excitationofEu3+,asshowninFigures13(b)and(c).Accordingly,Bi3+canefficientlytransferitsabsorp-tionenergytoEu3+.However,theprobabilityofenergytransferPSAisproportionalto(1)/(R6)(whereRrepresentsthedistancebetweenthesensitizerandtheactivator).Ifthedistancebetweenthesensitizerandtheactivatoristoolong,thenexcitationenergycannotbetransferredtoactivators.Thus,strongemissionofBi3+isobservedwhentheEu3+concentrationisverylow;andalmostnoBi3+emissionoccurswhentheEu3+concentrationisveryhigh,aspresentedinFigures4,5,6,and10.Therefore,theenergytransferinYVO4:Bi3+,Eu3+canbesummarizedasinFigure14(c).Additionally,the3T2f1A1emissionofVO43-largelyoverlapsthe7F0f5L9,5G9,5L6,5D3,and5D2excitationofEu3+,asdisplayedinFigures13(a)and(c),butYVO4exhibitsalmostnoabsorptionwhenitisexcitedby365nm,asshowninFigure13(a).ThisfactexplainsthesignificantdifferencebetweenFigure1andFigure2describedabove.Theabovecombinatorialresearchresultsrevealthats)0.040andt)0.005isapproximatelythecriticalconcentra-tionforthetransferofenergybetweenBi3+andEu3+inthe(Y1-s-tBisEut)VO4crystallattice.Accordingly,themeancriticaldistancebetweenBi3+andEu3+ionsforenergytransfercanbeestimatedusingtherelationthatwasproposedbyBlasseandGrabmaier,37Rc≈2

(3V4π󰀄(2)

cZ

)1/3

whereVisthevolumeoftheunitcell,󰀄cisthesumofthecriticalconcentrationoftheactivatorandthesensitizer,andZisthenumberofformulaunitsperunitcell.For(Y0.955Bi0.040Eu0.005)VO4phosphor,V)320.3158Å3,󰀄c)0.045,Z)4.Therefore,theobtainedRcvalueis15.0375Å.AlargerRcvaluecorrespondstoamoreefficientenergytransfer.

Conclusions

Insteadofdevelopinganovelredphosphorindividually,thisworkproposestheproductionofwhitelightbycom-bininganear-ultraviolet/ultravioletdiodechipwithablueandared-greendualcolorphosphorsoastoimprovethecolorrenderingofwhitelight-emittingdiodes.Thepossibilityofthisschemewasdemonstrated,andthedesireddual-colorphosphorhasbeensuccessfullyscreenedoutfromtheYVO4:Bi3+,Eu3+systemusingacombinatorialchemistryapproach.Whentheemissioncolorandluminousefficiencyarebothconsidered,theoptimalcompositionis0.040ese0.050and0594JournalofCombinatorialChemistry,2010Vol.12,No.4throughtheresonanceway;comparatively,Bi3+fEu3+playsamoreimportantroleintransferenergyforEu3+toproduceredlightunderNUVexcitation.

Acknowledgment.TheauthorswouldliketothanktheNationalScience,Taiwan,forfinanciallysupportingthisresearchunderContractNos.NSC97-2113-M-002-012-MY3andNSC97-3114-M-002-005).ThePostdoctoralScienceFoundationofChina(20090450802),thePostdoctoralRe-searchFellowofMaterialsScienceandEngineeringofHefeiUniversityofTechnology(103-035038)andtheScienceFoundationforExcellentYoungScholarsoftheMinistryofEducationofChina(20090111120001)alsosupportedthisresearch.EverlightElectronicsCo.LtdiscommendedforitsassistanceinpreparingtheLEDlamps.ReferencesandNotes

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