Operando SXRD of E-ALD deposited sulphides ultra-thin films: Crystallite strain and size

(1)

ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Full

Length

Article

Operando

SXRD

of

E-ALD

deposited

sulphides

ultra-thin

ﬁlms:

Crystallite

strain

and

size

Andrea

Giaccherini

a,∗

_,

_Francesca

_Russo

a

_,

_Francesco

_Carlà

b

_,

_Annalisa

_Guerri

a

_,

Rosaria

Anna

Picca

c

_,

_Nicola

_Ciofﬁ

c

_,

_Serena

_Cinotti

a

_,

_Giordano

_Montegrossi

d

_,

Maurizio

Passaponti

a

_,

_Francesco

_Di

_Benedetto

d,e

_,

_Roberto

_Felici

f

_,

_Massimo

_Innocenti

a

a_Department_of_Chemistry,_Università_Degli_Studi_di_Firenze,_Via_della_Lastruccia_3-13,_50019,_Sesto_Fiorentino_(FI),_Italy b_ESRF,_6,_Rue_Horowitz,_F-BP_220,_38043,_Grenoble,_Cedex,_France

c_Department_of_Chemistry,_Università_degli_Studi_di_Bari,_Via_E._Orabona,_4-70125_Bari,_Italy d_IGG-CNR,_via_G._La_Pira_4,_50121,_Italy

e_Department_of_Earth_Sciences,_Università_Degli_Studi_di_Firenze,_Via_La_Pira_4,₅₀₁₂₁_Firenze,_Italy f_SPINCNR,_Area_della_Ricerca_di_Roma₂₋_Tor_Vergata,_Via_del_Fosso_del_Cavaliere_100,₀₀₁₃₃_Roma,_Italy

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received8March2017

Receivedinrevisedform21July2017 Accepted31July2017

Availableonline5August2017 Keywords: SXRD ECALE E-ALD Photovoltaics Water-splitting Ultra-thinﬁlms

a

b

s

t

r

a

c

t

ElectrochemicalAtomicLayerDeposition(E-ALD),exploitingsurfacelimitedelectrodepositionofatomic layers,caneasilygrowhighlyorderedultra-thinfilmsand2Dstructures.Amongothercompounds CuxZnySgrownbymeansofE-ALDonAg(111)hasbeenfoundparticularlysuitableforthesolarenergy conversionduetoitsbandgap(1.61eV).Howeveritsgrowthseemstobecharacterizedbyamicrometric thread-likestructure,probablyovergrowingasmoothultra-thinfilms.Onthisground,aSXRD investiga-tionhasbeenperformed,toaddresstheopenquestionsaboutthestructureandthegrowthofCuxZnyS bymeansofE-ALD.Theexperimentshowsapseudosinglecrystalpatternaswellasapowderpattern, confirmingthatpartofthesamplegrowsepitaxiallyontheAg(111)substrate.Thegrowthofthefilm wasmonitoredbyfollowingtheevolutionoftheBraggpeaksandDebyeringsduringtheE-ALDsteps. BreadthandprofileanalysisoftheBraggpeaksleadtoaqualitativeinterpretationofthegrowth mecha-nism.ThisstudyconfirmsthatZnleadtothegrowthofastrainedCu2S-likestructure,whilethegrowth ofthethread-likestructureisprobablydrivenbythereleaseofthestressfromtheepitaxialphase.

1. Introduction

Theenergyissueofmodernsocietyislinkedtothelimited avail-abilityofnon-renewablefuelscoupledtotheincreasinglevelsof globalpollution, approaching an environmentalovershoot.This impliestheneedforarapiddevelopmentinmaterialsand tech-niquesforrenewableenergyapplications.Anincreaseinefﬁciency andsustainabilityofthesolarcellsnecessarilyrequirestheuseof materialsconsistingof elementswhoseavailability willsupport thediffusionofthephotovoltaictechnologyatagloballevelover theactualextentrepresentedbythesilicon-basedsolarcells.Atthe sametime,theuseofsuchmaterialsshouldensurealower environ-mentalimpact,inaFullLifeCycleAssessment(FLCA),andthedevice productionmustbedonethroughlessenergy-intensivemethods, thusincreasingtheEnergyReturnOverEnergyInvestment(EROEI). Forthesereasons,thescientiﬁccommunityisfocusingitsattention

∗ Correspondingauthor.

E-mailaddress:andrea.giaccherini@uniﬁ.it(A.Giaccherini).

onnewcompoundsbasedoneconomic andlow-environmental impactelementssuchasCu,Sn,FeandZn.Apossibilitytoobtain highqualitythinﬁlmsemiconductorsreliesonexploiting electro-chemicaldeposition.Thistechniqueisaveryattractivemethodto producecompoundsemiconductorswithdifferentstoichiometry andhighcrystallinitybecauseofitseasinessofimplementation,its relativelylowcost,itscapabilityofgrowingtheselectedmaterials ontosubstratesofanyshapeanddimensions.

Chalcogenide nanomaterials are interesting candidates for photovoltaic applicationsdue to thereasons well-explainedby Innocenti et al. [1]. Chalcogenides can be grown by means of ElectrochemicalAtomicLayerEpitaxy(ECALE)[2].Thistechnique consistsintheelectrodepositionofalternatingmetallicand non-metallicmonolayersinacyclewhichcanberepeatedseveraltimes andwhichexploitsreactionslimitedbythesurface(SLR−Surface LimitedReactions),includingUnderPotentialDepositions(UPD). This methodis alsoreferred asE-ALD (ElectrochemicalAtomic LayerDeposition),whenthedepositcannotbeconsideredstrictly epitaxial[3].

http://dx.doi.org/10.1016/j.apsusc.2017.07.294

(2)

damentalstandpoint,sincethetwocationsaresupposedtohave scarcestructuralandvalenceaffinitywheninsulfidesystems.In nature,CusulfidemineralsandZnsulfidemineralsarecommonly associatedinmostores;stilltheyhavepoororabsentsolubility. TheredoxpropertiesofCuinsulphides(i.e.theinstabilityofits divalentstateinsuchenvironment)seemstoplayacriticalrole inlimitingthestabilityofCu-Zn-Sstructuresin thermodynami-callyequilibratedphases.Thiscriticalpointcanbeovercomeunder out-of-equilibriumconditionsofsynthesis.Aswellasweknow, fewstudiesreporttheattemptofsynthesizingCuZnS2,(Cu,Zn)Sor Cu2Zn2S3[7–12].Uhuegbuetal.synthesizedCu-Zn-Scompounds throughaone-potsynthesiswithmetalchloridesandthioureain solution,assistedbyopportunecomplexingagents[7].Conversely, Adulojuetal.dopedZnSfromacontactsolutionofCu2+_,_the result-ingproductsbeinghomogenizedbyamildtemperaturetreatment (300◦C)Kitagawaetal.claimedtohavesuccessfullydepositedthin filmsofCu2Zn2S3byspraypyrolysis[10].Bagdareetal.claimed to have obtained a nanocrystalline thin films of CuxZn1-xS by meansofchemicalbathdeposition[11].Moreover,Adelifardetal. obtainedclaimedtohaveobtaineda biphasicCuS-ZnSthinfilm bymeansofspraypyrolysis[12].Unfortunately,thesestudiesdid notprovidedecisiveevidencesoftheattainmentofamonophasic compound,or,atleast,ofapartialsolidsolutionbetweenCu-and Zn-sulfidephases.Inthiscontext,E-ALD wasconsideredagood candidatetoovercomethethermodynamicsconstraintsinthe Cu-Znsulphidestoobtainametastablephase.Ourgroupattempted togrowCuxZnySbymeansofE-ALD onAg(111).However,the electrochemicalcharacterizationofthedepositedfilmhighlighted aZn-deficiency[6,13].It isworthtonotice thatasimilarresult waspointedoutwithreferencetoCdxZn1-xSwhereitwas corre-latedtotheCd-Znsolidsolution[14].Ingeneral,ternary E-ALD schemeswerefoundtogrowsemiconductingmaterialslayerby layerresultinginadeficiencyofthelessnoblemetal.Weshould reportthatonlyminormorphologychangesarefoundinCdxZn1-xS withrespect to thebinary films (CdS and ZnS). Moreover,the roughnesschangeswiththestoichiometry,reachingaminimum forx=0.28[15].Conversely,fortheCuxZnySfilmthemorphologyis foundtobedramaticallydifferentwithrespecttotheCu2S, present-ingathread-likestructureofnanowiresreachingthemicrometer scale[16,17],whereasforCu2Saroughnessof24Åisreported else-where[7].X-rayabsorptionspectroscopyexperimentspointedto thepossibleinfluenceoftheZndepositionstepintheformationof thenanowiresnetintheCuxZnySE-ALDscheme[16].

Thesestudiespointedoutthatadditionalstructural character-ization is needed toimprove the understanding of thegrowth processanddeterminethemainstructuralfeatures ofthefilms grownbyE-ALD.Thus,inthepresentstudy,wereporttheoperando SXRD (Surface X-Ray Diffraction) structural characterization of CuxZnySthin-filmsdepositedbyE-ALDonAg(111)electrodes,with theaimofstudyingthegrowingofthefilmandtoclarifytherole oftheZninthegrowthofthenanowiresnet.

Thethinﬁlmswererealizedstartingfromanalytical reagent

gradematerials:MerckCuCl2,andAldrichNa2S.MerckHClO470%

eMerckNH4OH83%solutionswereusedforpreparinga9.2pH

ammoniabuffer.Thesolutionswerefreshlypreparedjustbefore

eachsynthesis,usingwaterpuriﬁedbyMilli-Qlabwatersystem.In

Table1wereportthedifferentthinfilmsconsideredinthisstudy. Allsyntheseswerecarriedoutbymeansofanautomated deposi-tionsystem,consistingofadistributionsystemcomposedofPyrex bottles,electrochemicalcells,apotentiostat,acontrolsystemfor thesolenoidvalvesandthestabilityofthepotential[18,19].This automatedsystemallowsacombinationofthesolutionswithinthe electrochemicalcellensuringgoodfluiddynamicsandalow con-taminationduetoexposuretoair.Thefoursamplesweregrown intwodifferentelectrochemicalflowcells,thefirstonedesigned fortheE-ALDprocess,andthesecondspecificallydesignedfor cou-plingE-ALDwithanoperandoSXRDinvestigation[3,20,21].Inboth cases,thecellisdelimitedbytheworkingelectrodeononeside and bythecounterelectrode ontheotherside, whiletheinlet andtheoutletofthesolutionswereplacedonthesidewalls[19]. Athree-electrodeschemeisused,withanAucounterelectrode andanAg/AgCl(KClsaturated)asareferenceelectrode. Concern-ingthesynthesis, weusethefollowingnotationin referenceto theCu/Znratioandthecyclesgrown:asampleofCuxZnySwith n=m=1grownfor60cyclesisdepositedthroughthefollowing depositionsequence[(S/Cu/S)n(Zn/S)m]60.Thesamplereferredas “OP1:1”inTable1wasinvestigatedthroughoperandoSXRD dur-ingitsgrowth,whereassamplesEx20,Ex40andEx60weregrown inthelaboratoryofthe“AppliedElectrochemistry”groupatthe UniversityofFlorence(UNIFI)andex-situanalyzed.Forthe electro-chemicalparametersofthedepositionprocesswereferto[6,13]. Commercialsilversinglecrystalelectrodes,preparedbythe “Sur-facePreparationLaboratoryB.V.”,with(111)orientationwereused forthegrowthofsampleOp1:1.Theelectrodesurfacewascleaned underultra-highvacuumconditionsbyperformingseveralcycles ofAr+_sputtering_at₁_kV_followed_by_annealing_at₉₀₀_K_for_about 1min.Thecycleswererepeateduntilasharplowenergyelectron diffraction(LEED)patternwasobservedandnocontaminationwas observablebyX-rayphotoelectronspectroscopy(XPS).

Conversely,exsituEx20,Ex40andEx60samplesweregrown onsilversinglecrystalelectrodewith(111)orientationprepared accordingtotheBridgemantechnique, in thelaboratoryofthe “Applied Electrochemistry”group at the University of Florence

[16]andpolishedbyaCrO3 basedprocedure[22].Theoperando measurementswereperformedattheID03beamlineofEuropean Synchrotron RadiationFacility (ESRF).The measurements were performedbymeansofthesixcircleverticalaxisdiffractometer installedintheEH1hutch[3,23].Thediffractedintensityhasbeen registeredbymeansofaMAXIPIXfastreadoutarealdetector[24]. Theelectrochemicalcellwasmountedonthediffractometerstage. Allmeasurementswerecarriedoutusinga24keVX-rayradiation inordertoallowahightransmissionthroughtheECcellwallsand solutions[3,23].

(3)

Fig.1.AschematiclayoutoftheSXRDgeometry.Thesamplesurfaceisrepresented bytheyellowishdisc.x,y,zisthereferencesystemofthelaboratorywhilea1*,a2*

andc*_are_the_reciprocal_vector_of_the_Ag(111)_surface_cell_used_as_reference_system

ofthereciprocalspace.InSXRDthezaxisandthec*_vector_are_parallel._The_sample

surfaceisintheplanex,ywherealsothevectorsa1*anda2*lay.Theangle␻is

betweenthea1*vectorandthexaxisistheazimuthangle.Theincidentx-raybeam,

kin,isintheplanex,zatanangle␮withthesurface,Theexitbeam,kout,hasanexit

angle␥withrespecttothesurfaceandaninplaneangle␦withrespectthex,zplane. Tomeasurethediffractedintensityinaparticularpointofthereciprocalspacethe␦, ␥and␻anglesarepositionedwhile␮ismaintainedatﬁxedangle(typically0.5◦_–1◦

forthisexperiment).

2.2. Reciprocalspacemappingstrategy

WeperformedtheelectrodepositionofOp1:1whileacquiring SXRDdataoflinearscansand2Dmapsinthereciprocalspace,to understandtheevolutionofthestructuralfeaturesofthedeposited ﬁlmduringandafteritsgrowth.Thisoperandoprocedureallowed ustomonitorselectedpositionsinthereciprocalspaceusefulfor thecharacterizationofthestructuralpropertiesoftheCuxZnySﬁlm. Thescansandthemapsshowninthisstudyareparameterized withrespecttoacoordinatesystemreferredtothesurfaceunitcell oftheAg(111)substrate.Accordingtothisconvention,the corre-spondencebetweenthestandardfccbulkAgunitcellandthemetric ofthesurfacepseudo-hexagonalcell(a1,a2,c,␣,␤,␥)−wherea1 anda2vectorslayonthesamplesurface,whilecisperpendicular tothatsurface–isdeterminedbythefollowingrelations: ␣=␤=90◦,␥=120◦,|c|=√3a0,|a|=|b|= √a0

2

Wherea0isthelatticeparameterofthecubicfcccellofAg.Having adoptedthisconventionforthea1,a2andcvectors,thereciprocal spaceisparameterizedbythefundamentalreciprocalvectorsofthe pseudohexagonalAgstructure,a1*,a2*andc*.Wherea1anda2are respectivelythe[½−½0]andthe[0½−½]standardfccvectors anddeﬁnetheplaneparalleltothesamplesurface(referredas “in-plane”)andcisthestandardfcc[111]vectorsandperpendicular tothesamplesurface(“outofplane”).Anyvectorinthereciprocal spaceisrepresentedbytheh,kandlcoordinates,respectively, cor-respondingtothevectorscomponentalongthea1*,a2*andc*axis (Fig.1),distancesinthesamespaceareexpressedusingreciprocal latticeunits(rlu).Thelargesetsofthediffractiondatacollectedby thedetectorhavebeenreducedusingtheBinocularspackage:this procedureenablesacorrectevaluationoftheBraggpeak intensi-tiesandthemeshinginthereciprocalspaceofeachpixelofthe2D detector[24,25].

Forthestudyoftheﬁlmgrowth,wehaveregisteredl-scansin thereciprocalspaceacquiredevery15cycles.Thosearespectra

Fig.2. Operandoh,kintensitymapsatl=1.05measuredfortheOp1:1sample.

wherehandkarekeptconstant,whilelisletvaryinarangesuch astoexplorethemutualspacealongastraightline.

2Dscanshavebeencollectedafterthe60thcycle,attheendof thesynthesis,exploring(h,k)planesnormaltoc*tocomputelarge reciprocalspacemaps.

Moreover,XRPD(X-raypowderdiffraction)spectrahavebeen registeredbymeansof␦-scans(seeFig.1), correspondingtoan in-planescan ofthedetector.Thissetupallowstoexplore the presenceofBraggconesatthehorizonofthesample.Toimprove thesensitivitytothedepositedfilmstructurewithrespecttothe oneofsubstrateweusedgrazinganglesforboththeincidentand diffractedbeamsandwechooseanorientation␻ofthesample azimuthsuchtoavoidsubstrateBraggpeaksduringthe␦-scans. XRPDpatternswereregisteredevery15cyclesintherange8–25◦. XRPD datawere refinedby meansoffull-profile Rietveld algo-rithm,usingtheEXPO2014software[26].Theremovalofthediffuse scattering,introducedbythepresenceofthewaterinsidethe elec-trochemicalcell,hasbeenoperatedbyfittingthebackgroundofthe signalswithpolynomials(firstorderforthel-scansand18terms Chebyshevpolynomialforthe␦-scans)[27].

2.3. Singlelineproﬁleanalysis

Asinglelineanalysisofthediffractionprofilewasperformed byfittingaVoigtfunction.Wepresentaprofileanalysisofa rep-resentativeandreflection(0.73,0.73,4.18)alongthe(0.73,0.73,l) crystallographicdirectionbyusingaVoigtfunction,chosenonthe basisofitsintensityandsmalloverlappingwithothersignals.As calculatedbythegeometryofthediffractometer,wecanassumea negligibleinstrumentalcontributiontotheprofilebreadth,while thepeakwidthisdominatedbythecontributionsduetothedomain sizeandlatticestrain[22].Undertheseassumptions,the deconvo-lutionoftheVoigtprofilecanbecomputedassumingaGaussian profileforthestrainandaLorentzianprofileforthedomainsize

[28–32].Theparametersusedtoanalyzethepeaksandthe grow-ingmechanismaretheGaussianwidth(wG),theLorentzianwidth (wL),thepeakarea(A) andthepeakposition(xc).Therelation betweenthestrainandwG,aswellastherelationbetween crys-tallitesdimensionandwLis explainedbydeKeijser etal.[28]. Accordingtotheseauthors,wGisqualitativelycorrelatedtothe strainduetothedistributionofdefects(randomdistributionof defect-induced stresses).Relative uncertaintiesof theevaluated parameters(peakintensity,areaandwidths)werecalculatedby propagationoftheexperimentaluncertaintiesoftheintensitydata. TheselatterwereobtainedasRootMeanSquare(RMS)ofthe back-ground.Itisworthtonoticethatthedatasetshavebeenprojected onthehklspacewitharesolutionof0.001rlu[26,27].

(4)

Fig.3. (a)Rawin-planeXRPDpatternoftheOp1:1sampleat60cycles,(b)Backgroundsubtractedin-planeXRPDpatternoftheOp1:1sampleat60cyclescomparedwith thebestﬁtoftheRietveldanalysisandthepositionofthewurtzitereﬂections,(c)InplaneXRPDpatternsoftheOp1:1sampleand(d)Rietveldscalefactorsatdifferent growthstages.

2.4. SEM

SEMinvestigationswerecarriedoutonex-situCuxZnyS sam-pleswithdifferentstoichiometry,Ex20,Ex40andEx60(Table1), in order to determinethe morphology of the ﬁlms. The mea-surements were performed at the Interdepartmental center of ElectronicMicroscopyandMicroanalysis(MEMA)ofthe Univer-sityofFlorence.Toenhancethequalityofthemicrographsathigh magniﬁcation,samplesweregoldcoated.Secondaryelectron(SE) micrographswereobtainedusingaSEMZEISSEVOMA15,by apply-ingapotentialof20kVtotheelectronbeam.

3. Results

Bothpseudosinglecrystalpatternsandpowderareobservedfor theOp1:1sample.TheparametersobtainedbySXRDareusefulfor studyingthegrowthmechanismoftheOp1:1sampleandcarryout astructuralcharacterization,albeitpurelyqualitative.Thetrends fortheﬁtparameterswithdepositioncyclesallowedustoget infor-mationoncrystallitesstrainandsize.Moreover,theDebyerings havebeenanalyzedbymeansofstandardXRPDapproach. 3.1. Maps1:1

Thedifferentpeaksanalyzedduringthesamplegrowthcanbe observedinthe(h,k)intensitymapmeasuredataconstantLvalue of1.04rlu.ThemapsofOp1:1revealanapparentpseudosingle crystalhexagonalpattern(Fig.2),andthepresenceofrings.The dependenceoftheintensityalongtheldirectionshowsthatthe ringsareduetophasesalmostperfectlyorientedalongthe[001] direction.ThereﬂectionspresentasBraggpeakshavea periodic-ityandstructurecomparabletoanorderedphaseofCu2S,already

reportedbyourgroupelsewhere[16,17,33].Theoperando crystal-lographicstructureofthegrownCu2Sfilmshasstrongsimilarities withtheoneofthelowchalcocite[16,17,33],althoughthein-plane latticeparameterspointtoadifferentmetric[21].Thestructural modelofCu2Sproposedbytheseauthorsisbasedona hexagonal-close-packedframeworkofSatoms.Inthisframework,Cuatoms arepartially fillingtriangularvoids withintheSlayers perpen-dicularto[001],and partiallyintercalatedbetweenadjacentS layersparalleltothesurfaceinanarrangementsimilartonatural chalcocite’sstructural model [35]. We alsoregisteredthe pres-enceofthecharacteristicBraggpeaksinthepositionsexpected forwurtzite,i.e.(00.751.13).However,theseBraggpeaksarevery closetothepeaksoftheCu2S,i.e.(00.731.04).Accordingly,the proximityof ZnS andCu2Sreflectionsprevented a quantitative analysis.Eventually,superimposedtotheBraggreflections,aset ofDebyeringscanbeidentified(Fig.2).Thesewillbediscussedin thefollowingparagraph.

3.2. Powderscans

The␦-scanswereperformedontheOp1:1sampleatdifferent stages ofgrowth(0, 15,30, 45 and 60cycles). The experimen-talXRPDspectrumofthesampleOp1:1at60cyclesisshownin

Fig.3a.Allreflectionscanbeindexedaccordingtothestructureof monoclinicchalcocite(spacegroupP21/c,[34–40]).Thesmall dis-crepanciesbetweentheRietveldbestfitandtheexperimentaldata arecompatiblewithwurtzitestructure(spacegroupP63mc[41]). However,duetotheirsmallintensity,itisnotpossibletoperforma quantitativeanalysis.Nootherreflectionsduetospuriousproducts orrelicsoftheprecursorsareevident(Fig.3b).Thenondimensional R-BraggvaluesfortheRietveldfitsareintherangeof4–10.Under thepresentexperimentalconditions,theminimumdetectionlimit

(5)

Fig.4.ResultsoftheBraggpeaksanalysisfortheOperando2D-mapsat(0.73,0.73,4.18).(a)theBraggpeakswiththerelatedbestﬁtsand(b)theparametersresultingfrom thebestﬁts.

achievableforthemethodisnolessthan1wt%.The␦-scansare presentedin Fig.3caftertheremovalof thebackgroundsignal fromthediffusescattering.TheXRPDshows aconstantgrowth ofthedisorderedphasewhichseemtoincreaserapidlybetween the45thand60thcycles.Thistrendisquantitativelyconfirmedby theresultsoftheRietveldquantification,asillustratedbyFig.3d, wherewereportthescalefactor(i.e.afactorproportionaltothe totalamountofchalcocite)versusafunctionofthenumberofE-ALD cycles.Apparently,thegrowthofchalcociteproceedswithahigher ratebetweenthe15thand30thandbetweenthe45thandthe60th cycle,thanbetweenthe30thandthe45thdepositioncycle. 3.3. Singlelineprofileanalysis

Thegrowthprocessisstudiedbyfollowingthetrendsforthe ﬁtparametersobtainedbysinglelineproﬁleanalysisoftheBragg peaksintheOp1:1sample.Inthefollowing,wepresentthegrowth parametersrelatingtothepeakat(0.73,0.73,4.18)presentedin

Fig.4.Amongthemostintense,thispeakhasnooverlappingwith othersignalsandprovidesveryreliableinformation[21,33].The coordinatesofthispeakrefertoaCu2Schalcocite-likecrystal struc-turepreviouslyfoundinotherstudies.Thispeakallowsustostudy thegrowingprocessoftheorderedCu2Sphasewiththe deposi-tioncycles.Thepeak proﬁlesregisteredatthedifferentnumber ofdepositioncyclesareshowninFig.4a,togetherwiththe least-squaresbestﬁtcurvesobtainedusingtheVoigtfunction.Thereisa smallchangeintheBragg’speakspositionduringthegrowth(inthe range4.194–4.204)andwithrespecttotheCu2Sgrownbymeans ofthebinaryscheme(4.186rlu)[21],thesediscrepanciesareinthe range0.1%–0.4%comparabletotheuncertaintiesofthe position-ing.Still,thisdifferenceisnogreaterthantheuncertaintiesonthe positioningthatcanbeachievedwithoursetup.

Theevolutionofthemostrelevantparametersisshowninthe graphsof Fig.4b, where itis possibletoobserve thefollowing trends:wLdecreasesuntilthecycle45andthenincreasesslightly: thiscanberelatedtoanincreaseofcrystallitesizeatleastuptothe 45thcycle.ThetrendforwGisexactlytheopposite,thuspointing toanincreaseofstrainoftheﬁlmuntil45cycle.Thescale parame-terA,whichisrelatedtotheamountofdepositedmaterial,grows upto45cycleandthendecreasesslightly.

3.4. Morphology

AsalreadyreportedforCuxZnyS1:1(60cycles)theapparent threadofnanowiresonthetopoftheelectrode+ﬁlmsystempoints

tothepresenceofatleastapolycrystallinephase[16,17].InFig.5

themorphologyoftheEx20,Ex40andEx60samplescorresponding todifferentstagesofgrowthfortheCuxZnyS1:1.Theimagesreveal thelackofuniformityoftheﬁlm.Inparticular,after20deposition cycles,thesurfaceisnolongersmooth,asitoccursforCu2Sﬁlm

[42].Thefilm,infact,appearscoveredbyseveralgrainsstepping aheadfromitssurface.After40cycles,thesestructuresarereplaced bynanowireswiththelengthoftheorderof1–2␮m,andthe thick-nessintherangeoffewtensofnanometers.After60cycles,the number,densityandlengthofthenanowiresisdefinitelyincreased, whereasthewirethicknesshasremainedalmostthesamethan before, in full agreementwithpreviousstudies onthis type of film[16,17].Theseevidencescanberationalizedasitfollows:the sampleat20cyclesrepresentsanintermediatestageofthe reor-ganizationprocess;infact,onlystartingfromthe40thcycle,the formationofthread-likestructuresisobserved.Withthe increas-ingnumberofdepositioncycles,themorphologyoftheternary compound becomesmore complex,leading totheformationof nanoaggregatethread-likestructureswithlengthofseveral␮m.

4. Discussion

Thewholesetoftheexperimentaldataobtainedinthisstudy highlights,andconfirms,thepreviousfindingsontheE-ALDfilms realizedintheCu-Zn-Ssystem[16,17].Thedata(Figs.2and3),point outthatordered(pseudo-singlecrystal)anddisordered polycrys-tallinephasesareoccurringinthesampleOp1:1,aswellasthat thefinalproductdocontainstwodifferentCu2Sphases,with dif-ferent structuralarrangements,and wurtziteZnS.Thisevidence apparentlycontrastswhatalreadyobservedonotherbinary sul-phides,includingtheCu2Sitself[33,34],andtheternaryZn-Cd-S films[14,15].

The set of pseudo-single crystal Bragg reflections has been assignedtotheCu2S(chalcocitetype)phasealreadyreckoned dur-ingtheoperandoSXRDstudyoftheE-ALDgrownbinarycopper sulfide[33,34].Thegrowthoftheorderedphaseissimultaneously accompaniedbytheformationofadisorderedCu2Sphase,this lat-terfullycoincidentwiththenaturallowtemperaturechalcocite polymorph,thepresenceofwhichisalsoconfirmedbytheDebye ringspresentinthemapofFig.2.Thefinalassignmentofthis disor-deredphasewasconfirmedbymeansofRietveldanalysis(Fig.3).

ConcerningZnS,ourdatapointtotwopossiblephasesinthe sample,i.e.certainlyapseudosinglecrystalwurtziteandpossibly apolycrystallinewurtzite.Thisresultcanbeconsideredinlinewith

(6)

Fig.5.MorphologicalanalysisoftheCuxZnyS1:1at1000X(a),(b),(c)andat6500X(d),(e),(f).(a)and(d)refertoEx20,(b)(e)Ex40while(c)(f)Ex60.

thatof[16,17]:intheirstudy,infact,thepresenceoftwophases Cu2SandZnSwasderivedfromXASspectroscopy.Theseauthors haveattributedZnSeithertowurtziteortosphaleritepolymorphs, withpreferencetothelatter,buttheywerenotabletoﬁnallysolve thisissuebecauseoftheshortrangestructuralenvironment inves-tigatedbytheXAStechniquearoundZn.Thestudyofthetrends oftheCu2Scrystallites strainandsize (Fig.4a)depicts aneven morecomplexsituationoccurringduringthecrystalgrowth.These trendssuggestagrowthlessorderedthantheoneforCu2S sam-plesanalyzedinpreviousstudies[34].Thecrystallitestrainand sizealongthec-axisincreasewiththedepositioncyclesuntilthe 45thcycleandthenabruptlydecreases;thesametrendisfollowed bythevalueA,whichisrelatedtotheamountoftheorderedCu2S deposited.Itisworthtonoticethatafterthe45thcycle,theamount ofthepolycrystallinephaseincreases,whereasthatofthe epitax-ialphasedecreases.Inthiscontext,theeffectoftheZndeposition stepemergesclearly.Sincetheintercalationofcyclesdedicatedto thedepositionofZn-Slayersistheonlydifferencebetweenthe E-ALDschemeforthedirectgrowthofCu2S,theaccumulationofthe stressaswellasthegrowthofthenanowiresarestraightrelatedto theeffectofthedepositionofZn,andtheyarealsoprobablyjoined byacause-and-effectrelationship.Wesuggestthatthereleaseof thestressleadstotheabruptincreaseoftheamountofthe poly-crystallinephaseduringthelastpartofthegrowth.Anopposite trendisrecordedforthedisorderedpattern,forwhichtheRietveld scalefactorobtainedfromthe␦-scansincreasesabruptlyfromthe 45thcycletothe60thcycle(Fig.3b).Themicrographsforthe ex-situsamplesregisteredtheabruptincreaseofaverycomplexnetof nanowiresbetweenthe40thand60thcycles(Fig.5).Sincenoother diffractionsignalswererecordedonthissystem,thenanowiresare attributedtothedisorderedphaseobservedbythe␦-scans,i.e.to Cu2Schalcocite.Hence,themicrographsallowavisual conﬁrma-tionoftheabruptincreaseofthedisorderedphaseinthelastpart ofthegrowth.

5. Conclusion

Fromtheearlystages of thegrowth,at leasttwo phase are clearlypresent:an orderedphase (pseudo-single crystal)and a disordered(polycrystalline)phase,asconﬁrmedby

morphologi-calandXRPDdata.Ingeneral,thegrowthoftheCuxZnySbymeans ofE-ALDseemstobedrivenbytheoccurrenceofcopper-sulphides structures,asexpectedfromrecentstudiesthatrevealedastrong Zndeﬁciency [6,14]. No other phases involvingzinc sulphides, butwurtzite,areevidentinthepresentwork.However[16,17], havesuggestedtheoccurrenceofthecubicZnSpolymorph, spha-lerite.WerelyonthepresentattributionoftheZnSphase,asthe performedXRPDinvestigationproperlydealwiththelongrange arrangementofthestructure.

TheSXRDallowedthecharacterizationofthechangesinthe featuresofthecrystallitesofthepseudo-singlecrystalandinthe scalefactorsofboththepseudo-singlecrystalandthe polycrys-tallinephase.Theseresultspointstotheoccurrenceofacomplex growthprocessinvolvinganorientedandstrainedphase witha randomlyoriented,polycrystallinephasegrowingontopofit.After the45thcycle,most ofthestress isreleased byincreasingthe amountofdisorderedphase,likelybymeansofastructural rear-rangementorre-crystallization.ComparingthegrowthofOp1:1 withthedirectgrowthofCu2S[34]wereckonedageneralincrease ofthestrain,revealingahighconcentrationofrandomlydistributed structuraldefectsintheCu2Sepitaxialphase.Ourhypothesisisthat CuvacancyorZninclusioninadefectiveCu2Sstructureexplain theincreasedstrain.Inthiscontext,theeffectoftheZn deposi-tionstepemergesclearly,leadingtoaccumulationofstressthat seemstodrivethegrowthof thenanowires,henceleadingthe abruptincreaseoftheiramountduringthelastpartofthegrowth. ThisworkshedlightonthestructureandgrowthofCuxZnySand the origin of its complex morphology. Such results suggest an unexpectedly complexgrowthmechanismof theCuxZnySﬁlms depositedbymeansofE-ALD.Infact,asdiscussedlatelyinother similarpapers [21,43]theE-ALD growth,although surface lim-ited,isprobablynotonlytheresultofalayer-by-layerdeposition, butmostprobablytheproductofmultiplecompetitiveprocesses includingsurfacelimitedequilibria.Eventually,weshouldnotice thatsuchhighlystructuredsurfacesgrowsontopofaveryordered semiconducting surface and the overall product is obtainedby meansof very reproducible surfacecontrolled scheme.Thus, it couldbeengineeredtobeappliedinseveralﬁeldswherethegrowth ofsemiconductingmaterialwithhighsurfaceareaisrequired(e.g. aphotocatalyticmaterial[44]).

(7)

Acknowledgements

TheCentrodiServizidiMicroscopiaElettronicae Microanal-isi(MEMA),UniversitàDegliStudidiFirenze,isacknowledgedfor kindlygrantingtheuseoftheScanningElectronMicroscopy,as wellasMarioPaolieriand Maurizio Uliviareacknowledgedfor theirassistanceinthesameinvestigations.FDBandMIbeneﬁtedfor departmentalfunding(ex60%).CNRisacknowledged,forsupport. ThisworkwasfundamentallysupportedbytheassistanceintheAg singlecrystalsynthesisandpreparationbyFerdinandoCapolupo,at theElectrochemicalLaboftheDept.ofChemistryoftheUniversity ofFlorence.FerdinandoCapolupoiswarmlyacknowledgedforit.

References

[1]M.Innocenti,F.DiBenedetto,A.Lavacchi,N.Ciofﬁ,R.Felici,L.A.Pardi, Fabricatingenergydeviceswithlowenvironmentalimpacts,SPIENewsroom 2–4(2016),http://dx.doi.org/10.1117/2.1201512.006249.

[2]B.W.Stickney,J.L.Gregory,Electrochemicalatomiclayerepitaxy(ecale),J. Electroanal.Chem.300(1991)543–561.

[3]A.Giaccherini,R.Felici,M.Innocenti,OperandoStructuralCharacterizationof theE-ALDProcessUltra-ThinFilmsGrowth,in:M.Khodaei(Ed.),X-ray CharacterizationofNanostructuredEnergyMaterialsbySynchrotron Radiation(2017).

[4]EuropeanCommission,CriticalRawMaterialsfortheEU,2010.

[5]S.Caporali,A.Tolstogouzov,O.M.N.D.Teodoro,M.Innocenti,F.DiBenedetto, S.Cinotti,etal.,Sn-deﬁciencyintheelectrodepositedternaryCuxSnySzthin ﬁlmsbyECALE,Sol.EnergyMater.Sol.Cells138(2015)9–16,http://dx.doi. org/10.1016/j.solmat.2015.02.029.

[6]M.Innocenti,L.Becucci,I.Bencistà,E.Carretti,S.Cinotti,L.Dei,etal., ElectrochemicalgrowthofCu-Znsulﬁdes,J.Electroanal.Chem.710(2013) 17–21,http://dx.doi.org/10.1016/j.jelechem.2013.01.024.

[7]E.Acta,ElectrochemicalLayerbyLayerGrowthandCharacterizationof CopperSulfurThinFilmson,2011,http://dx.doi.org/10.1016/j.electacta.2011. 10.004.

[8]C.C.Uhuegbu,E.B.Babatunde,Thestudyofcopperzincsulphide(CuZnS2)thin

ﬁlms,J.Phys.32(2008)39–47.

[9]A.I.Mukolu,K.A.Aduloju,OpticalabsorptionandtransmissioninCuZnS alloys,Glob.J.PureAppl.Sci.15(2009)421–425.

[10]N.Kitagawa,Copperzincsulfurcompoundsolarcellsfabricatedbyspray pyrolysisdepositionforsolarcells,Nat.Resour.04(2013)142–145,http://dx. doi.org/10.4236/nr.2013.41A018.

[11]P.B.Bagdare,S.B.Patil,A.K.Singh,PhaseevolutionandPECperformanceof ZnxCd(1-x)SnanocrystallinethinﬁlmsdepositedbyCBD,J.AlloysCompd. 506(2010)120–124,http://dx.doi.org/10.1016/j.jallcom.2010.06.152. [12]M.Adelifard,H.Eshghi,M.M.BagheriMohagheghi,Synthesisand

characterizationofnanostructuralCuS-ZnSbinarycompoundthinﬁlms preparedbyspraypyrolysis,Opt.Commun.285(2012)4400–4404,http://dx. doi.org/10.1016/j.optcom.2012.06.030.

[13]M.Innocenti,S.Cinotti,I.Bencistà,E.Carretti,L.Becucci,F.DiBenedetto,etal., ElectrochemicalgrowthofCu-Znsulﬁdesofvariousstoichiometries,ECS Trans.161(2014)D14–D17.

[14]F.Loglio,M.Innocenti,G.Pezzatini,M.L.Foresti,Ternarycadmiumandzinc sulﬁdesandselenides:electrodepositionbyECALEandelectrochemical characterization,J.Electroanal.Chem.562(2004)117–125,http://dx.doi.org/ 10.1016/j.jelechem.2003.08.016.

[15]M.Innocenti,S.Cattarin,F.Loglio,T.Cecconi,G.Seravalli,M.L.Foresti,Ternary cadmiumandzincsulﬁdes:composition,morphologyand

photoelectrochemistry,Electrochim.Acta49(2004)1327–1337,http://dx. doi.org/10.1016/j.electacta.2003.08.032.

[16]F.Di,S.Cinotti,F.D.Acapito,F.Vizza,M.Luisa,A.Guerri,etal.,

Electrodepositedsemiconductorsatroomtemperature:anX-rayAbsorption SpectroscopystudyofCu-,Zn-,S-bearingthinﬁlms,Electrochim.Acta179 (2015)495–503,http://dx.doi.org/10.1016/j.electacta.2015.05.168. [17]F.DiBenedetto,S.Cinotti,A.Guerri,A.DeLuca,A.Lavacchi,G.Montegrossi,

etal.,PhysicalcharacterizationofthinﬁlmsofCuxZnySzforphotovoltaic

applications,ECSTrans.58(2013)59–65.

[18]B.M.Huang,L.P.Colletti,B.W.Gregory,J.L.Anderson,J.L.Stickney,Preliminary studiesoftheuseofanautomatedﬂow-cellelectrodepositionsystemforthe formationofCdTethinﬁlmsbyelectrochemicalatomiclayerepitaxy,J. Electrochem.Soc.142(1995)3007–3016,http://dx.doi.org/10.1149/1. 2048677.

[19]M.L.Foresti,M.Innocenti,G.Pezzatini,F.Forni,CdSandZnSdepositionon Ag(111)byelectrochemicalatomiclayerepitaxy,J.Electrochem.Soc.148 (2001)C357–C362.

[20]M.L.Foresti,A.Pozzi,M.Innocenti,G.Pezzatini,F.Loglio,E.Salvietti,etal., InsituX-rayanalysisundercontrolledpotentialconditions:Aninnovative

setupanditsapplicationtotheinvestigationofultrathinﬁlms

electrodepositedonAg(111),Electrochim.Acta51(2006)5532–5539,http:// dx.doi.org/10.1016/j.electacta.2006.02.031.

[21]A.Giaccherini,R.Felic,M.Innocenti,Operandostructuralcharacterizationof theE-ALDprocessultra-thinﬁlmsgrowth,in:X-RayCharacterizationof NanostructuredEnergyMaterialsbySynchrotronRadiation,InTech,2017,pp. 31–50.

[22]M.L.Foresti,F.Capolupo,M.Innocenti,F.Loglio,Visualdetectionof crystallographicorientationsofface-centeredcubicsinglecrystals,Cryst. GrowthDes.2(2002)73–77,http://dx.doi.org/10.1021/cg015537i. [23]M.Luisa,A.Pozzi,M.Innocenti,G.Pezzatini,F.Loglio,E.Salvietti,etal.,Insitu

X-rayanalysisundercontrolledpotentialconditions:aninnovativesetupand itsapplicationtotheinvestigationofultrathinﬁlmselectrodepositedonAg (111),Electrochim.Acta51(2006)5532–5539,http://dx.doi.org/10.1016/j. electacta.2006.02.031.

[24]C.Ponchut,J.M.Rigal,J.Clément,E.Papillon,A.Homs,S.Petitdemange, MAXIPIX,afastreadoutphoton-countingX-rayareadetectorforsynchrotron applications,J.Instrum.6(2011),http://dx.doi.org/10.1088/1748-0221/6/01/ C01069,C01069–C01069.

[25]A.Altomare,C.Cuocci,C.Giacovazzo,A.Moliterni,R.Rizzi,N.Corriero,etal., EXPO2013:akitoftoolsforphasingcrystalstructuresfrompowderdata,J. Appl.Crystallogr.46(2013)1231–1235,http://dx.doi.org/10.1107/ S0021889813013113.

[26]S.Roobol,W.Onderwaater,J.Drnec,R.Felici,J.Frenken,BINocularsData reductionandanalysissoftwarefortwo-dimensionaldetectorsinsurface X-raydiffraction,J.Appl.Crystallogr.48(2015)1324–1329,http://dx.doi.org/ 10.1107/S1600576715009607.

[27]J.Drnec,T.Zhou,S.Pintea,W.Onderwaater,E.Vlieg,G.Renaud,etal., IntegrationtechniquesforsurfaceX-raydiffractiondataobtainedwitha two-dimensionaldetector,J.Appl.Crystallogr.47(2014)365–377,http://dx. doi.org/10.1107/S1600576713032342.

[28]T.H.deKeijser,J.I.Langford,E.J.Mittemeijer,A.B.P.Vogels,UseoftheVoigt functioninasingle-linemethodfortheanalysisofX-raydiffractionline broadening,J.Appl.Crystallogr.15(1982)308–314.

[29]J.I.Langford,Accuracyinpowderdiffraction,Natl.Bur.Stand.Spec.Publ.567 (1980)255–269.

[30]J.I.Langford,Thevarianceandothermeasuresoflinebroadeninginpowder diffractometry.I.PracticalConsiderations,J.Appl.Crystallogr.1(1968)48–59,

http://dx.doi.org/10.1107/S0021889868005182.

[31]B.E.Warren,B.L.Averbach,Theeffectofcold-workdistortiononX-ray patterns,J.Appl.Phys.21(1950)595.

[32]J.I.Langford,R.Delhez,T.H.deKeijser,E.J.Mittemeijer,Proﬁleanalysisfor microcrystallinepropertiesbytheFourierandothermethods,Aust.J.Phys.41 (1988)173–187,http://dx.doi.org/10.1071/PH880173.

[33]A.Giaccherini,I.Bencista,S.Cinotti,G.Montegrossi,A.Guerri,F.DiBenedetto, etal.,Synthesisandtechnologicalapplicationofelectrodeposited

semiconductorsbyEC-ALD,ECSTrans.58(2014)35–41,http://dx.doi.org/10. 1149/05832.0035ecst.

[34]A.Giaccherini,S.Cinotti,A.Guerri,F.Carlà,G.Montegrossi,F.Vizza,etal., OperandoSXRDstudyofthestructureandgrowthprocessofCu2Sultra-thin ﬁlms,Sci.Rep.(2017)(In-press).

[35]H.T.Evans,Crystalstructureoflowchalcocite,Nat.Phys.Sci.232(1971) 69–70,http://dx.doi.org/10.1038/physci232069a0.

[36]S.Graˇzulis,A.Daˇskeviˇc,A.Merkys,D.Chateigner,L.Lutterotti,M.Quirós, etal.,CrystallographyOpenDatabase(COD):anopen-accesscollectionof crystalstructuresandplatformforworld-widecollaboration,NucleicAcids Res.40(2012)D420–D427,http://dx.doi.org/10.1093/nar/gkr900. [37]A.Merkys,A.Vaitkus,J.Butkus,M.Okuliˇc-Kazarinas,V.Kairys,S.Graˇzulis,

COD:CIF:Parser.Anerror-correctingCIFparserforthePerllanguage,J.Appl. Crystallogr.49(2016)292–301,http://dx.doi.org/10.1107/

S1600576715022396.

[38]S.Graulis,D.Chateigner,R.T.Downs,A.F.T.Yokochi,M.Quirós,L.Lutterotti, etal.,Crystallographyopendatabase−anopen-accesscollectionofcrystal structures,J.Appl.Crystallogr.42(2009)726–729,http://dx.doi.org/10.1107/ S0021889809016690.

[39]R.T.Downs,M.Hall-Wallace,Theamericanmineralogistcrystalstructure database,Am.Miner.88(2003)247–250.

[40]S.Graˇzulis,A.Merkys,A.Vaitkus,M.Okuliˇc-Kazarinas,Computing stoichiometricmolecularcompositionfromcrystalstructures,J.Appl. Crystallogr.48(2015)85–91,http://dx.doi.org/10.1107/S1600576714025904. [41]R.W.G.Wyckoff,CrystalStructures,1963.

[42]M.Innocenti,I.Bencistà,S.Bellandi,C.Bianchini,F.DiBenedetto,A.Lavacchi, etal.,Electrochemicallayerbylayergrowthandcharacterizationofcopper sulfurthinﬁlmsonAg(111),Electrochim.Acta58(2011)599–605,http://dx. doi.org/10.1016/j.electacta.2011.10.004.

[43]F.Carlà,F.Loglio,A.Resta,R.Felici,E.Lastraioli,M.Innocenti,etal., ElectrochemicalatomiclayerdepositionofCdSonAgsinglecrystals:effects ofsubstrateorientationonﬁlmstructure,J.Phys.Chem.C118(2014) 6132–6139,http://dx.doi.org/10.1021/jp405637g.

[44]T.Jafari,E.Moharreri,A.Amin,R.Miao,W.Song,S.Suib,Photocatalyticwater splitting—theuntameddream:areviewofrecentadvances,Molecules21 (2016)900,http://dx.doi.org/10.3390/molecules21070900.