X-ray tomography ptychographic by high-temperature materials Photonic for applications: Synthesis andcharacterization Applied Materials Today

characterization by X-ray ptychographic tomography

Kaline P. Furlan

, Emanuel Larsson

, Ana Diaz

, Mirko Holler

, Tobias Krekeler

, Martin Ritter

, Alexander Yu. Petrov

, Manfred Eich

, Robert Blick

, Gerold A. Schneider

, Imke Greving

,

Robert Zierold

, Rolf Janßen

aInstituteofAdvancedCeramics,HamburgUniversityofTechnology,Denickestraße15,21073Hamburg,Germany

bCenterforHybridNanostructures,UniversitätHamburg,LuruperChaussee149,22607Hamburg,Germany

cInstituteofMaterialsResearch,Helmholtz-ZentrumGeesthacht,Max-Planck-Strasse1,21502Geesthacht,Germany

dPaulScherrerInstitut,5232VilligenPSI,Switzerland

eElectronMicroscopyUnit,HamburgUniversityofTechnology,EißendorferStraße42,21073Hamburg,Germany

fInstituteofOpticalandElectronicMaterials,HamburgUniversityofTechnology,EißendorferStraße38,21073Hamburg,Germany

gITMOUniversity,49KronverkskiiAvenue,197101St.Petersburg,Russia

a r t i c l e i n f o

Articlehistory:

Received12July2018 Receivedinrevisedform 19September2018 Accepted7October2018

Keywords:

PtychographyX-raycomputedtomography 3Dimageanalysis

Low-temperatureatomiclayerdeposition Photonicmaterials

High-temperatureapplications

a b s t r a c t

Photonicmaterialsforhigh-temperatureapplicationsneedtowithstandtemperaturesusuallyhigher than1000^◦C,whilstkeepingtheirfunction.Whenexposedtohightemperatures,suchnanostructured materialsarepronetodetrimentalmorphologicalchanges,howeverthestructureevolutionpathwayof photonicmaterialsanditscorrelationwiththelossofmaterial’sfunctionisnotyetfullyunderstood.

Hereweusehigh-resolutionptychographicX-raycomputedtomography(PXCT)andscanningelectron microscopy(SEM)toinvestigatethestructuralchangesinmulliteinverseopalphotoniccrystalsproduced byavery-low-temperature(95^◦C)atomiclayerdeposition(ALD)super-cycleprocess.The3Dstructural changescausedbythehigh-temperatureexposurewerequantifiedandassociatedwiththedistinct structuralfeaturesoftheceramicphotoniccrystals.Otherthanobservedinphotoniccrystalsproduced viapowdercolloidalsuspensionsorsol-gelinfiltration,athightemperaturesof1400^◦Cwedetecteda masstransportdirectionfromthenanoporestotheshells.Werelatethesedifferentstructureevolution pathwaystothepresenceofhollowvertexesinourALD-basedinverseopalphotoniccrystals.Although theperiodicallyorderedstructureisdistortedaftersintering,themulliteinverseopalphotoniccrystal presentsaphotonicstopgapevenafterheattreatmentat1400^◦Cfor100h.

©2018TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).

1. Introduction

Photoniccrystalsare three-dimensional periodicallyordered structureswiththecapabilityofaffectingthepropagationofelec- tromagneticradiation bya photonic band-gap [1].The spectral range in which the reflectionof radiation occurs is defined by thespatialorderingofthestructureanditsrefractiveindex.This selectiveradiationpropagationbehavioris attractivefora vari- ety of technological applications, such as thermo photovoltaic energyconversiondevicesandalsonext-generationthermalbar- rier coatings(photonicTBCs) [2–4].However,when exposedto

∗Correspondingauthors.

E-mailaddresses:kaline.furlan@tuhh.de(K.P.Furlan),

rzierold@physnet.uni-hamburg.de(R.Zierold),janssen@tuhh.de(R.Janßen).

hightemperatures(usuallyhigherthan1000^◦C),photoniccrys- talstructuresmayundergoseveralmorphologicalchanges,suchas dimensionaldistortionduetosinteringshrinkageorphasetrans- formation,extensivegraingrowth,andin extremecaseslossof theperiodicalorder [5],thereby impairingitsphotonic proper- ties.Needlesstosay,theretentionofthesehigh-surfacearea3D structureforhigh-temperatureapplicationsremainsasignificant challenge[5].

Since the structural characteristics and 3D phase morphol- ogyof thesephotonicmaterialsdictate how theyinteractwith electromagneticradiation,informationandknowledgeaboutthe relationshipbetweenthematerial’s3Dstructureanditsphotonic properties,beforeandafterexposuretosuchhightemperatures, iscrucialforacompleteunderstandingofthematerial’sbehavior and its application. As thearrangement of thephotonic mate- rial’snanostructuremightdetermineitseffectivepropertiesand https://doi.org/10.1016/j.apmt.2018.10.002

2352-9407/©2018TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).

performance, there is a driving force for more precise, high- resolutioncharacterizationmethods.

Whenanalyzingthemorphologyofinverseopalphotoniccrys- tals,scanningelectronmicroscopy(SEM)isstillthemaintechnique used.Althoughbeingafastandsimpletool,severallimitationsarise whendealingwithoxide-basedinverseopalphotoniccrystals.First ofall,theyareusuallynon-conductive,therebypronetocharging andmechanicaldrift,whichcausesimageartifactsandscanning flaws.Furthermore,ifthesamplesaretobefurtherexposedtohigh temperatures, theycannot becoated withconductive material, whichcouldburn-out,meltorreactwiththestructure.Moreover, theroughnessassociatedtosuchaconductivecoating,oreventhe thickness(usually5–15nm),couldconcealsomeofthenanomet- ricstructuralfeatures ofphotonic crystals.Ontop ofthat, only qualitative 2Dinformation of either thetop viewor cross sec- tionswithalimitedareaofinvestigationareobtained.Meanwhile, transmissionelectronmicroscopy(TEM)imagingisonlycapableof analyzingverytinyareasofthewholeinverseopalphotoniccrystal structure,afterverycarefulandtime-consumingsampleprepara- tion.Besidesthat,TEMinvolvesastronginteractionbetweenthe electronsof theprimary beamand thematerialitself,which in turncaninfluencethestructureandleadtoartifacts,andrequires ahigh-vacuumenvironment.FIB-tomographyontheotherhand couldbeusedtoobtainhigh-resolution3Dstructuralinformation, howeverthis isadestructivetechnique.Furthermore,forhighly porousmaterials an infiltration step withepoxy or conductive materialisrequired,inordertoavoidexcessivesampledrift,which wouldcausevariationsintheprescribedslicethicknessandeven lossofinformationinthez-axisdirection(anisotropicresolution) [6].

Alternatively, X-ray computed tomography is a powerful non-destructivetechniquetoinvestigatetheinnerstructuremor- phologyofporousmaterials[7–9],suchastheinverseopalphotonic crystals,inamultilengthscaleapproach.Incaseofthesamples studiedherein,theresolution ofmicrotomography(micrometer scale)is notenough,but synchrotrontomography,more specif- ically, ptychographic X-ray computed tomography (PXCT), can imagedifferent structuralfeatures fromthemicrometer tothe nanometerscalewithhighresolution,eveninairatatmospheric pressure[10,11].

Thefabricationprocessdefinesthefinalstructureandcomposi- tionofaphotonicmaterial,andtherefore,itsphotonicproperties.

Inverseopalphotoniccrystalsareusuallyproducedbytheinfiltra- tionofapolymerictemplate,alsoreferredtoasadirectphotonic crystal,whichcanbeassembledbyavarietyofroutes[12].Infil- trationtechniques include sol-gel [13], colloidalroutes [14,15], chemicalvapordeposition(CVD)[16]andatomiclayerdeposition (ALD)[17].Inverseopalphotoniccrystalscanalsobefabricatedina single-stepapproach,describedasco-deposition[18,19].Depend- ingonthechosenfabricationtechnique,avarietyofceramicinverse opalphotoniccrystalmaterialscanbeproduced,suchassilica[19], titania[17],alumina[14],zirconia[15]andyttriastabilizedzirco- nia(YSZ)[18].Whenrankingthetechniques,ALDisbyfartheone withthehighestcapabilityforuniformcoatingofsuchhighaspect ratiostructures,bywhichveryhighfillingfractionsandinfiltration homogeneityarereported[20].Moreover,ALDbasesonsucces- sive,alternatingreactionsbetweenprecursorsandsubstrate,with excellentfilmthicknesscontrol(ontheÅngstrom-scale)andcom- positionalcontrol[21],evenforcomplexsystemssuchasternary andquaternaryoxides[22].ByALD,tailor-madeatomicallymixed systemscanberealized,suchasmullite,aknownrefractoryceramic systemwithhigh-temperaturestability.

Depositionofmullitefilmsonplanarsubstrates,notphotonic structures,hasalreadybeendemonstratedbyCVD[23],electron beamphysicalvapordeposition(EB-PVD)[24]andplasmaspray- ing[25]. However,alltheseprocesses run atconsiderablyhigh

orveryhightemperatures, i.e.600^◦C[24],1200^◦C [23]oreven higher(>1800^◦C)[25],whichmakesthemunviableforinfiltration of polymerictemplates.In ourpreviouswork [26],we demon- stratedthepossibilityofinfiltratingAl₂O₃ inverseopalphotonic crystalswithmullitebyALDsuper-cyclesperformedat150^◦C.This depositiontemperatureis already consideredlow forALD [21], howeverstilltoohighforpossibleinfiltrationofpolymerictem- plates,usuallybasedonpolystyreneor polymethylmethacrylate latexparticleswithglasstransitiontemperaturesofaround100^◦C [27].AttemperaturesusuallyusedinALD,polymerictemplates maynotonlydecompose,butalsobeoxidizedduringoxidantexpo- sure[28],especiallyincaseof ozone[29].Furthermore,already establishedALDcycleparametersforinorganicmaterials,maynot beadequateforpolymerictemplates,duetothedifferentsurfaces characteristics,resultinginpossibledifferentgrowthmechanisms [30].

In this work, we studied the feasibility of two very-low- temperature(95^◦C)atomiclayerdepositionprocessestoproduce mulliteinverseopalphotoniccrystalsbyinfiltrationofpolystyrene direct photonic crystals prepared by vertical convective self- assembly.The inverseopalphotoniccrystal 3Dstructureswere characterizedbeforeandafterheattreatmentbyX-raydiffraction (XRD),SEMandPXCT[10,11]ata3Disotropicresolutionof52nm.

Thehigh-resolutionimagingbyPXCTenabledthequantificationof 3Dstructuralmodificationscausedbythehigh-temperatureexpo- sure.Wedemonstratethatthisnon-destructivetechniqueenables thecharacterizationofthestructuralfeaturesofinverseopalpho- toniccrystalsatvariedscales,hardlyidentifiedbyothertechniques.

Furthermore,thequantificationofthestructuralchangesofdistinct featuresclarifiesthepathwayforstructuraldestabilizationwiththe temperature.Althoughsomestructuralfeaturesofthephotonic crystalpresentedrelevantdimensionalchangesafterheattreat- ment,aphotonicstopgapwasidentifiedevenafterheattreatment at1400^◦Cfor100h,anoutstandingbehaviorforaphotoniccrys- talconcerningthetemperaturestability[5]forhigh-temperature applications.

2. Materialsandmethods

Averticalconvectiveself-assemblyprocesswasusedtoform direct photonic crystals of monodisperse polystyrene particles (762±22nm, Microparticles GmbH) onto sapphire substrates (1−102,Ø30mm×0.53mm,CrystecGmbH).Beforeimmersion intothePS-watersuspensions(1.0mgml⁻¹),thesubstrateswere cleanedby1-hsonicationina1wt.%detergent(MucasolBrand, MerzHygieneGmbH)deionizedwater(diH₂O)solution,followed bybrushing,diH₂Orinsinganddryingwithnitrogengas.More- over,theywerefurthercleanedandtheirsurfaceswereactivatedby anoxygenplasmatreatmentfor20min(PolaronPT7160,Quorum Technologies).Thecolloidalcrystalgrowthwasperformedinside ahumiditychamber(MemmertHCP108),at70%RHand55^◦Cfor 168h.

Atomic layer deposition was performed at a very-low- temperature of 95^◦C in a super-cycle approach under a full exposuremodein aSavannah^TM 100reactor(Veeco-Cambridge Nanotech)andinahome-madereactor(UniversityofHamburg, PhysicsDepartment,CHyN–CenterforHybridNanostructures), usingnitrogenascarriergas.TheratioofAl2O3:SiO2intheinverse opal photonic crystals structure was varied by the number of internalloopswithinthesupercycle(refertoTableS1).Thepre- cursorsusedforthedepositionswerediH2O,trimethylaluminum, min.98%(TMA,StremChemicals),(3-aminopropyl)triethoxysilane, 98% (APTES, Sigma Aldrich), ozone (OzoneLab^TM, OL80W) andtris(dimethylamino)silane,99+%(TDMAS,Strem chemicals).

Fig.1.ComparisonbetweenthetopviewmorphologyofinverseopalphotoniccrystalsimagedbySEM(a,b)afterburn-outandafteravarietyofheattreatmentsperformed at(c,d)1000^◦Cfor10h,(e,f)1400^◦Cfor4hand(g,h)1000^◦Cfor10hplus1400^◦Cfor4h.OntoparesamplesproducedintheALDsupercycleM1-1andonthebottom supercycleM1-2.Scalebarscorrespondto500nm.

FurtherdetailsareavailableattheTableS1andtheDatainBrief relatedarticle.

Afterinfiltrationofthedirectphotoniccrystals(PStemplates) byALD, thesampleswereheatedup ina Muffle furnacein air for polymer burn-out (0.8^◦Cmin⁻¹, 500^◦C, 30min), generating theinverse opalphotoniccrystals, which werelater character- ized. Their thermal stability was assessed by heat treatments performedunder air atmosphere atdifferent temperaturesand dwelltimes upto1500^◦C (refer toTableS2). Theinverse opal photoniccrystalstopviewandcrosssection2Dstructuralmor- phologywasanalyzedbyscanningelectronmicroscopy(SEM,Zeiss Supra55VP).PhaseidentificationwasperformedbyGrazinginci- denceX-raydiffractionanalysis[31](BrukerAXSD8Advance,Cu K␣,40kV, 40mA, step size0.01^◦, step time 5s, incidentbeam glancingangle1.5^◦).Furtherdetailsofthecrystalstructureinves- tigations may be obtained from the Fachinformationszentrum Karlsruhe,76344Eggenstein-Leopoldshafen(Germany),onquot- ingthedepositorynumbersCSD-156191andCSD-66448(mullite), CSD-173625(alumino-silicate),CSD-36233(gippsite),CSD-39104 (eta Al2O3) and CSD-60419 (alpha alumina) or at the Crystal- lographyOpen Database(COD)onquoting thepatternnumbers indicatedin figurescaptionsand text.Opticalreflectionspectra weremeasuredwitha UV–vis–NIRspectrometer(Perkin-Elmer, Lambda1050)andcollectedinthe0.9–1.8␮mwavelengthrange, beforeandafterheattreatments.

CylindricallyshapedsampleswerepreparedforthePXCTmea- surements using a FEI Helios Nanolab G3 UC FIB (for further details checkthe Data in Brief related article).The PXCT mea- surementswere performedatthecSAXSbeamline oftheSwiss LightSourceatthePaulScherrerInstitut,Switzerland.Thetem- perature of the sample stage was90K and the measurements wereperformedat6.2keVphotonenergy.800projectionswere recorded,equallyspacedinanglesrangingfrom0^◦to180^◦.After alignmentoftheprojections,thetomographicreconstructionwas performed.Theisotropic3DresolutionwasevaluatedusingFourier shellcorrelation(asin[32])andresultedin54nmand52nmfor thesamplesbeforeandafterheattreatment,respectively.Image postprocessingandanalysiswereappliedonthereconstructed dataset.For thequantitative3Dimageanalysis,10 volume-of- interests(VOIs)withcubic shapewerechosenthroughout each sample(edgelengthof2.65␮m,totalvolumeof18.60␮m³),after arepresentativevolumeofinterest(RVI-test)hadbeenperformed.

A detailed description is found at the associated Data in Brief article.

3. Resultsanddiscussion

3.1. Inverseopalphotoniccrystalthermalstability

Bothtypesofinverseopalphotoniccrystals,producedbythe ALDsuper-cyclesM1-1andM1-2respectively,presentedthetypi- calappearanceofanorderedstructure,achievedafterburn-outof thePStemplate(111planeseeninthetopviewfromFig.1aand b).Thehighly-orderedstructureisnotaffectedbythemullitefor- mationheattreatment,carriedoutinairat1000^◦Cfor10h(Fig.1c andd).Althoughsimilartoadirectphotoniccrystaltopview(which isahighly-ordered3Dstructureofinterconnectedparticles),itis necessarytomentionthataninverseopalphotoniccrystalconsists ofastructureof3Dhighly-orderedmacro-poreswhichareinter- connectedthroughcontactpointsattheshell,highlightedinFig.2.

Insuchastructure,themacro-poresizeisdefinedbythesizeofthe formersacrificialpolystyreneparticle(sub-micronscale)andthe shellsizeisdefinedbytheALDsupercycle(nanometerscale).In caseofinverseopalphotoniccrystalsproducedbyALD,theinter- stitialsites(denotedasnanoporeshereafter–seeFig.2)willnever becompletelyfilled,duetothenatureoftheALDprocess,i.e.there isalimitforwhichtheprecursorscanreachthesesitesandcreate afilm.Beyondthislimit,theinterstitialsitewillbesurroundedby shells/coatingsandturnintoanisolatednano-poreinthemiddle ofthestructure.

Afterheattreatmentat1400^◦Cfor4h(Fig.1e–h)crystallite grainswereidentifiedinthemulliteinverseopalphotoniccrystals forbothpre-treated(1000^◦C10h)andnon-pre-treatedconditions.

Overall,alltheanalyzedstructurespresentedquitesimilarmor- phologyandnodifferentiationcouldbemadebetweenthedifferent ALDsuper-cycles.Theappearanceofaslightlydistortedlatticein Fig.1isjustrelated tothesamplespotanddomainorientation (self-assembly domains)in relationtothebeam,which is usu- allyrotatedforimageacquisition.Moreover,thesamples’structure remainedstableevenafter4hofheattreatmentat1400^◦C,whichis aremarkableachievementconcerningtemperaturestability,espe- ciallyincomparisontopreviousreports[5].Forinstance,areport presentedbyTangetal.describesthestructurespartialcollapse alreadyat850^◦CforSiO2 andTiO2 structuresproduced bycen- trifugation (PMMAspheresas template)and sol–gelinfiltration [33].Asol-gel routewasalsousedbySokolovetal.toproduce

␣-Al2O3 inverseopalphotoniccrystalsby infiltrationof PMMA templates,forwhich aheattreatmentat1300^◦Cfor4hcaused a distortion ofthe structure(0.93␮mand 1.58␮mmacropore

Fig.2.Structuralfeaturesinatypicalinverseopalphotoniccrystal.SEM,topviewshowingFCCplane(111)andthreehalf-cutshells.Macro-pore(PStemplateparticlesize) isaround1.5␮m.

Fig.3.Cross-sectionalviewofinverseopalphotoniccrystalsimagedbySEMafterheattreatmentat1400^◦Cfor4h(a,b)withoutand(c,d)with‘mullitization’pre-treatment at1000^◦Cfor10h;(a,c)mulliteinverseopalphotoniccrystals,cycleM1-1;(b,d)mulliteinverseopalphotoniccrystals,cycleM1-2.Thedebrisanddifferentviewingplanes inthepictureoriginatedduringsectioningduetomechanicalfracture.Scalebarsrepresent1␮m.

size)orevenfulldestruction,formingavermicular-likestructure (0.4␮mmacroporesize)[13].Amorestableinverseopalphotonic crystalofYSZ waspresentedbyLashtabeg etal. [34].Although theauthorsclaimstructuralstabilityupto1400^◦C,accentuated sinteringand graingrowthcanalready beobservedat 1100^◦C, whileinthestructureheattreatedat1400^◦Calossinthestruc- turalorderingisclearlyidentifiedattheSEMimages(reflectance measurementswerenotpresented,asthiswasnottheapplication focus).La0.8Sr0.2MnO3/YSZinverseopalphotoniccrystalsproduced bysol–gelinfiltrationofPStemplates(0.35␮mmacroporesize) werestudiedbyZhangetal.and,incontrast,athermalstabilityup toonly1100^◦Cinconventionalfiringwasreported[35].Titania[17]

andalumina[36]inverseopalphotoniccrystalsalsoproducedby ALDshowedpartialstructuralstabilityinairuptoonly1000^◦Cand

1200^◦C,respectively,howeverwithsignificantgraingrowth[17]

andcracks[36].Incontrasttotheseliteraturestudies,thestruc- turesofthemulliteinverseopalphotoniccrystalsproducedherein werestableoveritstotalthicknessasshownbythecross-section analysis(Fig.3).

Furthermore,delaminationoftheinverseopalphotoniccrystal wasonlyobservedforthesamplesproducedunderthecycleM1- 2andatisolatedspots,whichoverallindicatesagoodadhesion ofthemulliteinverseopalphotoniccrystalstothesapphiresub- strate.However,severalcrackswereobservedintheoverallinverse opalphotoniccrystalarea(Ø3cm).Cracksareexpectedinsucha system(mullite-sapphire)duringheatcycling,duetothediffer- entthermalexpansioncoefficientbetweenthesematerials[37],as wellastheconstraintimposedtotheinverseopalphotoniccrystal

Fig.4.Top-viewSEMimagesofmulliteinverseopalphotoniccrystalsshowingexamplesofcracksinbetweentheshells.Samplesheattreatedat1000^◦Cfor10hplus1400^◦C for4h,cycle(a)M1-1and(b)M1-2.Scalebarscorrespondto2␮mandto500nmintheinset.

bythesubstrate[38],whichcouldgeneratehighresidualstresses [39].Crackingwilloccurwhenthestresssurpassesthestructure ormaterialstrength(incaseofcrackinginbetweenthelayersor insidetheinverseopalphotoniccrystalsstructure),orthebonding strengthbetweentheinverseopalphotoniccrystalandsubstrate, for a delamination or buckling occurringwithout any previous crackformation,whichisveryunlikelytohappen.Itisimportant topointoutthattheALDprocessisbasedonchemicalreactions betweentheprecursorsandtheexposedsurfaces,which means thattheinverseopalphotoniccrystalsarechemicallybondedtothe sapphiresubstrates.Nonetheless,cracksareausualdefectinself- assembledpolymertemplates(colloidalspheresfilms),together withvacancies,Frenkeldefectsandscrewdislocations[12].Hence, acertainnumberofcracksarealreadyexistentinthepolymeric template(Fig.S4)beforetheALDsupercycleprocessandareonly relatedtothetemplatefabrication,andnottotheheattreatmentor theALDcycle.Nonetheless,imageanalysisofthemulliteinverse opalphotoniccrystals(seeFig.S5),showedanaverageincrease of6.7%(M1-1)and7.3%(M1-2)intheareaofcracksforthesam- plesheattreatedat1400^◦Cfor 4hwithprevious‘mullitization’

treatment(1000^◦C10h).Surprisingly,thesamplesheattreatedat 1400^◦Cfor4hshowedanaveragevalueofcracksareaveryclose tothevaluesobtainedaftertemplateburn-out(500^◦C),indicat- ingthatthetimeathightemperaturesmightbeakeyfactorfor structuredestabilization,evenmorethantemperature.

The inverse opal photonic crystal itself will have residual stressesaftertheALDdepositionprocess(valuesforAl₂O₃filmsare intherangeof250–470MPa[40,41]),whichcouldbemaximized attheshellcontactpointswiththesubstratewhencomparedto thecontactareaofasingledensefilm(assumingthesameoverall sample area). The inverseopal photonic crystal is then heated up,which couldresultin stressreduction (asin[41]), butalso stressincreaseduetothethermalmismatchbetweenthephotonic crystalandthesubstrate.Thenucleationandgrowthofthemullite crystallitesinsuchstructure,aswellasthephasetransformation ofaluminaphases,couldalsobeanadditionalsourceofresidual stresses.Moreover,during theheattreatment, theinverse opal photoniccrystalstructurestartstosinter,forwhichshrinkageand densificationwilloccur(valuesof9%[42]and6%[41]arereported forAl₂O₃films).Furthermore,astheinverseopalphotoniccrystal structureevolvesduringheattreatmentandcooling(seestructural featuresdescriptioninFig.2),theareasectionofthedifferentfea- turesmayvaryovertime(asshownlaterinthestructureevolution analysisbyhigh-resolutionptychography)andthustheabsolute stressvalues atsuchpoints.Inotherwords,theoverallresidual stressinthewholephotoniccrystalmightbebelowthemechanical resistanceofthe3Dstructure,but atsuchconcentrationpoints thestressesareexpectedtosurpasseventhematerialmechanical resistance, causing cracks and failure inside the inverse opal

Fig.5.Reflectancespectraof(a)mulliteinverseopalphotoniccrystalsbeforeand afterheattreatmentat1400^◦Cfor4handextensiveheattreatmentfor100h;(b) Al2O3inverseopalphotoniccrystalforcomparison,showingamuchhigherreduc- tioninreflectanceafterheattreatmentat1400^◦Cfor4hthanthemulliteinverse opalphotoniccrystal.

photoniccrystalstructure(seeFig.4),whichareentirelydifferent fromtheself-assembly process cracks(polymeric template, see Fig.S4).

The spectral positionof thereflectionpeaks atnormal inci- dencecanbedescribedbyasimpleequation=2×d111×neff [2],wherelambdaisthewavelength,d111istheinterplanarspac- ingin[111]directionoftheinverseopalphotoniccrystalandneffis theaveragerefractiveindexoftheopal.Thereflectionpeakposition of theas-fabricatedsamples(before anyheattreatment) corre- spondstothisequation.Althoughbothphotoniccrystalspresented astablestructureafterheattreatmentat1400^◦Cfor4h,onlythe samplesproducedunderthecycleM1-1stillpresentedaphotonic band-gap(Fig.5)witha significantlyhigherreflectancethanan aluminainverseopalphotoniccrystalheattreatedunderthesame conditions.Theshiftofthepeaktolongerwavelengthmightbea resultofincreasedneffafterthemixtureofSiO₂andAl₂O₃layers duringheattreatment,orincreaseoftheshelldensity,analogousto theresultspresentedbyWangetal.forALDAl₂O₃films[43].This isaninterestingopticaleffectwhichneedsfurtherinvestigation.

Furtherextensiveheattreatmentat1400^◦Cfor100hcausedthe reflectancepeaktobereducedinintensityandshiftedtotheleft.

Thisobservedshiftcanbeattributedtotheverticalshrinkageofthe structureandthusreductionofd111,whilethereductionininten- sitycouldbeassociatedtotheopeningofmicroscopiccracksdue tostructuresintering,aswellasthetextureofmullitegrainsonthe structureshells(compareFig.1aandbandgandh.Eventhough

thereflectancewasreduced andshifted,thepresenceofapho- tonicstopgapafterextensiveheattreatmentat1400^◦Cfor100his remarkable[5],especiallybecausetheinverseopalphotoniccrystal structureseemstobedisordered(Fig.S6).Thiscouldberelatedto somereminiscentperiodicalmodulationintheverticaldirection.

Afterextensiveheattreatmentat1400^◦Cfor100h,theinverse opalphotoniccrystalspresentedaclearlysinteredstructure,with theoccurrenceofabnormalgraingrowth(seewhitearrowsinFig.

S6)anddelamination.Besides,thesampleproducedunderthecycle M1-1(higherAl₂O₃content)presentedanapparentslightlymore refinedstructurethanthesamplesproducedunderthecycleM1-2.

Inbothsamples,grainswithvariedsizesandfacetedappearance wereobserved.Astonishingly,thesamplesarestillquiteporous.

Poreswerealsoobservedinthesamplesheattreatedat1500^◦Cfor 8h(Fig.S7).AlthoughthesampleM1-1stillpresentedtheshape oftheformerstructureshells,nophotonicband-gapwasidenti- fied(Fig. S8),associated tothesevere delaminationsufferedby thesesamples.ThesamplesfromcycleM1-2presentedthefor- mationofaciculargrains,resemblingthemullitemicrostructures encounteredincommonpowdermetallurgyproducts.

3.2. Mullitephaseformation

After ALD and burn-out, the inverse opal photonic crystals withinthisstudywerestillamorphous,althoughthesamplefrom thecycleM1-1(refertotheassociatedDatainBriefarticleforALD cycledetails)presentedonepeak(Fig.S1)thatcouldbeassociated toGippsite(transitionAl2O3phase,COD#9015976).Theverylow- depositiontemperatureusedinthisstudy(95^◦C)isfarbelowthe crystallizationtemperaturesforalumino-silicates,but theburn- outtemperature(500^◦C)couldalreadypromotecrystallizationof Al₂O₃ transition phases[44]. The heat treatmentperformed at 1000^◦Cfor10h(Fig.6a)promotedtheconversionofthe3Damor- phousstructurestomullite.Schmückeretal.[24]reporteddirect mulliteformation(alsocalledtypeI)inEB-PVDplanarfilms,when thelayerthicknessesoftheAl₂O₃andSiO₂laminatesweresmaller than5nm[24],whichwasalsothecaseinthisstudy.However, an␩-Al2O3phase(COD#1541582)wasalsoidentified,whichwas expected,astheALDsuper-cyclesweredesignedtofittheAl₂O₃- richregioninside theAl₂O₃–SiO₂ phasediagram[45].Sincethe mulliteinverseopalphotoniccrystalsaredesiredtobethermally stablestructures,thepossiblepresenceoffree silicaasaglassy phaseislikelytobemoredetrimentaltothispurpose[46],than thepresenceof alumina.Nevertheless,thetemperatureconver- sionofthemulliteinverseopalphotoniccrystalstructureisbelow thereportedtemperaturesforpowdermixtures[47]anddiphasic sol–gels[48].Theconversiontemperatureobservedinthisstudy (ofonlyabout1000^◦C)iscomparablewithtemperaturesassoci- atedtodirectmulliteconversionofmonophasicgels[48],however, withouttheexcessiveshrinkageassociatedtosol-gelbutwithALD inherentconformalcoatingof theinverseopalphotoniccrystal structures.Thislowconversiontemperatureisassociatedtothe highmixingofAl₂O₃andSiO₂inanatomisticscaleprovidedbythe ALDsuper-cycleprocess.

Moreover, although both samples presented the diffraction peaks associated to mullite, the inverse opal photonic crystals producedunderthesuper-cycleM1-1(refertoTableS1)fitteda mullitepattern(COD#7105575)withaslightlyhigherAlcontent than the cycle M1-2 (COD #9010159). Both patterns present phases which are very similar (most of the peak positions are nearlyidentical)and haveorthorhombiccells, beingtheformer slightlylargerinaandcdirections.Thisresultisinagreementwith theestimatedAl2O3:SiO2ratiobasedontheALDsuper-cyclesand expectedtobelarger(lowerSiO₂content)forthecycleperformed with3DMASthantheonewithAPTESassilicaprecursor,duetothe factthatforthecycleM1-2,thebinaryloopoftheALDsuper-cycle

Fig.6. XRDspectraofsamplesheattreatedata)1000^◦Cfor10hshowingtheconver- siontomullite(symbol¤–COD#7105575and#9010159)andsmallpeaksrelated toan␩-Al2O3phase(symbol§–COD#1541582)andb)1400^◦Cfor4hshowing thepresenceofmullite(symbol¤–COD#7105575and#9010159)andsmallpeaks relatedtoanaluminumsilicatephase(symbolо^{–COD#8103692).Thesymbol*} indicatessamples,whichwerepreviouslyheattreatedat1000^◦C(formullitecon- version).CyclesM1-1andM1-2areindicatedingreenandblue,respectively.COD standsforCrystallographyOpenDatabase.

comprehendstwiceasmore‘SiO₂’sub-cyclesthanthecycleM1-1.

Therefore,moresilicaisintroducedintotheinverseopalphotonic crystalstructure.Asthegrowthpercycleofthebinarysilicacycles aredifferentforeachprecursor,acycleperformedwiththeAPTES precursorwithonlyonepulseofeachbinarycyclepersupercycle (astheoneperformedfor 3DMAS, M1-1)wouldfall faroffthe stoichiometricrange for mullite, withestimated Al₂O₃ content ofmorethan91wt.%.Incontrasttoourpreviouswork[26],no silica-relatedmiddleband(2=22^◦)wasidentified,which once morecorroboratestheexpectedhigh-Al₂O₃contentofthemullite inverseopalphotoniccrystalsproducedinthiswork,whichisalso confirmedbyenergy-dispersiveX-rayspectroscopyanalysis(Fig.

S2).Theseresultsdemonstratethehighcapabilityoftuningthe materialcompositioninALDsupercycleprocess,evenatsuchalow depositiontemperatureof 95^◦C. Furthermore,mullite manufac- turedbyotherroutesoftenpresentundesiredresidualglassyphase fromsilicasources[48],evenwhendesignedtohaveahighAl₂O₃ content,whichisnotthecasefortheALDsupercycleprocessing.

Thepeaksassociatedtothemullitephasearegreatlyenhanced byfurtherheattreatmentoftheinverseopalphotonicstructures at 1400^◦C for 4h. Unlike thesamples heat treated at 1000^◦C,

Fig.7.3DrenderingofthePXCTtomogramsfromthemulliteinverseopalphotoniccrystals(a,c,e)beforeand(b,d,f)afterheattreatmentat1400^◦Cfor4h,showingsome ofthestructuralfeaturesquantifiedintheimageanalysis:(c,d)macroporeshighlightedindarkblue(e,f)imageskeletonsrepresentingtheinterconnectionsbetweenthe pores.Samples’volumesare(a,c,e)57␮m³and(b,d,f)52␮m³.

thesamplesdidnotpresentpeaksrelatedtoanAl₂O₃transition phase,butallthesamplesanalyzedpresentedpeaksidentifiedas an alumino-silicate phase, also orthorhombic (COD #8103692).

Nevertheless,thedetectionrangefortheX-raydiffractionanalysis should be taken into account [49], especially considering that the measurements were performed in grazing incidence mode (1.5^◦ as glancing angle) [31]. For this configuration, a volume associated witha smaller penetration depthis analyzed, when in comparison to the most common Bragg-brentano mode, reflecting in a globalsmallervolume ofanalysis, i.e.if a phase ispresent inthesample, butata volumefractionsmallerthan thedetectionlimit, thanitwill notbeidentified.Note thatthe mullite peaks are clearly identified even in the sampleswith- out previous heat treatment at 1000^◦C (curves with symbol * in Fig. 6b), thus indicating that mullite formation occurs even for samples heat treated directly at 1400^◦C (heating rate of 5^◦Cmin⁻¹),i.e. thereis noneedfor previousmullite formation treatment.

Thepeaksassociatedtothemullitephase arenarrowedand sharpenedbytheextremeheattreatmentat1500^◦Cfor8h(Fig.

S3).Thisobservationpointstotheprobablegrowthofmullitecrys- tallites[49],laterconfirmedbyelectronmicroscopyanalysis.Once again,no differencewas observed betweenthe samples previ- ouslyheattreatedat1000^◦Cfor10handthesamplesdirectlyheat treatedat1500^◦Cfor8h.Alternatively,atsuchextremeheattreat- ment,thesamplesproducedbytheALDsupercyclewithestimated higherAl₂O₃content(M1-1,3DMASassilicaprecursor)presented peaksclearlyassociatedtotheformationofalpha-aluminaphase (COD#1000032),reiteratingthehighAl2O3contentinthecom- positionofthisinverseopalphotoniccrystal.Incomparison,the samplesproduced by the ALD super cycleM1-2 (APTES as sil- icaprecursor),presentedonlypeaksrelatedtothemullitephase (COD#9010159).InourrecentworkwithAl₂O₃inverseopalpho- tonic crystals [36], we have demonstrated that the ␣-alumina phaseisformedalreadyat1100^◦C,beingtheonlyphasepresent afterheattreatmentat 1200^◦Cfor 1h. However,inthehereby presented work ␣-alumina phase wasonly identified afterthe extremeheattreatmentat1500^◦Cfor8h.Thisfactconfirmsthe metastablenature[45]ofthecompositionproducedbytheALD supercycleM1-1andalsosuggestsahindrancemechanismforthe nucleationandgrowthofthe␣-aluminaphase,whichinsol-gel routes,aswellasinpowdermetallurgyrouteisusuallyidentified atmuchlowertemperatures.Jakschiketal.haveshownthatthe

crystallizationbehaviorofALDAl₂O₃ filmsisdirectlyaffectedby thefilmthickness,forwhichgrainsweresmallerinthinnerfilms (forthesameheat treatmentconditions)[50].Theirexplanation wasrelatedtothehigherprobabilityofnucleationseedsinthicker films, thus reducingthe thermal budgetrequired toformcrys- tallites.A similartrendwasobserved duringannealing of6nm ALDAl₂O₃ films[41].Evenforthicker films(100nm)deposited at 250^◦C [51],crystallizationis identified onlyafterheat treat- mentat825^◦C.Itisimportanttopointoutthatthesepublications dealt with ‘pure’ Al₂O₃ films, while mullite is a ternary oxide and therefore a more complex system. Furthermore, the com- plexinverseopalphotoniccrystalmorphologicalstructurecould alsopresent,otherthanthelowfilmthickness,differentbehav- iorinjunctionpoints(interstitialsitesandcontactpoints)thanin theshells‘free’area(seeFig.2).Moreover,astheheattreatment evolvesthethicknessoftheinverseopalphotoniccrystalsstruc- tural featuresmayvary,asdiscussed laterin thePXCTanalysis section.

Fig.8. Periodicityoftheareafractionofinverseopalphotoniccrystalphaseand pores,accordingtothesampleheightobtainedthrough2D-analysisofasetofslices.

Inverseopalphotoniccrystal(a,d)beforeheattreatmentand(b,c)aftertheheat treatmentshowingthe(c,d)poresareafraction(macro-plusnano-pores)and(a, b)inverseopalphotoniccrystalareafraction.RefertoFig.S9forvisualizationofthe areafractionvariationindifferent2DslicesextractedfromthePXCTdataset.

3.3. 3Dstructuralchangesanalysisbyhigh-resolution ptychography

Imagingofthesamples’entirevolume(Fig.7,fordetailsrefer totheassociatedDatainBriefarticle)revealedahighlyintercon- nectedporousstructure,withthehighestlocalporosityreaching 75%forthesamplebeforeheattreatment,whichisslightlyabove thetheoreticalvalueforFCCclosepacking.Itisimportanttopoint outthat this maximum valuerepresentsthearea fraction(2D) occupiedbythemacroporesandtheircontactpoints,whilethe theoreticalvalueonlyconsidersthespacesoccupiedbythemacro pores,thusexcludingthecontactpoints(alsoreferredtoasnecks –refertoFig.2forstructuralfeaturesdefinition).Asexpected,the localporosity(areafractionofpores)in2Dvariesdependingonthe sampleheight(Fig.8)anditisinversetothephotoniccrystalarea fraction(i.e.ceramicoxideareafraction).Therelationshipbetween themaximumandminimumvaluesofareafractionofthesamples indicatesasinteringofthestructure,withconsequenttransportof matterintheinverseopalphotoniccrystalphase,resultinginrela- tivedifferentphotoniccrystalphaseareafractionvaluesafterheat treatment.Itisessentialtomention,however,thatthisisa2Danal- ysisoftheslicesobtainedfromthePXCTdatasetanditrepresents onlyanareafraction,therefore,comparabletotheplanardensityof thevarietyofFCCplanes.Whereasinthe3Danalysisavolumetric average(Perc.Vol.)isobtained,whichrepresentstheinverseopal photoniccrystalpackingfraction.

Aperiodof531nmwasidentifiedforthesamplebeforeheat treatment.Thisvalueisabovethetheoreticalvalueforthedistance between111planesinaFCCcellconsideringtheoriginalpack- ingoftheformerPSspheres,indicatingthattheoverall3Dpacking ofthespheresdiffersfromthetheoreticalclose-packedvalue.This observationwasconfirmedbythe3Dquantitativeanalysisofpack- ingfraction,whichresultedinavolumefractionof57.5±1.1%for themacroporesphase.Asmentionedabove,thisnumberisdirectly relatedtothevolumefractionoftheinverseopalphotoniccrys- talphase,whichwasfoundtobe41.4±1.0%forthesamplebefore heattreatmentand42.2±1.1%forthesampleafterheattreatment.

Thisquitesurprisingresult impliesthatthematerial’sphotonic responseisinfluencedbythestructuralshrinkage(especiallyof macropores)andmacrocracksopening(discussedintheprevious topic),ratherthantheoverallceramicoxidevolumefraction(here namedasphotoniccrystalphase).

Anoverall8% structuralshrinkage wasobservedfor distinct features in different scales and dimensions (Fig. 9). First, this valuecanbeextractedfromthe2DperiodicityshowninFig.8.

Whiletheperiodwas531nmbeforeheattreatment, itresulted in488nmafterheattreatment.Next,the3D quantitativeanal- ysisofthemacroporessize alsoresultedin ashrinkage of8%, from541±17nmto499±27nm.Finally,thecontactpointsanaly- sisalsoresultedinthesameshrinkagevalueof8%(135±30nm beforeand 124±26nmafterheat treatment).Nevertheless,the volumetricshrinkagevalueobtainedfromthequantitativeanal- ysisisclosetothevaluereportedforAl₂O₃ALDfilmsheattreated upto850^◦C(10%),associatedwithfilmdensificationmeasuredby X-rayreflectivity(XRR).

Although shrinkage was observed, the connectivity density ofthe3Dstructurewasincreasedfrom4.94±0.94␮m⁻³ before heattreatmentto6.48±0.49␮m⁻³ afterheat treatment, which indicatesanoverall openingof theinverse opalphotoniccrys- talstructure. Hereby, the connectivity density relatesthe pore interconnectivityofadjacentporesandisbasedonthenearestcon- nectingporeavailableattheshortestdistance,meaningthatthe nearestporeinthesame2Dslice,forexample,willnotnecessar- ilybethenearestconnectionpointfoundin3D(seeDatainBrief relatedarticle).

Thestructureopeningissupportedbytheanalysisofthenano- poresfeature(see Fig.2).Whereas shrinkagewasobserved for thestructural featuresdiscussed above,the nano-poresinstead presentedanincreaseindimension(hereafterdenotedasenlarge- ment), in the range of 110±46nm before heat treatment to 122±59nmafterheattreatment.Thehighstandarddeviationrep- resentstheheterogeneityofthisporousstructure(sphericityvalues were0.80±0.21and0.95±0.32beforeandafterheattreatment, respectivelyandrelatedtothefactthatanaveragesizeiscollected fromboththetetrahedralandoctahedralsitesnano-porespopu- lation(theoreticalsizeratiobetweenbothfeaturesisaround0.8) [52].Thisenlargement(Fig.9candh)isreflectedbytheopeningof theinverseopalphotoniccrystalstructureatsuchpointsandindi- catesamasstransportdirectionfromthesenanoporesregionsto theshells,whichisentirelydifferentfromthatreportedforinverse opalphotoniccrystalsproduced byinfiltrationofpolymertem- platesbypowdercolloidalsuspensions[53]orsol–gel[13,33,34].

Sokolovetal.[13]presenteda2DanalysisbySEMofAl2O3inverse opalphotoniccrystalsproducedbysol–gelafterheattreatment.

Apartfromthemacroporeshrinkage(whichwasalsoobservedin thiswork),acleargrowthoftheso-called‘vertexes’(locatedinthe sameplaceasthenano-pores,butfilledwithmaterial)together withthinningofthestruts(denotedasshellsinthiswork) was observed.

Aspointedoutbyourgroupinaformerpublication[36]and nowstronglysupportedbythePXCT3D analysis,werelatethe observeddifferencestothedifferentstartingstructures,namelythe presenceofadditionalnano-poresinourALD-basedinverseopal photoniccrystals.Thepossiblelowerhomogeneity(withlocalcon- centrationareas)forthesol–gelandcolloidalroutebasedinverse opalphotoniccrystalscouldalsocontributetotheobserveddiffer- entbehavior.Allthisculminatesintodifferentstructureevolution pathwaysfordifferentlysynthesizedphotoniccrystals.

Theenlargementofnano-poreswasalsoconfirmedbythe3D analysisofthevolumefraction,for whichanincreaseof67%in averagewasobserved(from1.2±0.2%to2.0±0.4%)andsupported byearlier studiesperformed byKingeryand Francois[54], and SlamovichandLange[55],forwhichthegrowthorshrinkageofa porewasrelatedtothedifferentialchemicalpotentialbetweenthe curvedporesurfaceandaflatsurfaceorthesurroundingsgrains, respectively.Inthefirststudy,convexpores(inthisstudyreferred toasnano-pores)werestatedtoalwaysgrow,whileinthelatter theauthorsclaimitcouldeithershrinkorgrowuptosomeequi- libriumsize.Inasecondstudy,SlamovichandLange[56]showed thattheporebehaviorhasaproportionalcorrelationtothepore coordinationnumberandintersectinggrainboundaries.However, thesestudieswereperformedinpowdermetallurgybulkmaterials, whichpresentawiderdistributionofbothporesandgrainssizes andshapes,whileforourphotoniccrystalsnarrowerdistributions areexpectedbothforpores(originalPSspheresandtheirpack- ing)andgrains(nucleationandgrowthfromaveryhomogeneous structuregeneratedbyALD).

Atlast,theinterpretationoftheabsolutevaluesforthestruc- turalfeaturesdiscussedabovemusttakeintoaccountthepossible contributionsfromthepartialvolumeeffects andgrayscaleval- uesdistribution,asdescribedintheassociatedDatainBriefarticle.

Nevertheless,X-rayptychography offersthepossibility of com- paringstructuresofrelativelylargesamplesbeforeandafterheat treatments(insituorexsitu),whichishardlypossiblebyother techniquessuch asFIB or TEMtomography. Depending onthe instrumentset-upitisalsopossibletoworkundervaryingtemper- atures,pressuresandchemicalenvironments.Thedevelopmentof theinstrumentationset-upandinsitumeasurementsofthestruc- turalchangesinphotonicstructuresinducedbythetemperature exposureisforeseenasfuturework.

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Fig.9. 3DrenderingofselectedVOIsextractedfromthePXCTdatasetsofthemulliteinverseopalphotoniccrystals(a–d)beforeand(f–i)afterheattreatmentat1400^◦Cfor4hshowingthestructuralfeaturesanalyzedduring imageanalysis:(a,f)inverseopalphotoniccrystalphase(b–d,g–i)macroporespicturedblueandinscribedblobsingreen;contactpointsinyellow;nanoporesinpurpleandimageskeletoninred(fordetailsseeFig.2andthe associatedDatainBriefarticle).Aperpendicularcuttingplanewasappliedtoallowthevisualizationofallthestructuralfeatures.VolumeoftheVOIsshownin(a,f)is18.6␮m³.