Functionalized Au Substrate How Critical Is the pH of Ag ( I ) - Lathika et al. - 2020 - Unknown

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pubs.acs.org/JPCLLetterDirectLayer-by-LayerGrowthofCrystallineAg-TCNQThinFilmsonFunctionalizedAuSubstrate:HowCriticalIsthepHofAg(I)Solution?AswaniSathishLathika,ShammiRana,AnupamPrasoon,PoojaSindhu,DebashreeRoy,andNirmalyaBallav*CiteThis:J.Phys.Chem.Lett.2020,11,10548−10551ReadOnlineACCESSMetrics&MoreArticleRecommendations*sıSupportingInformationABSTRACT:Wet-chemicalfabricationofacrystallineAg-TCNQ(TCNQ=7,7,8,8-tetracyanoquinodimethane)thinfilmonnon-Agsubstrateischallengingwherebythechemistrywaspoweredbyphotonenergyand/orelectricalenergy.Wereportforthefirsttime,directchemicalgrowthofaAg-TCNQthinfilmonafunctionalizedAusubstratebyemployingthelayer-by-layer(LbL)approachatambientreactionconditions.VariousAg(I)saltprecursorspreviouslyrealizedtobeunsuitableforthefabricationofAg-TCNQthinfilmsonnon-AgsubstratesultimatelygaverisetodenseanduniformthinfilmsofAg-TCNQ.ThecrucialknobregulatingthedirectformationofthethinfilmsofAg-TCNQwasidentifiedtobethepHoftherespectiveAg(I)solutions.RapidadvancesintechnologyinrecentyearshaveledtoahavereportedaAg-TCNQthinfilmonafunctionalizedAusurgeinthetrendofminiaturizingelectronics.Thishasinsubstrateviathelayer-by-layer(LbL)technique,however,atturnshiftedtheattentiontomaterialshavingpotentialthecostofaprefabricatedCu-TCNQthinfilmactingastheapplicationsintherealmofultrahighdensityinformation7sacrificialtemplate.ThefruitionofgeneratingaAg-TCNQ1−3storageandmemorydevices.Inthisregard,charge-transferthinfilmonafunctionalizedAusubstratemotivatedustocomplexesbasedon7,7,8,8-tetracyanoquinodimethaneinvestigatethefactorshinderingthedevelopmentofamore(TCNQ)resultinginM-TCNQcoordinationpolymers(Mfacilewet-chemicalrouteinvolvingAgsaltsandTCNQ.=Cu,Ag,Mn,Fe,Co,andNi)havereceivedrenewedHerein,forthefirsttime,wepresentadirectLbLgrowthof4interest.However,majoremphasishasbeenlaidonCu-adenseanduniformAg-TCNQthinfilmonathiolTCNQandAg-TCNQbyvirtueoftheirhighstructuralfunctionalizedAusubstrate.AnoptimalpHvalueoftheflexibility,semiconductingnature,reproducibleelectricalandAg(I)solutionwasfoundtobecriticalinachievingtheAg-5−8opticalpropertiesaswellasmemoryeffects.TCNQthinfilm.OurresultsnotonlyhighlighttheimportanceDownloadedviaUNIVOFCALIFORNIASANTABARBARAonMay16,2021at07:52:06(UTC).Seehttps://pubs.acs.org/sharingguidelinesforoptionsonhowtolegitimatelysharepublishedarticles.Ag-TCNQandCu-TCNQcanundergoreversibleandofpHinLbLgrowthbutalsoopenupanunexploredavenueofbistableswitchingofelectricalconductivityfromahightolowpHintheemergingdomainofthinfilmsofmetal−organicsimpedancestateinducedbyanelectricfieldand/oropticaland/orcoordinationpolymers(CPs),ingeneral.5,6excitation.AsforthethermallydrivenresistiveswitchingFabricationofaAg-TCNQthinfilmisschematicallyphenomenoninvolvingAg-TCNQandCu-TCNQthinfilms,depictedinFigure1.Briefly,aAusubstrate(functionalizeditwasascribedtothealterationoftheSchottkybarrieratthewithaself-assembledmonolayer(SAM)ofmercaptoundeca-metal−semiconductorinterfacebythermalenergy(annoicacid(MUDA))wassequentiallydippedintoAgOAc7interfacialphenomenon).Suchremarkableattributesmake(silveracetate)andTCNQsolutionsinethanoltogrowtheAg-TCNQthinfilmprototypestomoleculardevicesandAg-TCNQthinfilmatambientconditions(seetheSupportingstimulatesawiderangeofsyntheticprotocolstobedevised.Theserangedfromspontaneouscrystallization,electrolysis,electro-crystallization,photocrystallizationtovacuumvaporReceived:October26,2020depositionandthermalvapordepositionamongahostofAccepted:December1,2020others.4−6,9−17OfallthetechniquesdevelopedforthePublished:December9,2020fabricationofthinfilms,growthofAg-TCNQonanon-Agsubstratewasachallengethatwassuccessfullyovercomeinthe14,17seminalworkbyBondandco-workers.Morerecently,we©2020AmericanChemicalSocietyhttps://dx.doi.org/10.1021/acs.jpclett.0c0322910548J.Phys.Chem.Lett.2020,11,10548−10551

1TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLetter(002),(022),and(004)planesandarecharacteristicofa22phase-IAg-TCNQstructure.DirectformationofAg-TCNQthinfilmsonMUDAfunctionalizedAusubstratewithvariousAg-precursorsaltssuchasAgOAc,AgNO3,AgBF4,andAgClwasattemptedthroughtheLbLmethod(FigureS1).Interestingly,growthoftheAg-TCNQthinfilmwasachievedonlywiththeAgOAcsolution.Suchanobservationledustospeculatetheroleofthecounterion(acetate)asamodulatorforthegrowthofthethinfilm.Inordertofurtherinvestigatethesame,somecontrolexperimentswereperformedbyemployingtheLbLapproachundersimilarconditions.However,whenacetatesaltsofdifferentmetalssuchasCo(II),Ni(II),andMn(II)wereused,thecorrespondingM-TCNQthinfilmswerenotformed(FigureS2).Eventheadditionofaceticacid(CH3COOH)tothesolutionofAgNO3didnotresultintheformationofAg-TCNQthinfilm(FigureS3),therebyrulingoutthepossibilityoftheacetateionactingasamodulator.Surprisingly,formationofaAg-TCNQthinfilmwasconsistentlyobservedwhensodiumacetate(CH3COONa)wasaddedintotheFigure1.Schematicsofthelayer-by-layer(LbL)assemblyofaAg-solutionsofAgNO3andAgBF4,whichwerewell-characterizedTCNQthinfilmontheAusubstratefunctionalizedwithaself-byPXRDandFE-SEMtechniques(FigureS4).Therefore,assembledmonolayer(SAM).whatabouttheroleofpHoftheAg(I)solutionanditsimpactinassistingtheformationofAg-TCNQthinfilmviaLbL?Informationforexperimentaldetails).NotethatineachLbLTable1summarizesthecorrespondingpHvaluesofthegrowthcycle,multilayersofAg-TCNQweredeposited,whichrespectiveAg(I)solution(thevalueswerefurtheradjustedbyisconsistentwithourearlierobservationsonthinfilmsofCPs,7,18−21ingeneral.VisualinspectionofthebarefunctionalizedTable1.DirectRelationshipoftheApproximatepHValueAuandtheAg-TCNQthinfilmrevealedadistinctchangeinoftheAg(I)SolutionwiththeFormationofaAg-TCNQcolorfromgoldentodarkblue(Figure1).ThinFilmSurfacemorphologiesoftheAg-TCNQthinfilmwererecordedbyfield-emissionscanningelectronmicroscopy(FE-metalsaltpHobservationSEM)andrevealeduniformcoverageofAg-TCNQontheAuAgNO34.9nothinfilmsubstrate(Figure2a).ThewatercontactangleevidencedaAgOAc8.8thinfilmformedhighlyhydrophobicsurface,matchingwellwithapreviousAgBF44.8nothinfilm7report(Figure2a,inset).Furthermore,out-of-planeX-rayAgCl3.9nothinfilmdiffraction(XRD)patternsexhibitedthehighlycrystallineAgNO3+KOH7.3thinfilmformed22natureoftheAg-TCNQthinfilm(Figure2b).DiffractionAgOAc+CH3COOH4.4nothinfilmpeaksappearingat10.25°,14.5°,and21.5°correspondtoAgNO3+CH3COONa7.9thinfilmformedAgBF4+CH3COONa7.6thinfilmformedKOHorCH3COONaorCH3COOH).FromtheabovetableitcanbeinferredthatmetalsolutionshavingpH≥6.5supportstheformationofAg-TCNQthinfilms.Toprovethishypothesis,twocontrolexperimentswereperformed:(i)inthefirstexperiment,CH3COOHwasaddedtoAgOAcsolutiontoreduceitspHvaluewhile(ii)inthesecondexperiment,KOHwasaddedtoAgNO3toincreasethepHvalue(Figure3).WewouldliketopointoutherethatwhenonlyAgOAcsolutionwasusedasthesourceofAg(I)ion,thinfilmformationofAg-TCNQtookplace;however,uponadditionofCH3COOHtotheAgOAcsolution,nofilmformationwasobserved,whichcouldbeduetothereductionofthepHvalueoftheAg(I)solution.Also,onlytheAgNO3solutiondidnotallowformationoftheAg-TCNQthinfilm;however,uponadditionofKOHtotheAgNO3solution,formationofauniformAg-TCNQthinfilmtookplace,perhapsduetothemodifiedpHvalueoftheAg(I)solution.VariouscomplementaryexperimentsclearlysuggestacrucialroleofpH(≥6.5)indictatingthegrowthofAg-TCNQthinFigure2.(a)FE-SEMimageoftheAg-TCNQthinfilmobtainedfilm.TheseresultsalsoemphasizethefactthatmetalsaltwiththeinvolvementofAgOAcsolution(inset:contactangleofaprecursorsarenottheabsolutemarkersdictatingthefeasibilitywaterdroplet).(b)PXRDpatternofourAg-TCNQthinfilm.ofthethinfilmformation,rathermodulationofthepHofthe10549https://dx.doi.org/10.1021/acs.jpclett.0c03229J.Phys.Chem.Lett.2020,11,10548−10551

2TheJournalofPhysicalChemistryLetterspubs.acs.org/JPCLLetterTheprimaryroleofpHisthereforehypothesizedtodeprotonatecarboxylicacid(fromMUDAoftheSAM)moietiesandtoprovidecoordinationsitesfortheAg(I)ions,facilitatingnucleationwithgrowthofAg-TCNQcrystallitesonthefunctionalizedAusubstrate.TheherepresentedgrowthofAg-TCNQthinfilmsisnotaspreciseascoveringtheentiresurfacewithonelayerofAg-TCNQinonecycleofLbL,asinourearlierstudiesontheLbLgrowthofCu-TCNQthinfilms7,18,19,21undersimilarconditions.ThatiswhyweneedtokeepthepHoftheAg(I)solutionat≥6.5andcontinueLbLcyclesuntilthesurfaceiscompletelycoveredwithAg-TCNQcrystallitesfollowedbyformationofathinfilminsubsequentcycles.Interestingly,uponmixingtheTCNQsolutionwiththeAgOAcsolution(FigureS9)aswellastheTCNQsolutionwiththeAgNO3solution(FigureS10),solidAg-TCNQwasconsistentlyisolatedfromboththemixtures(FigureS9andFigureS10).Thus,apartfromtheroleofpHinprimarilyprovidingtheseedinglayersofAg-TCNQ,thereseemtobeadditionalimportantrolesofpHinthegrowthofaAg-TCNQthinfilmatthesolid−liquidinterface,whichneedsfurtherexploration.Insummary,thepHvalueofAg(I)solutionhasbeenrealizedtobeacriticalfactorforthedirectgrowthofAg-TCNQthinfilmonnon-AgsubstrateslikefunctionalizedAuFigure3.(a)FE-SEMimageoftheAg-TCNQthinfilmobtainedviatheLbLmethod.StartingfromdifferentAg(I)precursorwiththeinvolvementofAgNO3+KOHsolution(insets:contactsalts,uniformanddensethinfilmsofAg-TCNQcouldbeangleofwaterandPXRDpattern).(b)FE-SEMimageshowingnoobtainedonlyifthepHoftheAg(I)solutionwasmaintainedformationoftheAg-TCNQthinfilmwiththeinvolvementofAgOAcat≥6.5,whilethesamesaltscouldnotyieldathinfilmata+CH3COOHsolution(insets:contactangleofawaterdropletandmuchlowerpH.OurpH-assistedgrowthofAg-TCNQthinPXRDpattern).filmnotonlyshedsnewlightonanunexploreddomaininthefieldofM-TCNQthinfilmsbutalsoopensupanewavenuemetalionsolutionholdsthekeytoit.Sofarintheliterature,forthefabricationofthinfilmsofmetal−organicsingeneraltheeffectsofpHonthedimensionalityandgrowthofvianovelchemistryatsolid−liquidinterface.coordinationpolymerslikemetal−organicframeworks23(MOFs)areexplored.Tothebestofourknowledge,the■ASSOCIATEDCONTENTpresentworkisthefirstreportonapH-basedgrowthstudyin*sıSupportingInformationthedomainofM-TCNQthinfilms.TheSupportingInformationisavailablefreeofchargeatModulationofpHoftheAg(I)solutioncouldalsobehttps://pubs.acs.org/doi/10.1021/acs.jpclett.0c03229.carriedoutbyaddingCu(II)salts.TheuseofamixedmetallicExperimentaldetails,FE-SEMandopticalimages,PXRDsaltsolutioncomprisingCu(OAc)2andAgNO3(outlinedinandRamanspectra,I−Vdata,andexperimentsinthedetailintheSupportingInformation)ledtothealmostliquidphase(PDF)exclusiveformationofaAg-TCNQthinfilmviatheLbLmethod(FigureS5),whichcouldeitherbeduetoreactivitydifference(Ag(I)versusCu(II))orapHvalueof6.5.To■AUTHORINFORMATIONfurtherstrengthenourclaimonthemodulationofthepHCorrespondingAuthorvalue,amixtureofCu(NO3)2andAgOAcwasexplored.NirmalyaBallav−DepartmentofChemistry,IndianInstituteRemarkably,noAg-TCNQthinfilmformationwasobservedofScienceEducationandResearch(IISER),Pune411008,duetosignificantloweringofthepHvalueofthemixedIndia;orcid.org/0000-0002-7916-7334;solutionto3.1(FigureS6).Furthermore,wementionthattheEmail:nballav@iiserpune.ac.inthinfilmsofAg-TCNQpreparedfromamixed(Cu(II)+Ag(I))solutionwerefoundtobealmostidenticaltothoseAuthorsobtainedbythedirectgrowthusingAgOAcsolution(FigureAswaniSathishLathika−DepartmentofChemistry,IndianS7).Specifically,temperature-dependentcurrent−voltage(I−InstituteofScienceEducationandResearch(IISER),PuneV)measurementsconsistentlyrevealedthermallydriven411008,India7resistiveswitchingbehavior(FigureS8).ThepresenceofShammiRana−DepartmentofChemistry,IndianInstituteofAgnanoparticlesinourAg-TCNQthinfilms(aswellasAg-ScienceEducationandResearch(IISER),Pune411008,TCNQsoliddiscussedbelow)isconsistentwithpreviousIndia7,16reports.AnupamPrasoon−DepartmentofChemistry,IndianInstitutepKavaluesofsimplealiphaticacids(MUDAinthepresentofScienceEducationandResearch(IISER),Pune411008,study)usuallyliewithin4−5.TakinganaverageofpKavalueIndiaof4.5,fromtheHenderson−Hasselbalchequation(pH=pKaPoojaSindhu−DepartmentofChemistry,IndianInstituteof+log10([Salt]/[Acid])),onecaneasilyestimatethatalmostallScienceEducationandResearch(IISER),Pune411008,thecarboxylicacidmoietieswillbedeprotonatedatpH≥6.5.India10550https://dx.doi.org/10.1021/acs.jpclett.0c03229J.Phys.Chem.Lett.2020,11,10548−10551

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