Outline
• WhatisAFM?• HowdoesAFMWork?• 3Modes:– Contactmode– Non‐contactmode– Tappingmode
• Imaging• AdvantagesofAFM
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History• ThefirstAtomicForceMicroscope(AFM)wasdevelopedbyG.Binnig,Ch.Gerber,andC.Quatein1985– Glueda[nyshardofdiamondontooneendofa[nystripofgoldfoil
– Smallhookatendofthe[ppressedagainstsamplesurface
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WhatCanWeLookAt?• AFMiscurrentlythemostwidelyusedscanningprobemicroscopytechnique
• Materialsinves[gated:thinandthickfilmcoa[ngs,ceramics,composites,glasses,synthe[candbiologicalmembranes,metals,polymers,andsemiconductors
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Applica[ons• Toproduceatopographicalimageofasurfacewithatomicresolu[on
• Determineroughnessofasample’ssurface
• Measuretrenchestodeterminefilmthickness
• Imagenon‐conduc[ngsurfaces• Studythedynamicbehavioroflivingandfixedcells
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AFMEssen[alElements
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• 1.Laser–deflectedoffcan[lever• 2.Mirror–reflectslaserbeamto
photodetector• 3.Photodetector–dualelement
photodiodethatmeasuresdifferencesinlightintensityandconvertstovoltage
• 4.Amplifier• 5.Register• 6.Sample• 7.Probe–[pthatscanssample
madeofSi• 8.Can:lever–movesasscanned
oversampleanddeflectslaserbeam
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AFMPrinciples–LeonardJonesPoten[al
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Separa[ondistance
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AFMPrinciples
• Duringscan,featuresaffectlaserdeflec[onoffcan[lever
• Laserdetectorreadsdeflec[onandeither:– Translatesdeflec[onintotopographymap
– Movesstagetomaintainzerolaserdeflec[onandtranslatesstagemovementtotopography.
hNp://www.weizmann.ac.il/Chemical_Research_Support/surflab/peter/afmworks/9
3ModesofAFM
• ContactMode– Highresolu[on– Damagetosample
• Non‐ContactMode– Lowerresolu[on– Nodamagetosample
• Tapping/IntermiNentMode– Intermediateresolu[on– Minimaldamagetosample
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ContactMode• Measuresrepulsionbetween[pandsample• Commonly,forceof[pagainstsampleremainsconstant
• Feedbackregula[onkeepscan[leverdeflec[onatzero
• Voltagerequiredindicatesheightofsample• Problems:Willdamagesoqersurfaces
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Non‐ContactMode• Tipdoesn’ttouchsample• MeasuresVanderWaalsforcesbetween[pandsampletoacquiretopographydata
• Doesn’tdegradeorinterferewithsample‐beNerforsoqsamples
• Problems:Can’tusewithsamplesinfluid
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Tapping/IntermiNentMode• Tipver[callyoscillatesbetweencontac[ngsamplesurfaceandliqingoffat50‐500kHz.
• Oscilla[onamplitudereducedasprobenearssurface
• Advantages:overcomesproblemsassociatedwithfric[on,adhesion,electrosta[cforces
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Genera[nganImage
• The[prastersbackandforthinastraightlineacrossthesample(thinkoldtypewriterorCRT)
• Inthetypicalimagingmode,the[p‐sampleforceisheldconstantbyadjus[ngthever[calposi[onofthe[p(feedback).
• Atopographicimageisbuiltupbythecomputerbyrecordingthever[calposi[onasthe[pisrasteredacrossthesample.
ScanningTip
RasterM
o[on
www.nanodevices.comwww.fz‐borstel.de/biophysik/de/methods/afm.html
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• Radiusof[plimitstheaccuracyofanalysis/resolu[on
• Widerproberadiileadtofeatureinaccuraciesandroundingofsteps
EffectofTipResolu[on
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ImagingMaterials
Thakurdesai,M.;et.al.Vacuum2008,82,639–644hNp://ej.iop.org/images/0960‐1317/22/4/045012/Full/jmm399735f1_online.jpg
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ImagingBiologicalSamples
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Single‐strandedG4DNA
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OtherCapabili[es• Electrosta[cForceMicroscopy(EFM)
• Dis[nguishingdifferencesinsamplecomposi[on(crystalorienta[on,chemicalcomposi[on)
• Measuringconduc[vity(CAFM)
• Micro/nanoindenta[on
• PiezoresponseForceMicroscopy(PFM)
• Magne[cForceMicroscopy(MFM)
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Electrosta[cForceMicroscopy(EFM)
• Non‐contactmode– Constantheightorconstantdeflec[on
• Trackchangesin[poscilla[onsduetoelectrosta[cforcegradientsbetween[pandsample(atlargesepara[on).
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MoreEFMImages
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CarbonNanotubeFerrite(bright)andAustenite(dark)
AFMPhaseMapping
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• InTapping/IntermiNentmode,changesinmaterialleadtochangesinphaseangleof[poscilla[on
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Conduc[veAFM
• OperatesinContactmodetomapvaria[oninelectricalconduc[vityofsample
• Alsoabletocollectcurrent‐voltage(IV)spectraatspecificpointsonsample
hNp://www.azom.com/ar[cle.aspx?Ar[cleID=328529
CAFMImages
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GaAsQuantumDots DVDRecording
Micro/Nanoindenta[on
• Testmechanicalproper[esofmaterials:– Hardness(maxloaddividedbyindentarea)– Young’sModulus(elas[city)
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PiezoresponseForceMicroscopy
• Measuresthemechanicalresponsewhenanelectricalvoltageisappliedtothesamplesurfacewithaconduc[ve[p
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Magne[cForceMicroscopy(MFM)
• Asthe[pmovesoveranmagne[cfieldgradient,itiseitherpulledtowardorrepulsedawayfromthesample.
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AdvantagesofAFM
• AFMvs.STM(scanningtunnelingmicroscope):Imagesbothconductorsandinsulators
• AFMvs.SEM(scanningelectronmicroscope):Greatertopographiccontrast
• AFMvs.TEM(transmissionelectronmicroscope):Noexpensiveand[me‐intensivesampleprep
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