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RESEARCHARTICLECopyright2010AmericanScienticPublishersAllrightsreservedPrintedintheUnitedStatesofAmericaJournalofNanoscienceandNanotechnologyVol.
10,1–4,2010BrightGreenUpconversionFluorescenceofYb3+,Er3+-CodopedNaYF4NanocrystalsLiliWang12,WeipingQin1,YanWang1,GuofengWang1,ChunyanCao1,GuodongWei1,RyongjinKim1,DaishengZhang1,FuhengDing1,andChunfengHe11StateKeyLaboratoryonIntegratedOptoelectronics,CollegeofElectronicScienceandEngineering,JilinUniversity,Changchun130012,China2SchoolofBasicSciences,ChangchunUniversityofTechnology,Changchun130012,ChinaUniformhexagonalNaYF4:Yb3+,Er3+nanocrystals(NCs)withbrightgreenupconversionuores-cencewereprepared.
Thenanocrystalswerehighlyhydrophobicanddispersedincyclohexaneformingatransparentcolloidalsolutionduetotheuseofoleicacidasacappingligand.
Temper-aturedependenceforemissionofEr3+ionsinthissamplewasalsostudied.
ThegreenemissionintensityofEr3+ionshadamaximumat160Kunder980nmexcitation,whichwasattributedtothethermallyactivateddistributionofelectronsandthethermalquenchingeffect.
Thetransparentcolloidalsolutionisexpectedtobeagoodcandidateforresearchinbiologicalimagingordisplayapplications.
Keywords:NaYF4,RareEarth,NonradiativeRelaxation.
1.
INTRODUCTIONAnumberofrare-earth-dopeduoridenanocrystalsandsubmicrocrystalswithefcientinfrared(IR)radiationintovisiblelightthroughfrequencyupconversion(UC)atroomtemperaturehavebeenproduced.
1–6Duetothelowphotonenergy,therelativelyhighefciencyoftheUCprocessintheseuoridecrystals,andtheinexpensive980nmnear-IR(NIR)diodelasers,therealizationofefcientNIRtovisi-bleupconvertingnanocrystalsandsubmicrocrystalsshouldunlockarealmofnewpossibilitiesintheeldofbiosen-sors,displays,andshort-wavelengthlasers.
Itiswell-knownthatthesizeandshapeofinorganicnanocrystalshavegreatinuenceontheirphysicalproperties.
78Thus,furtherexplorationsofnovelnanomorphology,withcon-trolledsizeandshapebyconvenientsynthesismethods,isofgreatimportanceforthedevelopmentofnewfunctionaldevices.
Simultaneously,inordertorealizethebiologicalimagingordisplayapplicationsbasedontheUCprinci-pleinauid,itiscrucialtoinvestigatelanthanide-dopedUCnanocrystallinecolloidalsolutions.
Todate,thesuc-cessfulpreparationoftransparentcolloidalsolutionshasbeendemonstratedinafewNaYF4nanocrystals.
However,itstemperature-dependentluminescencebehaviorwassel-domstudied.
Inthisletter,wereportastudyonthetrans-parentNaYF4:Yb3+,Er3+nanocrystalcolloidalsolution.
Authortowhomcorrespondenceshouldbeaddressed.
Under980nmexcitationfromalaserdiode,thetrans-parentsolutionpresentedbrightgreenUCuorescence.
Itisexpectedtobeagoodcandidateforresearchinbio-logicalimagingordisplayapplications.
Wealsoinves-tigatedthetemperature-dependentcharacteristicsofEr3+ionemissions.
Theunderstandingofthisprocessishelpfulinthesearchforhighefciencyphosphors.
Thermalizationeffectsbetweenthe2H11/2and4S3/2levelsseparatedby765cm1intheseuoridenanocrystalswerealsoreported.
Theresultsshowthat520nmemissionisfavoredathightemperaturesbyitshigheremissioncrosssectionandthattheoverallgreenuorescenceincreasesconsequently.
2.
EXPERIMENTALDETAILSInatypicalpreparation,0.
6gNaOHwasdissolvedinasolutioncontainingoleicacid,ethanol,anddeionizedwater(10/5/4,v/v/v).
Then1.
5mmolKF,0.
39mmolY(NO33·6H2O,0.
10mmolYb(NO33·6H2Oand0.
01mmolEr(NO3)3·6H2Owereaddedtothesolu-tionundervigorousstirring.
Themixturewasagitatedfor30minandthentransferredintoa50-mLautoclave,sealed,andtreatedat180Cfor16h.
Subsequently,themixturewasallowedtocooltoroomtemperature,andthepowderwasobtainedbycentrifuging,rinsing,anddrying.
PhaseidenticationwasperformedviaX-raydiffrac-tion(XRD,modelRigakuRU-200b),usingnickel-lteredJ.
Nanosci.
Nanotechnol.
2010,Vol.
10,No.
xx1533-4880/2010/10/001/004doi:10.
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2010.
20851Vol.
10,1825–1828,201031533-4880/2010/10/1825/0051825RESEARCHARTICLEBrightGreenUpconversionFluorescenceofYb3+,Er3+-CodopedNaYF4NanocrystalsWangetal.
CuKradiation(=15406).
Thesizeandmorphol-ogyofthenanocrystalswerecharacterizedbytransmissionelectronmicroscopy(TEM,JEM,2000EX200kV).
High-resolutionemissionspectrawereobtainedat13–300Kwiththesamplemountedinaheliumexchangegascham-berofaclosedcyclerefrigerationsystem.
A980nmlaserdiodewasusedastheexcitationsource.
Theuorescencephotoofthecolloidalsolutionwith0.
2mg/mLconcen-trationincyclohexanewasacquiredwithadigitalcameraunder980nmexcitationfromalaserdiode.
3.
RESULTSANDDISCUSSIONThecrystalstructureandthephasepurityofthematerialwasobtainedbyXRD,asshowninFigure1.
AccordingtoJCPDSstandardcards,theNaYF4:Yb3+,Er3+nano-crystalsexhibitapurehexagonalstructure.
TheXRDpat-ternalsodemonstratesthattheNaionscanbeincorporatedintotheframeworkofYFn,formingNaYF4.
Figure2(a)showstheTEMimageofhexagonal-phaseNaYF4:Yb3+,Er3+nanocrystals.
Theimageclearlyrevealsthatthemorphologyoftheas-synthesizednano-crystalsishexagonalnanoplateswithameansizeof200nm.
Intheimage,thereisnoaggregationofnano-crystals,whichindicatesthegooddispersionofthissam-ple.
Inaddition,thesenanocrystalsarehighlycrystalline.
Theirhighcrystallinityisalsoinferredfromtheelec-trondiffractionimages(insertinFig.
2(a)).
Figure2(b)illustratesphotographsofhexagonal-phaseNaYF4:Yb3+,Er3+dispersedincyclohexanewiththeconcentrationof2mg/mL.
Wecanseethatthecolloidalsolutionisfullytransparentintheleftpicture,andtherightpictureshowsthatthecolloidalsolutionemitseye-visibleUClumines-cenceunder980nmexcitation.
Thissolutionisstableformonthswithoutanyvisibleprecipitate.
ThesurfacesofthesynthesizedNaYF4nanocrystalswerecoveredbyFig.
1.
PowderXRDpatternofhexagonal-phaseNaYF4:Yb3+,Er3+nanocrystals.
Fig.
2.
(a)TEMimageofthemonodispersedNaYF4:Yb3+,Er3+nano-crystals.
Insert:Theelectronicdiffractionpatternsofthesample.
(b)Pho-tographofhexagonal-phaseNaYF4:Yb3+,Er3+dispersedincyclohexaneshowseye-visibleupconversionluminescenceunder980nmexcitationwiththeconcentrationof2mg/mL.
Theleftcellshowsthatthecolloidalsolutionisfullytransparent.
thesurfactantoleicacid,sothenanocrystalswerehighlyhydrophobicanddispersedincyclohexane.
ThenormalizedUCemissionspectraofhexagonal-phaseNaYF4:Yb3+,Er3+nanocrystalsatthreediffer-enttemperaturesunder980nmexcitationareshowninFigure3.
Thelaserpoweris400mW.
Inthespectra,thegreenemissionspeaksareobservedintherangeof515–560nm,correspondingtothe2H11/2,4S3/2→4I15/2transitions,whiletheredemissionpeaksareobservedbetween640and680,correspondingtothe4F9/2→4I15/2transitions.
At13K,the2H11/2uorescenceintensityisnegligible,anditprogressivelyincreaseswithincreas-ingtemperature.
Thisisarepresentativebehaviorofpro-cessesinvolvingthethermallycoupled4S3/2and2H11/2asreportedelsewhere.
910Theupconversionluminescencemechanismandpopula-tionprocessinEr3+/Yb3+-codopedsystemsarepresentedFig.
3.
Upconversionemissionspectraofhexagonal-phaseNaYF4:Yb3+,Er3+nanocrystalsat13,190,and300K.
2J.
Nanosci.
Nanotechnol.
10,1–4,20101826J.
Nanosci.
Nanotechnol.
10,1825–1828,2010RESEARCHARTICLEWangetal.
BrightGreenUpconversionFluorescenceofYb3+,Er3+-CodopedNaYF4NanocrystalsinFigure4.
Following980nmirradiation,theEr3+ionisexcitedtothe4F7/2stateviatwosuccessiveenergytrans-fersfromtheYb3+ionsinthe2F5/2state.
Thus,oneYb3+ionwilltransferitsenergytoanEr3+ioninthegroundstate,therebyexcitingittothe4I11/2state.
ThisprocessisfollowedbyatransferofenergyfromanotherYb3+ionalsoinitsexcitedstate,resultinginthepopulationofthe4F7/2stateoftheerbiumion.
Theloweremittinglevelsarethenpopulatedviamultiphononrelaxation,andgreenandredemissionsarethenobserved.
Ofcourse,interactionsbetweentwoEr3+ionscannotnecessarilybeignored.
Anearinfrared(NIR)photonfromthepumpbeamwillalsoexciteanEr3+iontoits4I11/2state.
AnotherEr3+ionalsointhe4I11/2stateandincloseproximitywilltransferitsenergytotheinitialion,therebyexcitingittothe4F7/2state.
However,followingtheadditionofYb3+ions,thisprocessisgreatlydiminishedduetothelargeabsorptioncross-sectionoftheytterbiumions.
Thus,thesimultaneoustransferofenergyfromYb3+,whichpopulatesthe4F7/2state,isdominant.
Thedependenceoftheupconversionefciencyupontemperaturewasexamined,andtheresultsaredepictedinFigure5.
Ascanbeinferredfromexperimentaldata,thevisibleemissionefciencyincreasedbyafactorof4whenthesamplewascooledfromroomtemper-atureto160K.
Thisbehaviorreectsthecompetitionbetweenthetwomajorthermaleffectsactinginthesys-temasfollows.
Onethermaleffectisthetemperature-enhancedsidebandanti-Stokesexcitationofthesensitizer,whichisresponsiblefortheincreaseintheupconver-sionemissionefciency.
Theexpressionoftheabsorp-tioncross-sectionrelatedtothetemperatureiswrittenasT=0exp/kBT1E/,where0istheabsorptioncrosssectionat0Kandisthephotonenergy.
Theotherthermaleffectisthethermallyenhancednonradiativedecayratethatinhibitsradiativeemission.
ThenonradiativetransitionprobabilitybetweenlevelsforlowconcentrationsofEr3+ionsisduetomultiphononFig.
4.
SchematicdiagramofYb3+-sensitizedEr3+upconversionunder980nmexcitation.
Thesolidlinesrepresentradiativetransitions,andthedashedlinesrepresentnonradiativetransitions.
relaxationprocesses,andcanberelatedtothetemperaturethroughWNR=WNR01exp/kBTE/,1112whereWNR0isthenonradiativerelaxationrateat0K.
ItalsocanbeseenfromFigure5(b)thatthegreenemissionfrom4S3/2decreasesfasterthantheredemis-sionfrom4F9/2above180Kastemperatureincreases.
Asmentionedabove,therearetwomainfactors.
Onefactoristhethermalquenchingeffectofluminescence.
Theinter-mediatelevelofthegreenemissionis4I11/2,whilethatoftheredemissionis4I13/2.
Fortheredaswellasthegreenemission,thenonradiativerelaxationof4I11/2→4I13/2and4S3/2→4F9/2isinvolved,whichincreasestheelectronpopulationon4F9/2whiledecreasingthaton4S3/2.
Thatmakestheredemissionsdecreaseslowerthanthatofgreenemissions.
Asiswellknown,thenonradiativerelax-ationprobabilityissize-dependent.
Astheparticlesize(a)(b)Fig.
5.
(a)VisibleUCluminescencespectrainNaYF4:Yb3+,Er3+nanocrystalsatdifferenttemperatures.
(b)Theupconversionemissionintensityof4S3/2→4I15/2,4F9/2→4I15/2inNaYF4:Yb3+,Er3+nano-crystalsasafunctionoftemperature.
J.
Nanosci.
Nanotechnol.
10,1–4,20103J.
Nanosci.
Nanotechnol.
10,1825–1828,20101827RESEARCHARTICLEBrightGreenUpconversionFluorescenceofYb3+,Er3+-CodopedNaYF4NanocrystalsWangetal.
decreases,duetotheincreaseofthesurface-to-volumeratiocomparedwiththebulk,alargenumberofNO23andOHareinvolved,withvibrationalenergiesof1350and3350cm1,respectively,andhaveavailablelargevibrationalquantatoefcientlydepopulatetheexcitedstates4I11/2and4S3/2nonradiatively.
1314Thus,thenon-radiativerelaxationbecomesmoreefcientthanthermalexcitation.
Theotherfactoristhethermallyactivateddis-tributionofelectronsamong4S3/2.
Someofelectronscanbethermallyexcitedinto2H11/2from4S3/2asthetemper-atureincreases.
Thatalsodecreasesthegreenemissionsfrom4S3/2.
4.
SUMMARYInconclusion,Yb3+,Er3+-codopeduoridenanocrystalsweresynthesized.
Thesecrystalsdemonstratehighcrys-talqualityandexcellentdispersivityandcanbetrans-parentlydispersedinnonpolarsolvents.
Atthesametime,thetransparentcolloidalsolutionshowsefcientinfrared-to-visibleUCemission.
Thegreenupconversionluminescenceexhibitedafour-foldintensityenhancementwhenthetemperatureofthesamplewasvariedintherangebetween20Kand300Kwiththemaximumintensityattainedaround160K.
Theexcellentlumines-cenceofhexagonal-phaseNaYF4:Yb3+,Er3+nanocrystalsprovidesthebasisforamorethoroughinvestigationoftheiropticalandtemperaturepropertiesandmightenableapplicationsinnanoscalebiologicalimagingordisplays.
Acknowledgments:ThisworkwassupportedbytheNationalNaturalScienceFoundationofChina(NNSFC)(grants10874058and50672030).
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Received:9September2008.
Accepted:5April2009.
4J.
Nanosci.
Nanotechnol.
10,1–4,20101828J.
Nanosci.
Nanotechnol.
10,1825–1828,2010