Low Temperature Annealed Zinc Oxide Nanostructured Thin Film-Based Transducers

20 Pages·2015·2.99 MB·English

RESEARCHARTICLE
Low Temperature Annealed Zinc Oxide
Nanostructured Thin Film-Based Transducers:
Characterization for Sensing Applications
R.Haarindraprasad1,U.Hashim1*,SubashC.B.Gopinath1,2,MohdKashif1,
P.Veeradasan1,S.R.Balakrishnan1,K.L.Foo1,P.Poopalan3
1 BiomedicalNanoDiagnosticsResearchGroup,InstituteofNanoElectronicEngineering(INEE),Kangar,
Perlis,Malaysia,2 SchoolofBioprocessEngineering,UniversityMalaysiaPerlis(UniMAP),Kangar,Perlis,
Malaysia,3 SchoolofMicroelectronicEngineering,UniversityMalaysiaPerlis(UniMAP),KualaPerlis,Perlis,
Malaysia
* [email protected]
Abstract
OPENACCESS Theperformanceofsensingsurfaceshighlyreliesonnanostructurestoenhancetheirsen-
sitivityandspecificity.Herein,nanostructuredzincoxide(ZnO)thinfilmsofvariousthick-
Citation:HaarindraprasadR,HashimU,Gopinath
nesseswerecoatedonglassandp-typesiliconsubstratesusingasol-gelspin-coating
SCB,KashifM,VeeradasanP,BalakrishnanSR,et
al.(2015)LowTemperatureAnnealedZincOxide technique.Thedepositedfilmswerecharacterizedformorphological,structural,and
NanostructuredThinFilm-BasedTransducers: optoelectronicpropertiesbyhigh-resolutionmeasurements.X-raydiffractionanalyses
CharacterizationforSensingApplications.PLoSONE
revealedthatthedepositedfilmshaveac-axisorientationanddisplaypeaksthatreferto
10(7):e0132755.doi:10.1371/journal.pone.0132755
ZnO,whichexhibitsahexagonalstructurewithapreferableplaneorientation(002).The
Editor:YogendraKumarMishra,Institutefor
thicknessesofZnOthinfilmspreparedusing1,3,5,and7cyclesweremeasuredtobe40,
MaterialsScience,GERMANY
60,100,and200nm,respectively.Theincrementingrainsizeofthethinfilmfrom21to52
Received:March25,2015
nmwasnoticed,whenitsthicknesswasincreasedfrom40to200nm,whereastheband
Accepted:June17,2015 gapvaluedecreasedfrom3.282to3.268eV.BandgapvalueofZnOthinfilmwiththickness
Published:July13,2015 of200nmatpHrangingfrom2to10reducesfrom3.263eVto3.200eV.Furthermore,to
evaluatethetransducingcapacityoftheZnOnanostructure,therefractiveindex,optoelec-
Copyright:©2015Haarindraprasadetal.Thisisan
openaccessarticledistributedunderthetermsofthe tricconstant,andbulkmoduluswereanalyzedandcorrelated.Thehighestthickness(200
CreativeCommonsAttributionLicense,whichpermits nm)ofZnOfilm,embeddedwithaninterdigitatedelectrodethatbehavesasapH-sensing
unrestricteduse,distribution,andreproductioninany
electrode,couldsensepHvariationsintherangeof2-10.Itshowedahighlysensitive
medium,providedtheoriginalauthorandsourceare
credited. responseof444μAmM-1cm-2withalinearregressionofR2=0.9304.Themeasuredsensi-
tivityofthedevelopeddeviceforpHperunitis3.72μA/pH.
DataAvailabilityStatement:Allrelevantdataare
withinthepaperanditsSupportingInformationfiles.
Funding:TheauthorswouldliketothankUniversiti
MalaysiaPerlis(UniMAP),andMinistryofHigher
EducationMalaysiaforthefinancialsupportthrough
MTUN-COEgrant9016-00004toconductthe Introduction
research.Thefundershadnoroleinstudydesign,
datacollectionandanalysis,decisiontopublish,or Sensitivityistheprimedeterminingfactorofthequalityofsensors,andtheconstructionof
preparationofthemanuscript. nanostructureonthesensingsurfacecreatesanavenueforhigh-performancesensing[1].
Amongdifferentsensorsproposed,pHsensorhasbeenconsideredasmandatorytomaintain
CompetingInterests:Theauthorshavedeclared
thatnocompetinginterestsexist. thequalityoffoodbymeasuringtheaciditylevels,whichwillpreventfoodbornepathogens.
PLOSONE|DOI:10.1371/journal.pone.0132755 July13,2015 1/20
ZnONanostructure-MediatedSensingApplication
Previously,pHsensorshavebeenfabricatedviavarioustypesofelectrodesystembyimplement-
ingconventionalstrategy.Inordertoovercomesomelimitations,suchasrigidandbulksize
structureofglasselectrode,researchersdevelopedsmallerandrobustelectrodesystems,which
includesIon-SensitiveField-Effecttransistor(ISFET)andextended-gatefield-effect-transistor
(EGFET)[2,3].However,thecomplexityofthesenon-interdigitatedelectrodedevicefabrications
aswellaselectricaloperationsincreasesthecost,duetotheusageofmultiplemasksets,which
greatlyincreasesfabricationprocesses.Furthermore,ZnOintegrationencounterstheproblems
withconventionalCMOSprocessesandpossiblecontaminationofprocessequipment.Addition-
ally,thesethreeterminaldevicesrequirecomplexbiasingarrangementsincomparisontocapaci-
tiveinterdigitatedelectrode(IDE).AnattempthasbeenmadetodevelopIDEforpHsensing
applicationsinceIDEhasbecomepromisingfordevelopingbiosensorsystemduetoitshigher
stability,robustandsimplicity.Inaddition,ultra-lowvolumeofsolutionisrequiredformeasure-
mentsinIDEcomparedtootherelectrodesystemssuchasglasselectrode,ISFETandEGFET.
Zincoxide(ZnO)hasbecomeapopularchoiceforthedevelopmentofsensingplatforms
withvariousapplications,suchasphotovoltaics,solarcells,andoptoelectronicbiosensors[4–
7]duetoitsappealingcharacteristicssuchasalargebandgapof3.37eV[8]andbiocompati-
bility[9,10].IntensivereviewsindicatethatZnOhasbecomeanemergingcontenderforenvi-
ronmentalapplicationswiththeconsiderationsofreducedcost,non-toxicity,andhigh
reactivity.StudiesthatfocusedonthemorphologicalaspectsofZnOmaterialspreparedbythe
sol-gelmethodhaveledtonovelapplications.ZnOhavebeenattestedbyresearcherstobean
idealmaterialformakingnanostructures,duetotheireaseofsynthesis,resultinginahighcrys-
tallinitywithafewstructuraldefectsusingalow-temperatureprocess.ZnOalsopossesses
excellentelectricalcharacteristicsthatcanbetailoredforafastandaccuratesensorresponse
[11,12].ThebiocompatibilityofZnOmakesitconvenientforsurfacemodificationandinter-
facingwithchemicalandbiologicalcompoundsatextremepHconditions.
TheZnOseedsolution,whichisindexedtothewurtzitestructureassociatedwithhighcrystal-
linity,hasahightendencytosynthesizequasione-dimensionalnanowiresviabottom–up
approaches,suchasthevapor-liquid-solidmechanism,pulsedlaser,flametransfersynthesis,RF
magnetronsputteringandhydrothermalmethods[13–17].PriortotheformationofZnOnano-
structures,studiesonthecrystallographicandmorphologicalpropertiesofZnOthinfilmsarenec-
essarytoanalyzethevariousparametersoftheseedsolutionthataffectthethinfilmformation.
TheparametersrelevanttothesynthesisofZnOthinfilmsincludethethickness,pre-
annealingandpost-annealingtemperaturesmaintainedduringprocessing,sol-seedconcentra-
tion,andannealingperiod.Inthisstudy,aZnOseedsolutionissynthesizedviathesol-gel
method,andthethicknessoftheseedsolutionistunedtoinvestigatetheoptoelectronicand
structuralpropertiesofZnOthinfilmsthatarecoatedonglassandsiliconsubstrates.Further-
more,wecharacterizedtheopticalandbulkmoduluspropertieswithrespecttothicknessto
evaluatethetransducingcapacity.TheeffectofopticalpropertiesofZnOthinfilm(200nm)at
differentpHconditionshavebeencharacterizedbymeasuringthebandgapofthemembranous
ZnOthinfilm.WemadeanapproachofdevelopinganoptimizedZnOthickness,fortwoelec-
trodedetectionscheme(IDE)withsimplecurrentdetection.Withaviewforhigh-performance
sensingapplications,theelectricalbehaviorsonZnOthinfilmsofdifferentthicknessesunder
variedpHsfrom2to10wereinvestigated.
MaterialsandMethods
ConsumableMaterials
ZnOseedsolutionwaspreparedusingzincacetatedehydrate[Zn(CH3COO) .2H O](98%;
2 2
Sigma-Aldrich)asaprecursor.Isopropylalcohol(IPA;99.8%)wasfromSigma-Aldrich.
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ZnONanostructure-MediatedSensingApplication
Monoethanolamine(MEA;99%;Merck)wasusedasastabilizer.Siliconwafers(100)werepur-
chasedfromFilmetrics,andglassslideswerepurchasedfromFisherScientific.
SilverInterdigitatedelectrode(IDE)
AsilverIDEelectrodewasdepositedonthesiliconwafer<100>usingthetraditionalwetetch-
ingmethod.Apositivephotoresist(PR)wascoatedonthesiliconwaferfollowedbysoftbak-
ingfor90sec.UVlightwasappliedfor10sectopatterntransfertheIDEmask.The
developmenttook15secusingRD-6developer,andthesamplewasbakedat110°Ctoremove
excessmoistureandenhancetheadhesionforcebetweenthesilverandSiO layers.Theunex-
2
posedareawasremovedbysilveretchantappliedfor23sec,andthesamplewasthencleaned
withacetone.
ZnOsynthesis
TheZnOseedsolutionwassynthesizedaspreviouslyreported[18–20].Briefly,1632mgof
zincacetatepowderwasdilutedin40mlofisopropylalcoholtoobtain0.2MZnOseedsolu-
tion.Theseedsolutionwasthenstirredat1000rpmfor20minat60°C.Thevolumesofthesol
stabilizerandmonoethanolamine(MEA)weremeasuredtoensurethattheratiobetweenthe
solventandstabilizerwas1:1sinceF.Boudjouanetal.hasstatedthatthemostoptimumratio
ofMEAsolventis1:1forsynthesizingZnOsolgelsolution[21].Afterstirringthesolventfor
20min,theMEAwassubsequentlyaddedfor120minuntilaclearandtransparentsolution
appeared.ThepreparedZnOseedsolutionwaslefttoageatroomtemperaturefor24h.
ZnOseeding
TheZnOseedsolutionwascoatedonp-typesiliconandglasssubstratesofdimensions2x2
cm2.Bothsubstratetypesunderwentaninitialstandardcleaningprocesspriortocoatingwith
theZnOseedsolution.ThepreparedZnOseedsolutionwascoatedviathespin-coating
method.Thecoatedsampleswereheatedat60°Cfor20min,atpointwherethetemperature
wasraisedto150°Candkeptconstantfor10minbeforecoolingto50°C.Thesampleswere
heatedaftereachcoatingtoeliminateresidualmoistureandenhancetheadhesionbetweenthe
surfaceofthesubstrateandtheZnOseedsolution.Afterthecoating,allsamplesunderwentan
annealingprocessat300°Cfor2h.
CharacterizationofnanostructuredZnOthinfilms
ThemorphologicalcharacterizationofthenanostructuredZnOthinfilmswasconducted
usingfieldemissionscanningelectronmicroscopy(FESEM;CarlZeissAG-ULTRA55,Gem-
ini).ThestructuralanalysiswasconductedviaX-raydiffraction(XRD,BrukerD2Phaser).
Atomicforcemicroscopy(AFM;SPA400-SPI3800)wasutilizedtoexaminetheroughnessof
thefilmsurfaces.Theopticalpropertieswereexaminedbyultraviolet-visiblespectroscopy
(UV-vis,Lambda35,PerkinElmer),andphotoluminescence(PL)spectroscopy(PL,Horiba
Fluorolog-3,HORIBAJobinYvonInc;USA)wasusedtoobservetheeffectofthethicknesson
theopticaltransmittanceandcrystaldefects.Theelectricalcharacterizationwasconducted
usingaKeithley2400sourcemeter.
ResultsandDiscussion
Analysis
Azincoxide(ZnO)thinfilmwassynthesizedbyasimplechemicalroute(sol-gel)andopti-
mizedtoimproveitsinternalandsurfacematrixesforsensingapplications.Thecharacteristics
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ZnONanostructure-MediatedSensingApplication
Fig1.StructuralandgrainsizeanalysisofmatrixofZnOthinfilm.X-RaydiffractionanalysesonZnO
thinfilms.
doi:10.1371/journal.pone.0132755.g001
ofthethinfilmweremodifiedbythedepositionofdifferentthicknessesonthesurfaceofthe
sensingelectrode.Thestructural,morphological,opticalandelectricalchangesuponvarying
thethicknessesoftheZnOthinfilmwerecriticallyobserved.
Structuralanalysis
ZnOthinfilmsofdifferentthicknesseswereobservedbyX-raydiffraction(XRD)analysis,and
preliminaryinvestigationswereevaluatedforthegraincrystalsize,graingrowthorientation
andcrystallinequalityofthematrix.Fig1displaystheXRDpatternsofZnOthinfilmsofdif-
ferentthicknesses.TheXRDspectrashowsthatallfilmsconsistof(100),(002),and(101)
planepeaks.ThepresenceofthesepeaksindicatethatalldepositedfilmshaveonlyZnOmate-
rialsthatareindexedbyahexagonalwurzitestructure,whichisingoodagreementwiththe
standarddiffractionpattern(JCPDScard#36–1451).
Asthefilmthicknesswasincreased,theintensityofthediffractionpeaks[(100),(002),and
(101)]werealsoincreased,revealingthatthecrystallinitywasincreasedinthethinfilmdueto
thepreferentialgrowthofZnOalongthec-axis[22].TheScherrerequationwasusedtocalcu-
latethecrystallitesizeoftheas-preparedfilms:
kl
D¼ ð1Þ
FWHMcosy
where,κistheScherrerconstant,withavalueof0.9;λrepresentstheincidentX-raywave-
length;FWHMisthefull-widthathalf-maximumoftherespectivepeak;andθrepresentsthe
diffractionpeakangle.
Thegrainsizewasincreasedwiththenumberofcoatingcycles,asshowninTable1.The
grainsizeoftheseedsolutionwascalculatedbasedonthehighestpeakorientation.Thus,
Table1. Calculatedgrainsize,fullwidthathalfmaximum(FWHM),andstrainofZnOfilmswithdifferentthicknesslayersusingBraggequation.
ZnOthinfilmthickness(nm) Grainsize(nm) FWHM(°) a(Å) c(Å) Strain, ε ð%ÞC(cid:2)Co x100
Co
40 21.00 0.51 3.257 5.641 8.356
60 41.60 0.10 3.251 5.631 8.164
100 43.00 0.08 3.250 5.629 8.125
200 52.00 0.02 3.246 5.622 7.991
doi:10.1371/journal.pone.0132755.t001
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ZnONanostructure-MediatedSensingApplication
calculationswereconductedonthe(002)orientationbasedontheXRDdata.Thecrystallinity
oftheZnOthinfilmwasenhanceduponincreasingthefilmthicknesses,ascalculatedbythe
reductionoftheFWHMvalue[23].Thestrainpercentage[ε(%)]wasdecreasedwithincreas-
ingfilmthicknesses,whichismainlyduetolowtemperature(200°C)andlongerannealing
time.Consequently,itisalsosuspectedthatitcausethereductioninRMSsurfaceroughness.
Anincreaseinthefilmthicknessleadstoreducedsurfaceroughness,whichismoresuitablefor
thefunctionofatransducerinoptoelectronicapplicationsduetoitsuniformity.Strain,ε(%)
wascalculatedbasedonthefollowingequation:
C(cid:2)C
o x100% ð2Þ
C
o
whereCisthelatticeconstant,whichwascalculatedusingBragg’sequation,andCoisthelat-
ticeparameterforbulkZnO,whichhasafixedvalueof0.5206nm[24].
MorphologicalAnalysis
MorphologicalobservationswereconductedoncoatedZnOthinfilmsofvariousthicknesses.
FESEMcharacterizationwascarriedouttoinspectthegraingeometry(shapeandsize)ofthe
bipolarmatrixintheZnOthinfilm.Theuniformityandsurfacesmoothnessofthecoated
ZnOthinfilmwasalsoobservedbyAFMcharacterization.
Fig2showsFESEMimagesofZnOthinfilmsofdifferentthicknessesthatwerecoatedon
siliconsubstrates.TheaveragegrainsizeoftheZnOthinfilm,asobservedfromtheFESEM
image,wasintherangeof10to50nm.TheFESEMimagesrevealedthatthethinfilmsurface
showedspherule-shapedstructures,whichisinagreementwiththeresultsofBerrinand
Sumer[24].Thespheruleshapeoftheseedremainsunchangedwithadditionalcoatingcycles
ontheglasssubstrate.However,Aihuaetal.reportedthatthegrainshapeoftheZnOchanged
fromhexagonalsheet-liketowedge-shapedwiththeincreasingthicknessofthethinfilm[23].
Ourresultsaredifferentmainlybecauseoflowtemperatureandlongannealingtimewereused
inthecurrentprocess.Whereastheyuseddifferentcoatingmethod,andduetotheinvolve-
mentofN-AldopingintheZnOthinfilms,therewasachangeintheshapeoftheseededlayer.
Onthecontrary,withthecurrentbaking-annealingprocess,thegrainsizeoftheseedwas
enhancedwithincreasingcoatingcycles.
Fig2.MorphologicalinspectionofZnOthinfilmsofdifferentthicknesses.Geometricalgrainshape
analysisandsizeinspectionwereconductedtoobservetheeffectofdifferentthicknesses.FESEMimagesof
ZnOthinfilmscoatedwithdifferentthicknesses(a)40nm,(b)60nm,(c)100nmand(d)200nm.
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ZnONanostructure-MediatedSensingApplication
Fig3.Surfaceanalysisofcoatedthinfilm.Thesurfaceuniformityandroughnesswereinvestigatedby
AFM.The2DAFMimagesofZnOthinfilmsofdifferentthicknessesareshown.
doi:10.1371/journal.pone.0132755.g003
Thegrainwasseenasbright-whitestructureonthelowerrightcornerofFig2B,wasdueto
differencedegreeofcontrastinFESEMimage.Thegrainsizewasincreasedafterundergoing
severalcoatingcyclesbecauseoftheenhancedcoalescencebetweengrainsoftheZnOthin
film.Fig2A–2DdepictsthattheFESEMmagnificationataconstantlevel(150,000X)forall
fourfilmthicknessesproducedincreasinggrainsizeswithfilmthicknesses.Thisalsoleadsto
comparativeincrementintrapsites[25].
Theuniformityofthethinfilmwasimprovedwithincreasingcoatingcycles,whichisin
agreementwiththeAFMresultdisplayedinFig3.Thereducedsurfaceroughnessofthecoated
thinfilmisoptimalforvariousapplications,suchasphotovoltaicandsolarcells,becauseof
theirenhancedabilityforphotonabsorption[26].Fig3shows2D-AFMimagesofnanostruc-
turedZnOthinfilmsofdifferentthicknesses.AFMimageswerecapturedintappingmode
overanareaof500x500nmatascanningspeedof2MHz.Fig3alsodisplaysthatthegrain
particlesofthecoatedthinfilmexhibitchangesinsizeanddistributionwithincreasingthick-
nessesofthethinfilm.Thesurfaceuniformityofthethinfilmvariesnon-monotonically.The
AFMimagesshowedthatthethicknessofthethinfilmandthearrangementofthegrainparti-
clesstronglyaffectthesurfaceuniformityofthethinfilm.Thesurfaceuniformityisdominated
bythegrainparticleduetograincoordination.Thus,asreportedbefore,asuitablethinfilm
thicknessisneededforfurtheroptimization,togetherwithasynthesisofgoodsurfaceunifor-
mity[27].
Fig4showstherelationshipbetweenthethicknessofaZnOthinfilmcoatedonaglasssub-
strateandtherootmeansquare(RMSsurfaceroughness)valueofthesurface-coatedglasssub-
strate.TheRMSsurfaceroughnessvaluedecreaseswithincreasingthicknessofthethinfilm,
indicatingthatthedegreeofthesurfaceroughnessdecreasedbecauseoftheincrementinthe
grain-sizeparticlesoftheZnO[28].Xuetal.reportedsimilarresultsregardingtherelationship
betweenthethicknessoftheZnOthinfilmandtheRMSsurfaceroughnessvalues[29].
Ghayouretal.statedthatanincreaseinthethicknessofaZnOthinfilmwillcauseareduced
roughnessfrequency[30].
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ZnONanostructure-MediatedSensingApplication
Fig4.SurfaceroughnessmeasurementonZnOthinfilm.ThicknessofZnOthinfilmversusRMSsurface
roughnessvalue.
doi:10.1371/journal.pone.0132755.g004
Fig5shows3DimagesofcoatedZnOthinfilmsthatunderwentvariouscoatingprocesses.
Whenthecoatingcyclesontheglasssubstratewereincreased,thethicknessesofthethinfilm
werealsoincreased.Fig5alsoshowslargenumbersofgrainparticlesatthelowthicknesslayer
oftheZnOthinfilm;thus,thenumberofgrainparticlesdecreaseduponincreasingthethick-
nessofthethinfilm.Whenthenumberofcoatingcyclesincreasedfrom1to7,theZnOthin
filmsurfaceuniformitywasgraduallyimproved.
Opticalanalysis
ThepreparedZnOseedsolutionwascoatedonglasssubstratestoinvestigatethetransmission
andabsorptionperformancesbyUV-visspectroscopy.Thequalityofthethinfilmwasinvesti-
gatedusingphotoluminescence(PL)atroomtemperature.Thestructuraldefectsintheinter-
nalmatrixoftheZnOthinfilmcanbeidentifiedthroughPLcharacterization.Fig6showsthe
Fig5.Thicknessmeasurementsofthinfilmsundergoingdifferentnumbersofcoatingcycles.3-D
AFMimageofsurfaceofglasssubstratecoatedwith1,3,5,and7coatinglayersofZnOseedsolution.
doi:10.1371/journal.pone.0132755.g005
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ZnONanostructure-MediatedSensingApplication
Fig6.OpticalcharacterizationofthinfilmstoevaluatetheirperformancesforUVtransmission
application.InsertshowsthemagnifiedscaleoftransmissionspectraofZnOthinfilmsofvarious
thicknesses.
doi:10.1371/journal.pone.0132755.g006
effectofthicknessonthetransmissionspectrumoftheZnOthinfilm.TheZnOthinfilm
showedexcellenttransmissionpercentage,whichismorethan80%.Thetransmissionpercent-
ageofZnOthinfilmreducesasthethicknessofthethinfilmincreasesfrom40to200nm.The
standardBeer'sLawequationexplainsthereductionoftransmittanceofZnOthinfilmwith
increasingthicknessofZnOthinfilm:
I ¼I e(cid:2)ad ð3Þ
o
whereαistheabsorptionco-efficientanddisthefilmthickness.
Thefollowingequationisusedtodeterminetheabsorptioncoefficient(α):
lnð1=TÞ
a¼ ð4Þ
d
whereTrepresentstransmittanceofZnOfilmsanddrepresentsthefilmthicknessofZnOthin
film.
Thereductioninthetransmissionwithincreasingfilmthicknessmaybeattributedtothe
effectonthesurfaceroughnessandgrainsizeboundaryofthethinfilms,asdescribedearlier
[31–34].ResultsobtainedfromFigs2and3werevalidatedanddepictedthatwithincreasing
filmthicknesses,thesurfaceroughnessandthetransmissionwerereduced.ZnOthinfilms
withauniformsurfaceexhibitedsuperiorabsorptioncapabilitiestothosewitharoughsurface.
Fig7showsthebandgapcurvesofZnOthinfilmsofdifferentthicknesses.Thebandgapwas
obtainedbyplottingaTauccurve,andthevaluesweredecreasedfrom3.282eVto3.268eVas
thethicknessesoftheZnOfilmwereincreased(40to200nm).
ThedirectbandgapvaluesofthesamplestreatedwithdifferentZnOthinfilmthicknesses
wereobtainedbythefollowingformula:
ahv ¼bðhv(cid:2)EgÞn ð5Þ
wherehvrepresentsphotonenergy,βisfixedconstant,Egisthebandgapenergy,andnisthe
alloweddirectbandwithavalueof1/2.
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Fig7.Bandgapobservationofcoatedthinfilmtoinvestigatetheoptoelectronicperformance.Tauc
plotofZnOthinfilmsofdifferentthicknesses.
doi:10.1371/journal.pone.0132755.g007
Thedecreaseinthebandgapmaybeattributedtoareduceddeteriorationrateofthelattice
structureoftheZnOthinfilmwithincreasingthicknesses[35].Previousreportsstatedthat
voidsinthestructureofthethinfilmcouldenhancethedeteriorationrate[36],andthesestud-
ieshaveillustratedthatthebandgapofaZnOthinfilmcanbealteredwithoutintroducingfor-
eigndopants.
Tofurthervalidatethecrystallinequalityofthethinfilms,photoluminescenceemission
spectroscopywasused,andtheresultsareillustratedinFig8.PLspectrademonstratedthatall
theZnOfilmsexhibitedtwopeaks,oneisintheUVrangeandtheotherisinthevisiblerange.
ThesmallpeakintheUVregioncorrespondstothenearbandedge,attributedtotheradiative
recombinationoffreeexcitons,andthebroadpeaksinthevisibleregionareassociatedwith
Fig8.Photoluminescenceanalysisatroomtemperaturetoinvestigatetheeffectofthicknesson
structuralqualityofZnOthinfilmmatrix.PhotoluminescencespectraofZnOthinfilmsofdifferent
thicknessesareshown.
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ZnONanostructure-MediatedSensingApplication
structuraldefects[37].Structuraldefectsofaseedsolutionexhibitedinthevisibleregionare
referredtoasnativepointdefectsandincludezincvacancies(V )andoxygenvacancies(V ),
zn o
whichpresentdeep-energylevelsofthebandgap[38,39].VoidsandimbalancesintheZn-O
ratiocancauseZnandOvacancies,leadingtostructuraldefectsinthethinfilm[40].Thevisi-
blerange(from500to700nm)hadasignificantdecreaseinintensitywiththeincreasingthick-
nessoftheZnOthinfilm,whichshowsareductioninthestructuraldefectdensityinthethin
film.ThisresultrevealsthatincreasingthethicknessesofZnOthinfilmfrom40to200nm,
couldimprovethestructuralquality,whichcancontributeitsusageasatransducerinbiosen-
singapplications.
Optoelectronicperformanceanalysis
Therefractiveindex‘n’isconsideredavitalphysicalparameter,anditcanbeapproachedfrom
itsrelationshipwiththedensityorthelocalpolarizability[41–46].Thevalueof‘n’isnot
dependentontemperatureorincidentphotonenergy.Ravindraetal.[47]expressed‘n’asalin-
earfunctionofE :
g
n¼a þbE ð6Þ
g
whereα=4.048andβ=-0.62eV-1.Fromthesimplephysicsoflightrefractionanddispersion,
HerveandVandamme(1995)formulatedanempiricalrelation[48]
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u !
u 2
t A
¼ 1þ ð7Þ
E þB
g
Where,A=13.6eVandB=3.4eV.Ghoshetal.[49]usedadifferentstrategybyconsidering
bandstructureandthequantumdielectricapproachesofPenn(1962)[50]andVanVechten
[51].Byindexing‘A’asanimportantcontributionofthevalenceelectronsand‘B’asaconstant
additiveforthebandgapwiththelowestE ,thehigh-frequencyrefractiveindexcanbe
g
expressedas:
A
n2(cid:2)1¼ ð8Þ
ðEgþBÞ2
whereA=25E +212,B=0.21E +4.25,and(E +B)referstothematerialwithanappropriate
g g g
averageenergygap.Verificationoftherelationshipwasperformedbycalculatingthedielectric
constantε1,inwhichε1=n2[52],anditisinagreementwiththeexperimentalvalue[47,53].
Theintenseabsorptionandminimalreflectionrepresentsanincreaseintheefficiencyofgener-
atingoptoelectronicdevices.
Thebulkmodulusrepresentsthematerialstiffness,asobservedpreviously[54–59],andit
canbecomputedaccuratelybyusingsolidstructuralandelectronicproperties.Amoreempiri-
calderivationwasdesignedwith‘ab’initiocalculations[60].Cohen[61]formulatedanempiri-
calformulatocalculatethebulkmodulus‘B0’basedonthenearest-neighbordistanceunder
agreementwiththeexperimentalvalues.Lametal.[62]designedananalyticalexpressionfor
thepressurederivative‘B0’thatisdifferentfromtheempiricalformulainstructure,butpro-
ducessimilarnumericalresults.Dourietal.[63]followedaconceptbasedonthelatticecon-
stantandestablishedanempiricalformula.Areasontostudy‘B0’isduetothecleardifferences
betweenthelatticeconstantsofdifferentZnOsamples,asshowninTable2.
ThedominanteffectrepresentsthedegreeofcovalencythatcharacterizesPhillips’homopo-
largap,‘Eh’[60],aqualitativeconcept,suchasbulkmodulus.Theseapproacheswereusedin
PLOSONE|DOI:10.1371/journal.pone.0132755 July13,2015 10/20