W doped SnO2 growth via sol-gel routes and characterization: Nanocubes

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ContentslistsavailableatScienceDirect

Optik

j o ur na l h o m e p a g e :w w w . e l s e v i e r . d e / i j l e o

W

doped

SnO

2

growth

via

sol–gel

routes

and

characterization:

Nanocubes

Eyüp

Fahri

Keskenler

a,∗

_,

_Güven

_Turgut

b

_,

_Serdar

_Aydın

b

_,

_Seydi

_Do˘gan

c

a_Recep_Tayyip_Erdo˘gan_University,_Faculty_of_Engineering,_Department_of_{Nanotechnology,}₅₃₁₀₀_Rize,_Turkey

b_Atatürk_University,_Kazım_Karabekir_Education_Faculty,_Department_of_Physics,₂₅₂₄₀_Erzurum,_Turkey

c_Department_of_Electrical_and_Electronics_Engineering,_Faculty_of_Engineering_and_{Architecture,}_Balikesir_University,_Balikesir_10145,_Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received18September2012 Accepted15February2013 Keywords: SnO2 W-doping Nanocube Sol–gel Cubicphase

a

b

s

t

r

a

c

t

TheeffectsofWdopingonthecharacteristicalpropertiesofSnO2thinfilmspreparedbysol–gelspin coatingmethodwereinvestigated.TheSnO2thinfilmsweredepositedatvariousWdopingratiosand characterizedbyvariousmeasurements.XRDstudiesindicatedthattheundopedandWdopedSnO2films hadcubicandtetragonalphases.TheSEMimagesofWTOthinfilmsshowedcubicshapednanocubes correspondingtocubicphaseandthesmallerparticlescorrespondingtotetragonalphasewereformed onthefilmsurfaces,andtheirdistributionsandsizesweredependentontheWdopingratio.EDX spec-troscopyanalysesshowedthatthecalculatedandparticipatedatomicratiosofW/(W+Sn)(at.%)inthe startingsolutionandintheWTOthinfilmswerealmostclose.Itwasfoundthatthesheetresistance dependedonWdopingratioand2.0at.%WdopedSnO2(WTO)exhibitedlowestvalueofsheetresistance (7.11×103_/cm2_).

1. Introduction

Tinoxide(SnO2)hasvarious applicationareas[1] duetoits

uniquepropertiessuchaslow electrical resistivity,highoptical transmittanceinthevisibleregion,highinfraredreﬂectivity, chem-icallyinertandmechanicallyhard[2–5].SomepropertiesofSnO2

thinﬁlmscanbeimprovedbysuitabledopantelementssuchas antimony(Sb),ﬂuorine(F),vanadium(V),andtungsten(W).Among thesedopants,tungstenhasoxidationstatesapttoW6+_ion_state,

andtheradiusofW6+_is_close_to_that_of_Sn4+_(W6+_:₆₇_pm,_Sn4+_:

71pm),which makeiteasytoreplacetheSn4+_ions._Therefore,

tungsten-dopedtin oxidemaybe expectedtohave a potential prospect[6].

Undopedand dopedSnO2 thinﬁlmshave beenpreparedby

variousexperimentaltechniquessuchaselectrochemical deposi-tion[7],hydrothermalmethod[8],polymerizing-complexingand sol–gel[9]techniques.However,tothebestofourknowledge,the fabricationofWdopedSnO2thinﬁlmshavenotbeenreportedup

tonowbysol–gelspincoatingmethod.Therefore,inthisstudy,we aimedtoinvestigatetheeffectofWdopingonstructural, morpho-logical,opticalandelectricalpropertiesofSnO2thinﬁlmsprepared

bysol–gelspincoatingmethod.

∗ Correspondingauthor.Tel.:+9004642236126x1764;fax:+9005376104849.

E-mailaddress:keskenler@gmail.com(E.F.Keskenler).

2. Experimental

Inthepresentstudy,W-dopedtinoxide(WTO)thinﬁlmswere preparedbysol–gelspincoatingmethodonglasssubstrateusinga solpreparedwithstannouschloridedihydrate(SnCl2·2H2O),

tung-stenhexachloride(WCl6),monoethanolamine(C2H7NO,MEA)and

2-methoxyethanol(C3H8O2,2-MTE),asstartingmaterial,dopant

source, stabilizer, and solvent, respectively. The molar ratio of MEAtometalsaltswasmaintainedat1:1inallsolutions.Various amountsofthestannouschloridedihydrateandtugsten hexachlo-ridewerecombinedtoachievedifferentW/W+Snatomicratios changingfrom1.0at.%to4.0at.%with1.0at.%step.Theprecursor solwasstirredat80◦Cfor24hinatightlyclosedflasktoobtain aclearandhomogenoussolution.Theglasssubstratesfirstlywere keptinboilingchromicacidsolutionandthentheywererinsed withdeionized water. Finally,they werecleaned withacetone, methanolanddeionizedwaterbyusinganultrasoniccleanerand driedwithnitrogen.Theresultantsolutionbeingdroppedonglass substratewasrotatedataspeedof3000rpmfor30sbyusinga spin-coater.Aftertheglasssubstrateswerecoated,theyweresinteredat 200◦Cfor5mintoevaporatesolventandremovetheorganic sed-imentsandthenspontaneouslycooledtoroomtemperature.This procedurewasrepeatedfor4timesandfinally,thesampleswere annealedinairat450◦Cfor30min.

ThestructuralcharacterizationoftheWTOthinfilmswas car-riedoutbyX-raydiffraction(XRD)measurementsusingaRigaku MiniflexIIdiffractometerwithCuK␣radiation(=1.5418 ˚A).The diffractometerreflections weretaken atroom temperatureand thevaluesof2werealteredbetween15◦ and80◦. Morpholog-icalpropertiesoftheW-dopedSnO2thinfilmsweredetermined

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Fig.1.XRDspectraforundopedandWdopedSnO2thinﬁlms.

withJeolNano-SEM.Theopticaltransmittanceofthesampleswas recordedinspectralregionof300–1000nmat300KusingaUV-Vis spectrophotometer(Perkin-Elmer,Lambda40)whichworksinthe rangeof200–1100nm.Thesheetresistancevaluesofﬁlmswere measuredbymeansoffourpointprobetechnique.

3. Resultsanddiscussion 3.1. X-raydiffractionresults

ThecrystalstructureofWTOthinﬁlmswasinvestigatedby X-raydiffraction(XRD)patterns.Fig.1showsXRDspectraofWTOthin ﬁlms.Thesespectraindicatethatthesampleshave(111)plane cor-respondingtoSnO2cubicphase(JPDS50-1429)and(110),(101)

planescorrespondingtoSnO2 tetragonalrutilephase (JPDS

41-1445).Noadditionalpeakwhichimpliesoxidesoftungstenwere notobserved.AsseeninFig.1,thestrongestorientationis(111) lineofcubicstructureand(111)peakintensityvalueofundoped samplehasdecreasedcontinuouslywithincreasingWdopant con-tent.ItcaneasilybeconcludedthatthecrystallineofWTOthin ﬁlmsarebeingdeterioratedbyincreasedWcontent.

AlthoughSnO2 crystallizeinthetetragonalrutilecrystal

sys-tem,therearealsoorthorhombicandcubicphasesofSnO2.Asa

mineral,SnO2beingformedattetragonalrutilephaseisalsocalled

Cassiterite[10].Suitoetal.[11]reportedthephasetransitionfrom tetragonaltoorthrombicstructure.Shieh[12],HainesandLegér [13]observedSnO2 cubic phase.Phasetransformations ofSnO2

haveusuallybeenstudiedunderhighpressure.Jiangetal.[14]have foundﬂuoritecubicphasewithFm3mspacegroupabove18GPa pressure,andOnoetal.[15]haveobservedpeaksof(111),(200), (220),(222)belongingtothecubicstructure underhigh pres-sureandtemperature(about23GPaand1000K).Inrecentyears, cubicandotherphaseshavebeenreportedfromtheexperiments studiesbasedonthesolutionsperformedatatmosphericpressure [16–18].CubicphaseforSnO2thinﬁlmspreparedbyreactiveradio

frequencymagnetronsputteringwithdifferentsputteringpower (about>100W)havebeenreportedbytheSongetal.[19].

Inthisstudy,SnO2cubicstructurehasbeenobtainedat

atmo-sphericpressure,andtothebestourknowledge,thisistheﬁrst result obtainedfor SnO2 ﬁlms grownby thesol gel technique.

Intheliterature,ethanole[20,21],1-propylalcohol[22],mixture ofwaterand alcohol[23,24]aregenerallyusedasasolventfor preparation of SnO2 ﬁlms via sol–gel route. Huang et al. [25]

havesynthesizedWdopedSnO2 thinﬁlmfromsol–gelsolution

preparedby mixinga weighed quantityofSnCl2·2H2Owithan

ethanol/water,WCl6solutedinmixture.Thereasonforobtaining

thecubic structureinthis studymay,atﬁrst,betheusageofa solpreparedwithstannouschloridedihydrate(SnCl2·2H2O),

tung-stenhexachloride(WCl6),monoethanolamine(C2H7NO,MEA)and

2-methoxyethanol(C3H8O2),asstartingmaterial,dopantsource,

stabilizerandsolvent,respectively.

Forthesamples,theobserved‘d’valueswhicharethe inter-planerdistances are presented in Table1 and thesevalues are comparedwiththestandardonesfromtheJPDS50-1429dataﬁles. Thelatticeconstant‘a’forcubicstructureisdeterminedbyrelation [26]. 1 d2 =

h2₊_k2₊_l2 a2

(1) where(hkl)ismillerindices.Thecalculatedandstandardlattice constantsarealsogiveninTable1.Thecalculated‘a’valuesagree withJPCDScardno:50-1429(a=4.87 ˚A).AscanbeseeninTable1, thelatticeconstantvalueforundopedsampleis4.8745 ˚A.Thisvalue decreaseswithWdopingupto2.0at.%,andthenitincreases con-tinuouslywiththeWdopingconcentrationintheﬁlms.Thiscan beexplainedifitisconsideredthat:Whasmanyoxidationstates suchas+6,+5,+4,+3,+2[27,28]andwithdecreasingtheoxidation numberofW,itsionicradiiincreases[28,29].Atthelowdoping levels,W6+_presumably_substitutes_with_the_Sn4+_and_causes

lat-ticeconstantdecrease.WiththeincreasingofWdopinglevelin SnO2lattice,W5,4,3,2+oxidationstatesalsosubstitutetoSn4+and

havingincreaseinlatticeconstant.Similarly,inearlierstudies,it wasfoundthatsomeoftheSn4+_ions_in_the_lattice_were_replaced

bySb5+_at_low_doping_level,_however_Sb3+_substituted_to_Sn4+_at

highdopinglevel[20]. 3.2. SEMandEDXresults

ThecompositionofWTOthinﬁlmswasdeterminedbyenergy dispersiveX-rayspectroscopy(EDX).EDXspectraofWTOthinﬁlms andthecompositionofelementsinSnO2structure aretogether

giveninFig.2.ThesespectraclearlyconﬁrmtheexistenceofSn andWelementsintheWTOthinﬁlms.TheSi,Na,MgandCa

ele-Table1

Thestructural,electricalandopticalvaluesofundopedandWTOthinﬁlms.

Sample Cubic(hkl) Cubic-dst.(Å) Cubic-dobs.(Å) Cubic-a(Å) Rs(×103/cm2) Eg(eV)

UndopedSnO2 (111) 2.8120 2.8143 4.8745 48.22 4.105

1.0at.%WdopedSnO2 (111) 2.8120 2.8140 4.8739 12.96 4.112

2.0at.%WdopedSnO2 (111) 2.8120 2.8236 4.8626 7.11 4.115

3.0at.%WdopedSnO2 (111) 2.8120 2.8146 4.8750 8.24 4.099

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Fig.2.EnergydispersivespectroscopyanalysisofWdopedSnO2thinﬁlms:(a) undoped,(b)1.0at.%Wdoped,(c)2.0at.%Wdoped,(d)3.0at.%Wdoped,(e)4.0at.% Wdoped,(f)elementalconcentrationsintheﬁlmwithdifferentWdopinglevelsin SnO2lattice.

ments,insolidﬁlms,areresultedfromtheglasssubstrates.EDX

analysesshowthatthecalculatedandparticipatedatomicratiosof

W/(W+Sn)(at.%)inthestartingsolutionandintheWTOthinﬁlms

arealmostclose.Theseresultsconﬁrmthereliabilityofthesol–gel

spincoatingmethodusedinexperiments.Thescanningelectron

microscopy(SEM)imagesofWTOsthinﬁlmsgiveninFig.3.Itcan

beseenthatthecubicshapednanocubesareformedonsurfaces oftheﬁlms,andtheirdistributionsandsizesofnanocubesdepend ontheWdopingratio.Fortheundopedsample(Fig.3a),thesizes ofcubicnanoshapevarybetween300and400nm.Forthe1.0at.% WdopedSnO2 (Fig.3b),thesizesofnanocubesdecreasetothe

valueofabout250–350nm.AscanbeseenfromFig.3c,thesizes ofnanocubessharplydecreasetoabout70nmforthe2.0at.%W dopedSnO2anddistributionofnanocubesismorearranged

com-paredtoundopedand1.0at.%WdopedSnO2.Above2.0at.%W

dopinglevel(Fig.3dande),thesizesofnanocubesincreasesharply toabout200–250nmandtheﬁlmsurfaceconsistoflotof non-uniformeddistortednanocubeshapes.Also,fortheallsamples,it shouldbenotedthatthesenanocubescanbeformedbyan aggre-gationofsmallparticlesobservedonglasssubstrates.Theseresults areinharmonywithtendencyoflatticeconstant‘a’calculatedfrom XRDresultsfortheWTOthinﬁlms.ThesimilarstructuresofSnO2

cubicphaseandcubicshapewereobtainedfortheﬁlmsgrownby hydrothermalmethod[8].

3.3. Electricalandopticalproperties

Electricalpropertiesoftheﬁlmswereinvestigatedbyfourpoint probemethod.AscanbeseeninTable1,there isadecreasein thesheetresistancevaluesupto2.0at.%Wdopinglevels,thenit increaseswithW-dopantconcentrationincrease.Thevariationin thesheetresistanceofSnO2withWdopingcanbeexplainedon

thebasisofthepresenceofdifferentvalancestateofWelement. WhenitisdopedwithW,someoftheSn4+_ions_in_the_lattice_can

Fig.3. SEMimagesundopedandWdopedSnO2thinﬁlms:(a)undoped,(b)1.0at.%

Wdoped,(c)2.0at.%Wdoped,(d)3.0at.%Wdoped,(e)4.0at.%Wdoped.

bereplacedbyW6+_,_resulting_the_sheet_resistance_[6,25,30]_._Hence,

areductioninthesheetresistanceisobserveduntiltheWdoping levelreaches2.0at.%.Beyond2.0at.%ofWdoping,apartoftheW6+

ionsarereducedtothelowvalancestatessuchasW4+_,_W3+_,_W2+_,

resultingintheformationofacceptorstatesandalossofcarriers. Thusanincreasingisobservedatsheetresistance.Inearlierstudies, itwasfoundthat3.0at.%Wdopingreducedthesheetresistanceof SnO2 andWatomswerefullyoxidizedtothevalenceof6+,and

mostofW6+_were_incorporated_inside_the_SnO

2 structureatlow

dopingratiofromXPSstudy[6,25,30].

TheopticalpropertiesofWTOthinﬁlmswereinvestigatedby usingUV-Visspectrophotometer.Transmittancespectraaregiven in Fig. 4. As can be seen, the transmittance values are varied

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Fig.5.Thevariationof(˛h)2_vs._h_for_undoped_and_W_doped_SnO

2thinﬁlms.

between60 and75%invisible region.It isquiteclearfromthe transmittancespectrathattransparencyhasslightlychangedwith Wdoping.Theanalysisofthedependenceofabsorptioncoefﬁcient versusphotonenergyinthehighabsorptionregionsiscarriedout toobtainmoredetailedinformationabouttheenergybandgaps. Theabsorptioncoefﬁcient(˛)isdeterminedbytheequation[31]; ˛= ln(1/T)

d (2)

whereTistransmissionanddisﬁlmthickness.Theopticalband gapofWTOsisobtainedbyfollowingrelation[32];

˛h=A(h−Eg)1/2 (3)

wherehandAarephotonenergyandtheconstant,respectively.Eg

valuesweredeterminedbyplotting(˛h)2_vs._h_and_{extrapolating}

ofthelinearregionoftheplottozeroabsorption((˛h)2₌_0)._The

bandgapvaluesofundoped,1.0,2.0,3.0and4.0at.%WdopedSnO2

thinﬁlmswerefoundas4.105,4.112,4.115,4.099,4.097eV, respec-tively.ItisclearlyseeninFig.5thattheincreasingWcontentin theSnO2structuresupto2.0at.%,thebandgapvaluesincreaseand

thencontinuouslydecreasewithfurtherincreasingWdopinglevel. ThereasonforincreasingbandgapwithWcontentmayprobably beexplainedasfollows;SnO2isoneofdegeneratesemiconductors

[10],whichtheFermilevellieswithintheconductionband[27]. Thus,theopticalbandgapsarerelatedtotheexcitationofthe elec-tronsfromthevalancebandtoFermilevels[33,34].Thismeans thatFermilevelisliftedsomemoreintotheconductionbandofthe degeneratesemiconductorduetotheincreaseinthecarrier den-sity.Thisleadstotheenergybandbroadening(shifting)andsomeof theSn4+_ions_in_the_lattice_are_replaced_by_W6+_(or_W5+_),_and_this_is

calledMoss–Bursteineffect[35].Adecreaseintheenergybandgaps beyond2.0at.%WdopingcanbecausedthatapartoftheW6+_ions

isreducedtolowervalancestatesofWlikeW4+_,_W3+_,_W2+_,_which

canresultintheformationofdefects,impuritiesorcompletelynot substitutionofdopantwithhostatomsorinterstitialandlattice strain[36–39].Thecalculatedlatticeconstantvaluessupportthat W6+_are_being_replaced_with_Sn4+_at_low_doping_contents_(for_1.0

and2.0at.%). 4. Conclusions

Thisstudypresentscubicphasebeingobtainedfortheﬁrsttime forWdopedSnO2thinﬁlmsgrownbysol–gelspincoating,using

methanolamine,2-methoxyethanol,stannouschloridedihydrate, andtungstenhexachloridebasedonprecursorsolution.XRD stud-iesindicatethattheundopedandWdopedSnO2 havegrownat

cubicandtetragonalphases.EDXanalysesshowthatthecalculated andparticipatedatomicratiosofW/(W+Sn)(at.%)inthestarting solutionandintheWTOthinﬁlmsarealmostclose.TheSEMimages

ofWTOsthinfilmsshowedthatthevariousdistributedandsized cubicshapednanocubeswereformedonthesurfacesofthefilms, dependingonWdopingratio.Allresultshadshowedthat2.0at.% WdopedSnO2filmhadthebestoptoelectronicproperty.Thus,the

resultsofthis studyindicatethatthestructural,morphological, electrical,andopticalpropertiesoftheSnO2thinﬁlmsprepared

bysol–gelmethodcanbestronglyaffectedbytheincorporationof WelementsinSnO2thinﬁlmsandWTOthinﬁlmscanbeusefulfor

microandnano-sizedoptoelectronicdeviceapplications.

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