Blue-to-Orange Tunable Luminescence from Europium Doped Yttrium–Silicon–Oxide–Nitride Phosphors

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CHIN.PHYS.LETT.Vol.25,No.9(2008)3444Blue-to-OrangeTunableLuminescencefromEuropiumDopedYttrium–Silicon–Oxide–NitridePhosphors

YANGHu-Cheng(󰀁󰀁󰀁)1,LICheng-Yu(󰀁󰀁󰀁)2,PANGRan(󰀁󰀁)2,G.Lakshminarayana1,ZHOUShi-Feng(󰀁󰀁󰀁)1,TENGYu(󰀁󰀁)1,QIUJian-Rong(󰀁󰀁󰀂)1∗∗

1StateKeyLaboratoryofSiliconMaterials,ZhejiangUniversity,Hangzhou310027

2StateKeyLaboratoryofApplicationofRareEarthResources,ChangchunInstituteofAppliedChemistry,Chinese

AcademyofSciences,Changchun130022

(Received24May2008)Europium-dopedyttrium–silicon–oxide–nitridephosphorsaresynthesizedbycarbothermalreductionandnitri-dationmethod.ThecrystalstructureofthephosphorschangedgraduallyfromoxideY2Si2O7tonitrideYSi3N5

statewithincreasingdosageofSi3N4andcarbonpowder.TheY2Si2O7:Euphosphorshowsablueemissionat

465nmwith300nmexcitationandacharacteristicredemissionofEu3+at612nmwith230nmexcitation.TheYSi3N5:Euphosphorshowsabroademissionbandcentredat595nmwithsomesharppeaksofEu3+with325nmexcitation.Theabsorptionofthestudiedphosphorsincreasesfrom450to700nmwithanincrementinnitrogencontent.Blue-to-orangetunableluminescenceisobservedwith390nmexcitation.

PACS:78.55.Hx,61.66.Fn,78.40.HaInrecentyears,solidstatelighting(SSL)hasbe-comeaninterestingfieldofresearch.Incompari-sonwithincandescentandfluorescentlamps,InGaN-basedSSLshowsmanyadvantages,suchaslonglifetime,smallbulk,toxicity-freeandenergy-saving,etc.[1]Now,theluminescentefficiencyofwhitelightemittingdiodeshasalreadyexceededthatofincan-descentlampsandisexpectedtobemorehigherforreplacingtheconventionalfluorescentlamps.[2,3]Un-doubtedly,SSLisachallengeforconventionallighting.Threemethodscanbeusedtorealizewhitelightemittingpresently:combiningtricolourlightemit-tingdiode(LED)chipsdirectly;combiningblueLEDchipswithyellow(orgreenandred)wavelengthcon-versionphosphorandcombiningnear-ultraviolet(orpurple)LEDchipswithtricolourwavelengthconver-sionphosphors.[4−7]Inthesemethods,thelasttwoaremoreeconomicandeasytobeproducedforlight-ing,inwhichphosphorsplayakeyroleinSSLfieldsastheydidinfluorescentlamps.For“blue-yellow”method,thetotaloutputcolourisaffectedbybothchipsandphosphors,soitiscomplextorealizesta-blewhitelightemission.Fornear-ultravioletmethod,theemissioncolourisaffectedmainlybythephos-phor,thusstablewhitelightcanbeobtainedbyem-ployingsuitablephosphor.However,manypresently-usedclassicphosphorscannotbeemployeddirectlyinSSLfieldbecausetheirmaximumexcitationbandisusuallyfarfromtheemissionofInGaNchips(nearultraviolettobluelight),resultingintheirlowlumi-nescenceefficiency.Thus,itisnecessarytosearchfornovelphosphorsoradjustthemaximumexcita-tionwavelengthofpresently-usedphosphors.Theexcitationoremissionbandsofsomerareearthions,suchasCe3+andEu2+aredueto4f-5dtransitions,whicharesensibletotheenvironmentandcanbeobviouslyadjustedbychangingcrystalfieldenvironment.[8]Theluminescenceofsomerareearthionsinnitridesappearsinlonger-wavelengthregionthanthatinoxides;usuallyforthered-shiftofenergygravitycentreandthehigherformalchargeofN3−

comparedtoO2−.[9]Therefore,thechangeinenergylevelsoftheserareearthionsisexpectedfromoxidetonitridehostandsomeresultshavebeenreportedsuchthattheirluminescencehasshownred-shiftinnitridesoroxy-nitridescomparedwithpureoxides.[10,11]

Additionally,nitrideoroxy-nitridephosphorsusu-allyexhibitexcellentmechanical,chemicalandther-malstability,whichmakesthemmoresuitableforthehightemperaturecausedbyInGaN-chipsinthesmallbulk.Uptodate,manyinvestigationsarefocusedondevelopingsuitablephotonconversionphosphorforSSLapplication.YttriumsilicateY2Si2O7isusuallystudiedforitsdielectricpropertiesandpolymorphousnature.[12,13]However,italsohasotherequallyin-terestingpropertiesrelatedtoitsluminescencewhendopedwithvariousrareearthionsduetoitshighphysicalandchemicalstabilitywithrespecttotimeandtemperatureascomparedwithotherwell-studiedphosphorssuchasZnSandCdS.[14,15]Therearenotoxic,hazardouswastesorgasesinvolvedduringitspreparation.However,Eu2+ioninY2Si2O7showsbluecolouranditsexcitationbandisfarfromtheemissionofInGaN-chips,whichmakesitunsuitablefortheSSLapplication.Presently,blue,green,andyellowemittingphosphorshavebeendevelopedsuffi-ciently.Howeverexcellentlongerwavelengthemittingphosphorisstilllack.Thuswemadeanattempttorealizethisgoalasmentionedabovevianitridationmethod.Forsynthesizingcomplexsiliconlanthanideni-trides,itisdifficulttoachieveanabsolutelyoxygen-freeenvironmentwhenworkingwithnitridestartingmaterials.Insomecases,carbothermalreductionand

∗∗Towhomcorrespondenceshouldbeaddressed.Email:qjr@zju.edu.cn