Atomic Force Acoustic Microscopy AFAM and Ultrasonic Atomic Force Microscopy UAFM Both these techniques are combination of atomic force microscopy AFM and acoustic waves We have used commercial piezoelectric PZT PbZrTiO ceramic to elucidate the capa ID: 27043
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ContactResonanceImagingusingAtomicForceMicroscopyWehaveshownthatonecandeterminelocalelasticpropertiesofsurfacesusingcontactresonanceimagingie.,AtomicForceAcousticMicroscopy(AFAM)andUltrasonicAtomicForceMicroscopy(UAFM).Boththesetechniquesarecombinationofatomicforcemicroscopy(AFM)andacousticwaves.WehaveusedcommercialpiezoelectricPZT,Pb(Zr,Ti)Oceramic,toelucidatethecapabilityofthethesetechniquestoimagethedistributionoflocalstiffnessoverthesamplesurface.Wehaveshownboththetechniquesgivesimilarresults.But,UAFMhasanadvantageoverAFAMbecauseitdoesnotrequirethesampletobebondedtoanultrasonictransducer.IntheUAFMtechniquethecantileveritselfcanbeexcitedbyapermanentlyattachedultrasonictransducer.WehaveshownforthefirsttimethatUAFMgivessimilarcontrastinversionimagesliketheAFAMimages.InbothAFAMandUAFMtechniquethetipoftheAFMcantileverisincontactwiththesampleandtheneitherthesampleisexcitedusingaultrasonictransducerbondedunderneaththesample(AFAM)asshowninFig.1(a)orthecantileverisexcitedusinganultrasonictransducerattachedtothecantilever(UAFM).Inboththecaseseithertheamplitudeorfrequencyofcantilevervibrationismonitored.Dependingonwhethertheeffectiveelasticconstantbetweenthetipandthesamplesurfaceishigher(stiff)orlower(soft)thepeakoftheresonancecurvewillshifttowardsahigherorlowerfrequency,respectivelyasshowninFig.1(b)andFig.1(c).Thefixedendofthecantileverwillhaveonenodeofthevibrationatafixedposition(substrateend)andthepositionoftheothernodewilldependonthelocalrigidityofthesamplesurface(forthesofterregionthenodewillbedeeperintothesample,whereasfortheharderregionthenodewillbeclosetothesamplesurfaceasshownschematicallyinFigure1(b)).Thuswhenthetipisincontactwiththesofterregion,thewavelengthofthecantilevervibrationwillincrease(),resultinginadecreaseofthecantilever'svibrationalfrequency(f).Also,forthesofterregionthenodeatthesampleaccompaniedbyareductionoftheamplitudeattheresonance.Thus,asthetipmovestowardsthesofterregion,thefrequencyofvibrationdecreasesorincreasesifthetipmovestowardstheharderregion.Iftheexcitationfrequencyofthetransducer,whichisalsousedasthereferencefrequencyofthelock-inamplifier,issetslightlyabovethepeakvalueasshowninFig.1(c),andifthetipincontactwiththesurfacemovestoahigherelasticconstantregionthentheamplitudeofoscillationofthecantileverwillincrease(markedbyarrow1)andifthetipmovestoalowerelasticconstantregion,thentheamplitudeofoscillationofthecantileverwilldecrease(markedbyarrow2).Thusanincreaseintheamplitudeofoscillationofthecantilevercorrespondstoahigherelasticconstantregionandadecreaseintheamplitudeofoscillationcorrespondstoalowerelasticconstantregion.Ifwenowchoosethetransducerexcitationfrequencyandthereferencefrequencyforthelock-inamplifierbelowtheresonancepeakthenthesituationshouldbereversed.Thatis,theamplitudeofvibrationwilldecrease/increaseinregionswithhigher/lowerelasticconstantcomparedwiththereferencepoint.Thisismarkedbyarrow3andarrow4,respectively,inFig.1(c).Theimagestakenatfrequencyaboveandbelowtheresonancepeakwillhavecontrastinversionoftheimages.Wehaveseenthatthisinfactisobservedandfromthiswecanknowthedistributionoflocalelasticityatnano-scaleoverthesurfaceofthesample.endisnotverysharplydefinedascomparedwiththeharderregion,resultinginabroadeningofthepeak(increaseinfullwidthathalfmaximum(FWHM)correspondingtoFig.1:(a)SchematicdiagramoftheexperimentalsetupofAFAMandUAFM.(b)Schematicrepresentationofcantilevervibrationwhentipisincontactwithahardsurface(thenodesarewelldefinedatboththeendsofthecantilever)andasoftsurface(c)Schematicrepresentationoftheresonancecurveandtheshiftoftheresonancecurveforhigherandlowereffectiveelasticconstantregionsmarkedassoftandstiff -inamplifierSignalGenerator ACHIEVEMENT Fig.2:AFAMimageshowingin(a)stiff(bright)andsoft(dark)striperegionsandin(b)thestiff(dark)andsoft(bright)striperegions.(c)and(d)showstheschematicrepresentationofthePZTunitFig.3:AFAMandUFMimagesofpolishedbulksinteredpolycrystallinePZTsample.(a)and(b)aretheAFAMimagesobtainedwithoperatingfrequencyabove(1387kHz)andbelow(1370kHz)theresonancepeakrespectivelyand(c)and(d)aretheUFMimagesobtainedwithoperatingfrequencyabove(1392kHz)andbelow(1362kHz)theresonancepeak,respectivelyInFig.2(a)and2(b)theimagesobtainedonPZTsampleusingAFAMwithoperatingfrequencyaboveandbelowthecontactresonancepeakvaluerespectivelyareshown.WealsoshowschematicallyinFigures2(c)and2(d)thatdependingontheorientationoftheaxisoftheunitcellwithrespecttotheprobingtip,thesoftandthehardaxescanbeimaged.Sincethepolarizeddomainsareorientedindifferentaxesanddirections,theelasticconstantalongthataxisanddirectionarealsodifferentandhenceweseeadistributionofferroelectricdomainsonthesamplesurface.Wehaveshownthatusingcontactresonancetechniquei.e.,bothAFAMandUAFMtechniquesonecanimagethelocalelasticpropertiesofthesampleatnanoscaleorder.Wehavealsoexplainedthecontrastinversionoftheimageusingasimpleargumentbasedontheresonancecurve,wheretheamplitudeofcantileveroscillationchangesdependingonthe1.S.Banerjee,N.Gayathri,S.Dash,A.K.Tyagi,andBaldevRaj;,(2005)211913.2.S.Banerjee,N.Gayathri,S.R.Shannigrahi,S.Dash,A.K.TyagiandBaldevRaj;(2007)Appl.Phy.LettJ.Phys.D:Appl.Phys.,Furtherinquiries:Dr.S.BanerjeeandDr.A.K.Tyagi,MaterialsScienceDivisionMetallurgyandMaterialsGroup,IGCAR,e-mail:akt@igcar.gov.inTocomparetheAFAMandUAFMtechnique,inFigs.3(a)and3(b)weshowtheAFAMimagesandinFigs.3(c)and3(d)weshowtheUFMimagesobtainedforthePZTsample.Figs.3(a)and3(c)showstheimageswhentheoperatingfrequencywasabovethecontactresonancepeakfrequencyandFigs.3(b)and3(d)showtheimageswhentheoperatingfrequencywasbelowthecontactresonancepeakfrequency.WeobserveclearlybrightanddarkregionsontheAFAMandtheUAFMimagesandalsocontrastinversionwhentheoperatingfrequencyischanged.WehaveshownthatUAFMwhichdoesnotrequireaseparateultrasonictransducerforsampleexcitationalsogivesthesameinformationandcanbeusedtoimagethelocalstiffnessofthesample Wehaveexplainedthecontrastinversionoftheimageusingasimpleargumentbasedontheresonancecurve,wheretheamplitudeofcantileveroscillationchangesdependingonthelocalstiffnessofthematerialdescribedschematicallyinFig.1(b)andFig.1(c). ADDITIONALINFORMATIONABOUTAFAMGENERALEXPLANATION:COMPARISONOFAFAMANDUFAMBRIEFDESCRIPTIONOFTHEORETICALBACKGROUNDPUBLICATIONSARISINGOUTOFTHISSTUDYANDRELATEDWORK