【doc】 由ZnO胶体球生长光子晶体(英文)
由ZnO胶体球生长光子晶体(英文)
:H?049026756(2005)0z220183204
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PhotonicCrystalsGrowthFromZnOColloidalSpheres
XIEJuan
3
,DENGHong,XUZi2qiang,LIYan,HUANGJun
(SchoolofMicroelectronicsandSolidStateElectronics,UniversityofElectronic
ScienceandTechnologyofChina,Chengdu610054,China)
Abstract:AnovelmethodforthesynthesisofmonodisperseZnOcolloidalspheresispresented.
Thesesphereswereself2assembledintophotoniccrystals.Themicrostructureswerecharacterized
bythescanningelectronmicroscope.Theresultsshowthatthephotoniccrystalsexhibitaclear
bandgapinspecularreflectivitymeasurements.Thepositionoftheobservedr
eflectancepeak
scalesremarkablywellwiththediameterofthespheres,indicatingtherewasanintrinsicfeature
ofthephotoniccrystals.Thepositionsofthebandgapareshiftedtotheshorterwavelengthwith
decreaseofZnOparticlessize.
Keywords:zincoxide;photoniccrystals;colloidalspheres;self2assembly;blue2shift
CLCnumber:TN304.2+
1H?H?H?Documentcode:A
1H?Introduction
Recently,photoniccrystals(PCs)haveattractedmuchattentionfrombothfundamentalandpractical
pointsofview[1]
.PCsareperiodicdielectricstructuresdesignedtocontrolthepropagationofelectromagnetic
(EM)wavesbydefiningallowedandforbiddenenergybandsinthephotondispersionspectrum[2]
.Oneofthe
characteristicpropertiesofPCsisthepresenceofaPBG.AtthePBGwavelength,propagationofphotonsis
inhibitedinsidethePCstructure.Thisopensintriguingperspectivesinphoton
icapplicationslikeopticalfibers
orhighbendinganglewaveguides[3]
.However,itisagreatchallengetofabricatethesePCswithaPBGat
opticalandnear2infraredfrequencies,andtoshowasignificanteffectonthespontaneousemissionrate.Nu2
meroustechniqueshavebeendevisedinanefforttoproducetheseperiodicarrays
[4]
.Manytechniqueshave
beenusedtofabricatePCs,suchaslithography,top2downGcapproach.However,theseapproachesarerela2
tivelyexpensiveandtime2consumingprocesses.Incontrast,chemicalassemblyofordereddielectricsprovides
asimplerandlesscostlyapproachtogeneratinglarge2scalePCs.Thesetechniquesmakeuseofthespontaneous
assemblyofsubmicronparticlesintospecificwell2orderedlatticestructures
[5]
.ZnOisanimportantwide2
bandgapsemiconductormaterial,whichhasahigherrefractiveindex(2.1??2.2inthevisibleregime)than
othermaterials(1.4??1.5forSiO2andmostpolymers).Therefore,ZnOisanotherpromisingcandidatefor
opticallyactiveself2assembledphotoniccrystals.However,littleresearchhasreportedonZnOPCs
[6]
.
H?H?Inthispaper,anovelmethodforsynthesisofZnOcolloidalspheresispresented.Inaddition,wehave
proposedatunablePC,inwhichthePBGcanbetunedasdesiredbycontrollingparameterssuchasZnOcol2
2005??10?(
?J42?????)2
?????p??????(???A?????-)
JournalofSichuanUniversity(NaturalScienceEdition)
Oct.2005
Vol.42H?Issue2
:9711009409007
:.
Email:hdenguestc.edu.cn.
loidalspheressize.ThereflectancespectraofthePCshiftsystematicallywiththespheressize,providingevi2
denceofphotoniccrystaleffects.
2H?Experimental
ThemethodofsynthesisZnOcolloidalspheresinvolvestwosteps.Inaprimar
yreaction,zincacetatede2
hydrate(ZnAc,0.02mol.)wasaddedinto150mLdiethyleneglycol(DEG).Thisreactionsolutionwas
heatedunder180w4.Afterreachingtheworkingtemperature,precipitationofZnOspheresoccurred.Thein2
termediateproductwasplacedinacentrifuge.Thepolydispersepowderwasdiscardedandthesupernatant
(DEG,dissolvedreactionproducts,andunreactedZnAcandwater)wasusedinthesecondstep.Asecondary
reactionwasthenperformedtoproducethemonodisperseZnOspheres,whichbeganinthesamewayasapri2
maryreaction.ZnAc(0.02mol.)wasaddedinto150mLDEGandthereactionsolutionwasheatedunderre2
flux.Priortoreachingtheworkingtemperature(at170w4),differentvolumeoftheprimaryreactionsuper2
natantwasaddedintothesolution,3mL,5mL,8mL,10mL,15mL,20mL,respectively.Following
thisaddition,therewasatemperaturedrop,andprecipitationwouldtypicallyoccuratalowertemperature
thanwithoutsuchanaddition.Afterreaching180w4,thereactionwasstirredforonehour.ThentheZnO
colloidalsuspensionwasprepared.ThesizeoftheZnOcolloidalsphereswere
controlledbyvaryingtheamount
ofsupernatantadded.
H?H?PhotoniccrystalswereproducedfromtheZnOcolloidalspheresusingaself2assemblysedimentationpro2
cess.Thereactionsolutionwasdroppedontoaglasssubstrateattemperature,rangedfrom30w4to300w4,
andasthesolventevaporated,theparticlesspontaneouslyassembledintoperiodicarrayswithdifferentparticle
sizes.
ThesurfaceoftheZnOPCswasobservedwiththescanningelectronmicroscope(SEM,JSM25900LV)
andtheopticalpropertieswereanalyzedbyUV2VIS2NIRdoublebeamspectrophotometer(UV22550,SHI2
MADZU).
3H?Resultsanddiscussion
TheSEMimagesinFig.1revealthattheorderedstructuresofZnOsynthesizedusingthistechnique.
Andthesizeofthecolloidalspheresvariesinverselyandmonotonicallywiththeamountofprimarysuper2
natantadded.TheSEMimagesindicatethatthecolloidalspheresinthefilmarehighlyorientedwiththe
FH?SMfZOy
2fx22
yyTfyy
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ig.1Emicrographsonphotoniccrstal
closepackedplanesparalleltothesubstrate.Preerentialorientationalsoeistsintheclosepackedplaneprob
ablduetostressesdevelopingduringthedringprocess.hisalignmentocolloidalspheresmaprovever
usefulforapplicationsandmeasurementsespeciallyincaseswhereafullphotonicbandgapdoesnotexistinall
directionsofpropagationinthecrystal.
ThemajoropticalsignatureofZnOPCsisfoundinreflectancemeasurements.Thespecularreflectanceat
near2normalincidencefromourorderedstructuresisshown[Fig.2]fordifferentsizesofcolloidalspheres.
TheinitialspheresizesweremeasureddirectlyfromSEMimagesoftheorderedstructuresofZnOspheres.
Theprominentfeatureisaspecularreflectivitypeakforeachstructure,whichsystematicallyshiftsfrom
400to900nmovertherangeofphotoniccrystals.Thewavelengthofthespecularpeakcorrespondsverywell
tothesizeofZnOcolloidalspheres.Thewavelengthsofthereflectivitypeaksc
overtheentirevisiblespectrum
andextendwellintothenearinfraredspectrum.Inaddition,thereisagradualbutfeaturelessincreaseinre2
flectivityatlongerwavelengthsinseveralsamples[Fig.2].ThisisduetotheroughsurfaceofthePCGcsap2
pearingsmootherwhenprobedatlongerwavelengths.Thisincreasesthespecularreflectivity,withanaccom2
panyingdecreaseofthediffusereflectanceatlongerwavelengths.
Fig.2H?TransmissionspectraforperiodiclayersofZnOspheresofvarioussizes
Thepositionoftheobservedreflectivitypeakscalesremarkablywellwiththediameterofthespheres
[Fig.3],indicatingthatitisanintrinsicfeatureofthephotoniccrystals.Withthedecreaseofthesizeofcol2
loidalspheres,thereflectancespectraofthesesorderedstructuresblueshift.Thisisanobservationoftheop2
ticalsignatureofaphotoniccrystaltogetherwiththerequiredscalingwithspheresize,whichhasbeenseenin
sometypicalPClikepolystyrene[7]
.
4H?Conclusion
Thesuccessinthegrowthoflarge2areaZnOopticalphotoniccrystalsusingra
pid,economical,andsimple
techniqueswillopenthewaytowardstheexperimentalobservationofmanyinterestingeffectsinvolvingthe
controloflightemissionandpropagationinthesematerials.Thespecularreflectanceexperimentsyieldedor2
deredcrystallinestructure.Itisfoundthatwiththedecreaseofthesizeofcolloidalspheres,thereflectance
spectraofthesesorderedstructuresblueshift.
Acknowledgements
TheauthorswishtothanktheChineseNatureScienceFundamentalCommittee(60390073)andNation2
BRf(53Z)ff
581???)2?F?l?/:?gZnO?????4?l???/??????
alasicesearchProgramoChina11009orinancialsupport.
References:
[1]H?ChabanovAA,JunY,NorrisDJ.Avoidingcracksinself2assembledphotonicband2gapcrystals[J].Appl.Phys.Lett,
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