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Lasersseemtobeanobviouschoiceforthistask,butwere neveremployedforthispurposebefore.ThereasonisthatAAO isnearlytransparentforcommonlaserwavelengths.Asaresult effective photo-thermalheating of AAOis impossibledueto low absorptionevenfor high intensityirradiation.Wesolved thisproblembyequippingAAOwithcarbonnanotubes(CNT) asasacrificialabsorberthatenableslaser-inducedphase trans-formationofAAOto␣-Al2O3withcuttingedgeprecision.
2. Experimental
FabricationofAAO/CNT.Highpurityaluminumfoil (Good-FellowAl99.999)wasdegreasedin5wt% sodiumhydroxide at60◦Cfor1min,rinsedwithdeionizedwater,pickledin1:1 nitricacid/waterandfinallyrinsedwithdeionizedwater. AAO-membranesexaminedbyX-raypowderdiffraction(XPD)and irradiationintensitystudieswereanodizedin10vol%sulfuric acid solutionat 4◦C with35V.27 AAOs for dotwise pattern-ing wereanodized in0.3M oxalicacidsolution at 3◦Cwith 100V.17RemainingAlwasremovedbyetchingincupric chlo-ride/hydrochloricacidsolution.AspreparedAAOsampleswere rinsed with deionized water and dried at 50◦C for several days.
AAO-templateassistedCNTgrowthwasconductedbya ther-malCVDprocessusingconditionsdescribedbyYang.28Briefly, AAOsampleswereplacedinatubefurnaceandrampedatarate of5◦/min inareducingenvironmentcomprising25%H2and 75%N2.At750◦Cthe samplesweresubjectedtoagas mix-tureofH2/C2H2atflowratesof500sccmand150sccmfor2h.
Finallythefurnacewascooledatarateof5◦/minunderAr-flow.
Details concerning fabricationandhandling of AAO/CNT membranesaresuppliedintheSupplementaryInformation.
Chromiumdoped␣-Al2O3was prepared byimmersion of AAO/CNTinan aqueous solutionof 1M Cr(III)NO3 for 1h priortoirradiation.Titaniumdopingwasachievedby immer-sioninasolutioncontaining12%Ti(III)Cl3in8%hydrochloric acid(Sigma–Aldrich)for10min.Afterdraininganddryingthe substrateswereirradiatedtoproducemicropatternsofAl2O3:Cr andAl2O3:Ti,respectively.Finallythesubstrateswerepurged indistilledwatertoremovemetalsaltsfromunaffectedareas.
LaserirradiationofAAO/CNT.ANewportExplorerXP 532-5 DPSS laser emitting TEM00 (M2<1.1)at awavelength of 532nm was used for photo-thermal treatment of AAO/CNT samples. The laser beam was steered over the samples by a SCANgine14 galvanometer scanner equipped with a Roden-stockRonarF-Theta100mmlens.Largescaleirradiation for XPDsamplepreparationwasconductedat3.6Watarepetition rateof300kHz.Dopedandpure␣-Al2O3dotpatternswere gen-eratedat100mWand300kHzrepetitionrate.Dwellingtimes usedperdotrangedfrom1to100ms,dependingonthedesired dotsize.
XPD measurements were carriedout on a Philips X-Pert MPDPropowderdiffractometerequippedwithX-PerttubeCo LFFoperating at30kVand30mA. Spectrawereobtainedin therangeof25◦≤2θ≤85◦atroomtemperature.Dataanalysis andpostprocessingwasconductedbyX-PertHighScorePlus v2.2c.
Fluorescencemicroscopyof microstructuredsamples was performedonaCarlZeissLSM5Pascalundermercurylamp illuminationapplyingfluorescencefilterF-Set25.
Topological mapping of substrate surfaceswas performed with the same microscope operating in laser-scanning mode (LSM).
PhotoluminescenceemissionofAl2O3:CrandAl2O3:Tiwere exited bycw-laserirradiation at 532nmandmeasured by an OceanOpticsUSB2000fiberspectrometer.
Scanningelectron microscopy(SEM)wasperformed ona JeolJSM7500FfieldemissionmicroscopeequippedwithSEI, LEIandBSEmultisensor-matrix.
3. Resultsanddiscussion
Likemost non-metals,alumina ceramicscanbe heatedby infraredlasersduetoresonantphononcoupling.Unfortunately typicalIR-sourceslikeCO2-lasersgivepoorspatialresolution attributedtotheirlongwavelengthof10.6mwhichis unfavor-ablefor micropatterningregarding theAbbe diffractionlimit.
Likewise, the attempt to use shorter wavelengths for photo-thermalheatingencountersotherproblems:alumina ceramics aretransparentinthespectralrangefromnear-IRtonear-UV;
bandgapsaresolelylocatedinthevacuum-UV.29Inshort,laser wavelengths covered by available high power lasers are not absorbedandhighpowerlasersinthevacuumultravioletspectral rangearebarelyavailable.Acceptingthefactthatthisproblem cannotbesolvedbyanappropriatechoiceoflaserwavelength thealternativeistochangethe substrateabsorption character-istics. Several modifications increasing the opticaldensity of AAOweretested:Organicdyes soakedinto thenanoporesof AAOgavegoodopticalabsorptionbutlaserirradiation experi-mentsshowedthatphasetransitionofAAOto␣-Al2O3isnot possiblebecauseorganicmoleculesdecomposebeforetransition temperatureisreached.Anobvioussolutionforthisproblemis theuseofinorganicabsorbers,whichindeedenabledfor effec-tivephoto-thermalheating.Naturally,thisapproachresultsin absorbercontaminated␣-Al2O3whichisundesiredbecausethe physicochemical properties of ␣-Al2O3 are very sensitive to contamination.Finallywetookadvantageofthecatalytic capa-bilityofAAOtogrowcarbonnanotubesinitsnanopores30(see SupplementaryInformation for details).CNTsfeature higher temperaturestabilitythanorganicdyesandsuperioroptical den-sity.Wefoundthat photo-thermalheatingof suchAAO/CNT substrates readily induces surface temperatures allowing for phaseevolutionof ␣-Al2O3.XPDspectraofAAO/CNT sam-plesirradiatedatalaserwavelengthof532nmareillustratedin Fig.1togetherwithblankaluminum,anodizedaluminumand non-irradiatedAAO/CNTforcomparativepurposes.
AAOandAAO/CNTsamplescanbeconsideredamorphous apart from peaks ascribable to metallic aluminum, denoted bygraymillerindices.Laser treatmentofAAO/CNTwithan irradiation intensityof 100J/cm2 turns small amountsof ini-tiallyamorphousaluminainto␥-Al2O3.McQuaigandKirchner observedthistransitionto occurbetween 900 and970◦C by ovensinteringofanodicaluminamembranes.26,31The thresh-oldirradiation intensityfor␣-Al2O3generation wasfoundat
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Fig.1.Photo-thermalphaseevolutionfromAAO/CNT.Aluminumblank(a), AAO(b),AAO/CNT(c),AAO/CNTirradiatedwith100J/cm2(d),AAO/CNT irradiatedwith350J/cm2(e),andAAO/CNTirradiatedwith700J/cm2(f). Char-acteristicintensitiesoftransitionaluminaphasesaregivenatthebottom:ICDD card43-1484–␣-Al2O3(black),ICDDcard29-1486–␥-Al2O3(purple),ICDD card16-0394–␦-Al2O3(blue),ICDDcard21-0010–-Al2O3(pink),ICDD card35-0121–-Al2O3(green),ICDDcard04-0878–-Al2O3(red),ICDD card34-0493–-Al2O3(orange).(Forinterpretationofthereferencestocolor inthisfigurelegend,thereaderisreferredtothewebversionofthearticle.)
350J/cm2eventhoughthepreferred␣-phaserepresentsjusta minor amount of the mixed crystalline alumina polymorphs.
This changes radically when AAO/CNT is irradiated at an intensity of 700J/cm2. Distinct ␣-Al2O3 peaks, denoted by
blackmillerindices,toweraboveallotheraluminapolymorphs.
However,pure␣-Al2O3couldnotbeachievedbyhigher irra-diationintensitiesduetolaserinducedsubstratedamage.This resultsfromthefactthattheAAO/CNTto␣-Al2O3transitionis thermo-dynamicallyfavoredfortemperaturesbyfarexceeding thethermalstability of CNTsinoxidizingenvironments.32,33 Consequently, CNTs act as a sacrificial absorber, which is pyrolyzedduringthephoto-thermalprocess(seeSupplementary Informationfordetails).Ontheonehandthe rateofabsorber degradationisafunctionoftemperatureontheotherhand tem-peraturedependsonirradiationintensityandabsorberactivity.
Evidentlyabreak-evenpointispresentinthesystemwhich lim-itsthemaximumtemperatureavailableforphasetransitionand thustheyieldof␣-Al2O3.Inordertolocatethispointwe con-ductedaseriesofirradiationscoveringtheintensityrangefrom 400to700J/cm2.ResultsareillustratedinFig.2.
Irradiation at 400J/cm2 left the surface unaffected from amorphological pointof view – nanopores are still present.
500J/cm2changedthesurfacetoacinderymorphology, indi-catingthatthemeltingpointofaluminawasexceeded.Samples irradiatedat600J/cm2showmicrometersizedaluminagrains fusedtoamosaic-pattern.At700J/cm2the mosaic-like mor-phology is fragmented. In conclusion it can be stated, that AAO-embeddedCNTsshowgoodabsorberactivityfor irradi-ationintensitiesupto600J/cm2.Furtherincreaseinintensity causes absorber degradation and thus deeper penetration of photonsintotheAAO-matrix.Photo-thermalheatingofdeeper regionsinducesmechanicalstressresultinginsurfacewrinkling andrupture.Thepotentialtocreatespecificsurface morpholo-gies by variation of irradiation intensity may be interesting
Fig.2.Morphologicaleffectsofphoto-thermaltreatment. Dual-sensorSEMinspection(SEI+LEI)ofAAO/CNTafterphoto-thermalprocessing.Irradiation intensitiesaredenotedinyellow.(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthearticle.)
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Fig.3.Microstructureperformanceofphoto-thermalphasetransitionto␣-Al2O3.Photoluminescencespectraandmicro-patternsofAl2O3:Ti(a–c), photolumine-scencespectraandmicropatternsofAl2O3:Cr(d–f),greenarrowsindicatecw-laserexcitationat532nmwavelength.(Forinterpretationofthereferencestocolorin thisfigurelegend,thereaderisreferredtothewebversionofthearticle.)
for, e.g., biomedical applications. However, our objective is structuraltransformationofAAOideallywithoutchangingthe nanoporousmorphology;hencewekeptirradiationintensities below500J/cm2forourstudies.
␣-Al2O3 is the host crystal for gemstones like ruby. The replacementofafewAl3+-ionsbyCr3+-ionsinvolvesarigorous changeofitsopticalproperties.Lietal.reportedstrong photolu-minescencefromCr3+-dopedAAOafterthermaltransformation to ␣-Al2O3.34 We introduced this feature to our process by soakingCr3+-ionsintothenanoporousstructureofAAO/CNT prior to photo-thermal treatment. After transformation to ␣-Al2O3thosesamplesshowasharpphotoluminescencepeakat 694.2nm as showninFig.3,the R1-lineof Ruby.35 Repeat-ingthesameexperimentwithTi3+-dopingresultedinabroad photoluminescence emission band from 600 to 950nm with
amaximum intensityat 730nm,which is inagreement with spectraofAl2O3:Ti-laser-crystals.36,37
Thephoto-thermalheatingofAAO/CNTproofedtobehighly effectivefor thegenerationof metal-iondoped␣-Al2O3.The photoluminescence of Al2O3:Cr and Al2O3:Ti presents also a convenient solution for microstructure inspection. Owing tohighphotoluminescence efficiency laserexcitation is actu-ally not necessary; instead we used fluorescencemicroscopy formicrostructure imaging.MicrodotarraysofAl2O3:Crand Al2O3:Ti, shown in Fig. 3, demonstrate that AAO/CNT to
␣-Al2O3 transformation can be achieved with cutting-edge precision.SEMinspectionoftheblackringssurroundingthe flu-orescentspotsresultedinthefindingthatCNTswerepyrolized inthisregion.Thisheataffectedrimsexpandabout10mfrom thefluorescentedgewhichiscomparativelylowconsideringthe
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Fig.4.Schematicrepresentationofpatterntransferprocessusingselectiveetching.
Fig.5.Deptheffectofphoto-thermal␣-Al2O3 evolution.SEM-inspectionof␣-Al2O3-micropatternobtainedbypointwiseheatingofAAO/CNTwith25m dot-spacing(a,c,andd).Topographicillustrationof␣-Al2O3-micropatternbylaser-scanning-microscopy(b).
highthermalgradient.Ontheonehandthetight confinement of photo-thermally induced heat can be explained by means oflow thermalconductivity ofAAOowingtoitsnanoporous morphology.38,39OntheotherhanditmustbenoticedthatCNTs featureextraordinarilyhighthermalconductivity.40Weassume theverycombinationofaluminananoporesandCNTstobe ben-eficialforcloselylocalizedheatconfinement.Transversalheat diffusioninAAO/CNTis suppressedbythe coaxial arrange-mentofaluminananopores,actingasheat-diffusionbarrier,and CNTs, actingas coolingpipes, whichrapidly spreaddiffused heatinlongitudinaldirection. Additionaltothat,the simulta-neousdecompositionofCNTsisanenergyconsumingprocess, thusfavoringheatconfinementaswell.41
SuperficialconversionofAAOto␣-Al2O3byphoto-thermal treatmentwasdemonstratedsofar.Inthefollowingsectionwe giveinsightsinstructureformationbelowthesurface.Thesketch illustratedinFig.4summarizesthe entireprocessappliedfor thisinvestigation.
IncontrasttoAAO,whichisanamphothericoxide,␣-Al2O3
can neither be etched by acids nor by bases. This advanced
chemicalstabilitywas utilizedfor selectiveetchingof photo-thermally patterned AAO/CNT. ␣-Al2O3 micropatterns were fixed by a PMMA top coating prior to AAO-dissolution in aqueousNaOH-solution.
CNTswereremovedbyultrasonicationinanaqueous solu-tionof sodiumdodecylbenzene sulfonatefollowedbyrinsing withdistilled water.42 SEM pictures of the residual ␣-Al2O3
micropatternsonPMMAareshowninFig.5.Thedistinctregular morphologyof␣-Al2O3-micropatternsdemonstratesthat photo-thermaltransformationofAAOto␣-Al2O3providesverysharp depthresolution.Pointwiselaser-heatingincloseproximity pro-ducedinterconnectedhumpswith8mpeak-to-valleyheight.
MoreoverthenanoporousmorphologyofAAOismaintainedas asubstructureinthesemicro-humps.
4. Conclusion
Laser controlled photo-thermal phase transformation of AAO/CNTis an efficientmethod for the generation of hier-archically structured␣-Al2O3 patterns. CNTs featureperfect
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absorberperformanceenablingtheAAOto␣-Al2O3 transfor-mationundermaintenanceof the nanoporousAAOstructure.
CNTs act as sacrificial absorbers providing ␣-Al2O3 free of contaminants. This is absolutely mandatory to enable spe-cific ␣-Al2O3-doping by soakinge.g., Cr or Ti ions into the AAO/CNTprecursorandreceivingthecharacteristicruby-and Ti-sapphire photoluminescence emissions. Potential applica-tions for the presented systems encompass e.g., fluorescent imaginginbiomedicalarrays,microlasers,microlens-arraysand manymore.
Acknowledgment
FinancialsupportfromFederalMinistryofEconomicsand Technology (BMWi)throughgrantKF2307201MK9is grate-fullyacknowledged.
AppendixA. Supplementarydata
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/
j.jeurceramsoc.2013.01.005.
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