ATOMIC FORCE MICROSCOPY

Transcription

1 B47 PhysikalischesPraktikumfür Fortgeschrittene Supervision: Prof.Dr.SabineMaier ATOMICFORCEMICROSCOPY Version:E1.3 firstedit:15/09/2015 lastedit:16/12/2016

2 2 0BIntroduction CONTENT 1. Introduction Preparation TheeasyScan2AFM Experiment ChangingtheCantilever MeasurementsinDynamicMode Scanparametersettingsandtipapproach Measurementofthecalibrationgrid:optimizationofthescanparameter...12 XYScannercalibration Calibrationofthefreevibrationamplitude A mplitudedistanceandresonancecurves Contactmodemeasurements Exchangingthecantilever Measurementofanopticalstoragedisc MeasurementofCollagen DataAnalysis DynamicMode ContactMode References...21

3 ExperimentB47:AtomicForceMicroscopy 3 1. INTRODUCTION The atomic force microscope (AFM) is a crucial analysis instrument in modern nanotechnology. Surface propertiesliketopography,elasticityoradhesioncanbedeterminedbyafmmeasurementsonmicroscopic scales.theafmwasinventedin1986bybinningandrohrer(ibmresearchlabzürichandstanforduniversity). Likethescanningtunnellingmicroscope(STM)theAFMbelongstothefamilyofthescanningprobemethods. Allmembersofthisfamilyhavethesamebasicworkingprinciple:Amicroscopicprobescansoverasurface andinteractswithit.byrecordingameasurementparameter,whichisrelatedtotheinteractionbetweentip andsurface,alocalmapofthesurfacepropertiesiscreated. InthecaseoftheAFM,theprobeisatipwithalengthofsomemicrometersandatipradiusofabout10nm. Thetipismountedata50450μmlongcantileverthatisusedasforcegauge(Fig.1). Figure1:ScanningelectronmicroscopeimageofarectangularAFMcantileverwiththedimensions l=445μm,w=43μm,t=4.5μm,h=14.75μm(figureadaptedfromref.[1]) Usually the whole system(cantilever plustip) isproduced as onepartby etching it from a siliconpiece. In ContactMode AFM the tip is approached to the surface until the (repulsive) interaction forces lead to a deflectionofthecantilever,whichcanbedetectedandusedassignal.toimagethesurface,thedeflectionof thecantileveriskeptconstantbyafeedbackcontrol.indynamicmodeafm,thecantileverisoscillatingand the change in the amplitude or the frequency of the oscillation is measured. The dynamic mode can be subdividedintodifferentclasses,especiallyintotappingmodeandnoncontactafm.inthisexperimentyou willusecontactmodeandtappingmodetocharacterizedifferentsamples.

4 4 1BPreparation 2. PREPARATION Tocarryoutthisexperimentsuccessfully,anunderstandingofthefollowingtopicsisnecessary: TheworkingprincipleandsetupofanAFM MeasuringmodesofanAFM,especiallythedynamicvs.staticmode Propertiesofadampedharmonicoscillatorunderinfluenceofanexternalforce Forcesbetweentipandsample Youshouldbeabletoanswerthefollowingquestions: 1. HowdoesanAFMwork? 2. Whataretheimportantmeasuringmodesandtheiradvantagesanddisadvantages? 3. Howdoesthefeedbackcontrolsystem(PIcontroller)workwhileimagingwithanAFM? 4. Whichforcesexistbetweentipandsample?Whichonesareshortrangedandwhichlongranged? 5. How can the corresponding potential be approximated? How does the force look like? In which rangesofthepotentialarethedifferentmeasurementmodessettled? 6. Whatisthetypicalappearanceofaforcedistancecurve?Whatarethedifferenceswhenmeasuringit underambientconditions,inwater,andinvacuum? 7. Whendoinstabilities("jumpin","jumpout")takeplaceinaforcedistancecurveandhowcantheybe explained? 8. InwhichorderofmagnitudearetheforcesmeasuredbyanAFM? 9. Howdoestheresonancepeakofaonedimensionalharmonicoscillatorchangeundertheinfluenceof aconstantexternalforceoraconstantexternalforcegradient?theassociatedequationofmotionis: m x x kx F ext (x,t).

5 ExperimentB47:AtomicForceMicroscopy 5 Literatureforpreparation(allliteraturechaptersarealsoavailableinprintedformfromthesupervisorson request): [1]B.Bhushan,SpringerHandbookofNanotechnology,Springer,Berlin2007 AsEBook: Chapter22:PrincipleofOperation,Instrumentation,andProbes Chapter27:DynamicmodesofAFM [2]NanosurfeasyScan2AFMOperatingInstructions(seebelow,StudOn) [3]E.Meyer,H.HugundR.Bennewitz,ScanningProbeMicroscopy:TheLabonatip,SpringerVerlag [4] Furthermaterialfortheexperimentincludingsoftwareandmanuals: OnStudOn( )»OnlineAngebote»4.Nat»4.5Physik»PhysikderKondensiertenMaterie»Professurfür Experimentalphysik(Rastersondenmikroskopie)»Rasterkraftmikroskopie Password:B47AFM

6 6 2BTheeasyScan2AFM 3. THEEASYSCAN2AFM TheeasyScan2AFMfromNanosurfisusedinthisexperiment(Figure2).Itiseasytohandleandmanagesmany different measurement modes. In contrast to most other AFMs the easyscan2 uses an electromagnetic scannerinsteadofapiezoelectricone.theadvantagesofthissystemareahighlinearityandthefact,thata highvoltagesupplyisnotnecessary. (a) (b) Figure2:(a)OverviewoftheeasyScan2(b)Topandbottomviewofthescanhead(Figuresadaptedfrom[2]). TheforcedetectionoftheeasyScan2isbasedonadeflectionsensor(seeFigure3).Alaserspotisfocusedon thefrontpartofthecantileveranditsreflectioniscenteredonatwosegmentphotodiode.themonitored signalisthedifferenceofbothphotocurrents.beforescanning,thesystemisnormallyadjustedtozero.due tothebendingofthecantileverduringthescan,thereflectionofthelaserspotmovesverticallyonthediode andchangesthedifferencebetweenthephotocurrents.thesignofthedifferenceisrelatedtothedirection ofthebendingandthevaluetothemagnitude.

7 ExperimentB47:AtomicForceMicroscopy 7 (a) (b) Figure3:(a)SchematicofanAFMwithdeflectionsensor.(b)AssemblyoftheeasyScan2scanhead(Figureadaptedfrom[2]). Attention: The laser of the easyscan2 is a class 2M laser: DO NOT STARE INTO THE BEAM OR VIEW DIRECTLYWITHOPTICALINSTRUMENTS. The easyscan2 scan head has to be placed during the entire experiment either on the scan table or the storageplate.

8 8 3BExperiment 4. EXPERIMENT 4.1 CHANGINGTHECANTILEVER ThefirstmeasurementisperformedinthedynamicmodewithaBudgetSensorTap190AlGcantilever (f R =190kHz;k=48N/m). ExchangethecantileverONLYunderthesupervisionofthesupervisor.Nevertouchthecantileverwith barehands.itcanbeeasilydamagedordestroyed. Alwaysusetweezerswhenhandlingthecantilever. Always mount the Dropstop (see Figure 4a) before exchanging the cantilever. Otherwise the cantilever coulddropintothescanheadandcausedamage.inadditionthedropstopshieldsthelaser. Thepriceofonecantileveris40Euro. Howtoexchangethecantilever: 1. Cantileverexchange:Removetheoldcantilever(Figure4) Switchoffthescanelectronic TurnthescanheadupsidedownandmounttheDropstop(Figure4a). UsethePushRodtopushdownthecantileverfixingspring(Figure4b). Removethecantileverwithtweezersandputitintothestoragebox(Figure4c). Figure4::(a)MountingtheDropstop.(b)MountingthePushRod.(c)Replacingthecantilever(Figureadaptedfrom[2]).

9 ExperimentB47:AtomicForceMicroscopy 9 2. Insertinganewcantilever Usetweezerstotakeanewcantileverfromthestoragebox. Placethecantilevercarefullyonthealignmentchip(Figure5). Figure5::Left:Alignmentchiponthemicroscope.Right:Bottomsideofthecantileverchip(Figureadaptedfrom[2]). Carefullymovethecantileverchipintothecorrectpositionbyslightlytappingthecantileverchipon itstopside.thepositioniscorrect,whenthecantileverchipsymmetricallyfitstothealignmentchip (Figure6,left). Figure6::Left:Correctcantileverposition.Thecantileverchipandalignmentchipformsmalltrianglesintwocornerswhich shouldbesymmetric.thelightreflectioncontinuous.middle/right:wrongcantileverpositions. RemovecarefullythePushRod.Ifthecantileverchipstillmoves,itisnotinsertedcorrectly. RemovetheDropstop. Placethescanheadonthescantable. Connectallcableswiththescanheadandactivatetheelectronics.IMPORTANT:Alwaysactivatetheelectronic beforestartingthesoftwaresinceotherwiseapropercommunicationbetweencomputerandelectronics cannotbeassured.inthecase,thereisnoconnectionbetweencomputerandelectronics,thesoftwaresays "Simulation"inthebottomstatusbar,andbothelectronicsandcomputerhavetoberestarted.

10 10 3BExperiment 4.2MEASUREMENTSINDYNAMICMODE Scanparametersettingsandtipapproach Usethevideooptioninsideview. Figure1:Videoimageofthecantilever. Placethecalibrationgridonthescantablebyusingtweezers(NEVERtouchthesurfaceofthesample). Aligntheedgesparalleltothescantable. Usethefollowingparametersettings: Figure2:Parametersettings for dynamicmode measurements. Checkthe"SPMParameters"byclicking"More "andchoosingthe"imaging"tab:

11 ExperimentB47:AtomicForceMicroscopy 11 Approachthecantilevertothesurfacebyusingthefollowingprocedure: 1. Use the adjustment screws to approach the cantilever to a cantileversurface distance of 12 mm. Makesurethescanheadstaysparalleltothescantable. 2. Use"Advance"tofurtherreducethedistancebetweencantileverandsample.Usetheshadowofthe cantileveronthesurfaceasorientation:agapbetweenshadowandcantilevershouldstillbevisible. 3. Nowactivatetheautoapproachfunctionofthemicroscopebyclicking"Approach". 4. Aftershorttimeapopupwindowindicatesthattheapproachwassuccessful: 5. ChecktheLEDsatthecontrolunit:The"probestatusLED"shouldbegreen.IncaseofaredLEDthe approachfailed.thenretractthecantilever,checktheplacingofthecantileveronthealignmentchip andtryitagain. Important: Touching the scanhead or scantable while the cantilever is engaged to the sample can distort the measurement,andmostlikelydamagescantileverandsample.

12 12 3BExperiment Measurement of the calibration grid: optimization of the scan parameter Plane correction: The surface of the sample (i.e. measurement plane) and the plane of the xy scanner are ideally parallel to each other. In practical application this is rarely the case. A mismatch between both planes reduces the efficiency of the zcontroller, and thereby also reducestheabilitytoresolvesmalldetailsofthe sample. A slope correction is able to correct this mismatch: Slopecorrectionprocedure: Startascanandtakeameasurementwithoutslopecorrection. Measuretheanglebetweenthehorizontalandascannedlineinxdirectionduringscanning.Usethe anglemeasurementtoolfromtheanalysispanelandapplyittoalinescaninthecrosssectionwindow. Correcttheslopeinxdirectionbyinsertingthemeasuredangleinthe"SlopeX"fieldinthe"Imaging"tab ofthespmparameters(click"more...",choosethe"imaging"tab). Rotatethescanangleby90 torepeatthesameprocedurefortheydirection. Tasks: Measurethecalibrationgridbeforetheslopecorrection. Doaslopecorrectionandmeasurethecalibrationgridagain. Whatisthedifferencebetweenthe"linefitfilter"andtheslopecorrection? Whyaretherestripeswhenthe"linefitfilter"isactivated? Change the scan parameter (time per line, free vibration amplitude, P/I gain) in a controlled way and find optimal settings. Describe your observations for too high and too low settings of the controller gain. Saveoneimagewithoptimalparameters.

13 ExperimentB47:AtomicForceMicroscopy XYScannercalibration Thescannermovesinxandydirectionbyapplyingavoltagetothescanner.Inordertoconnectadistanceto theappliedvoltage,thescannerhastobecalibratedbyusingacalibrationgridwithknowndimensions(hint: Aparallelalignmentbetweenscandirectionandgridedgessimplifiesthecalibrationofthexyscanner.Ifthisis notthecase, Retract,changetheorientationofthegridandapproachagain). Thexyscanneriscalibratedbyusingthefollowingprocedure: Usethe"MeasureLength"toolfromthe"Analysis"paneltodeterminethesizeofthecalibrationgridin yourlastmeasurement. Calculatethecalibrationfactorsforxandydirectionbydividingthereferencesi measuredsizeofthegrid. Openthe"Scanheadcalibrationeditor"("Settings">"Calibration">"Edit") zeofthegridbythe Insertthefactorsforxandydirectioninthecorrespondingfieldsbyclicking"Set".Confirmbyclicking "Set". Closethe"Scanheadcalibrationeditor"againbyclicking"OK".Thescannershouldbecalibratednow. Measurethecalibrationgridagain.Donotforgettosavethemeasurement. Checkthesizeofthemeasuredgridbyusingthe"MeasureLength"tool. Task: Calibratethexyscannerusingtheproceduredescribedabove.Writeyourcalibrationfactorsdown?

14 14 3BExperiment Calibrationofthefreevibrationamplitude Thefreevibrationamplitude(deflectionofthefreeoscillatingcantilever)isbydefaultgivenasavoltage.In order to determine its value in nanometer, a conversion factor needs to be derived from an amplitude distancecurve. Theamplitudecalibration: AftereachsuccessfulapproachtheeasyScan2softwareusesthecurrentzpositionasreferenceandsetsitto zero.anegativevaluemeansazpositionabovethesamplesurfaceandapositivevaluemeansazposition below the surface. Always begin with "Start values" well above the surface (i.e. negative ones) and then decreasethe"startvalue"stepwise.avoidpressingtoostronglyonthesurface. Changetothespectroscopymodebyclicking"Spectroscopy"inthelowerleftpanel. Use the following parameters: Start value 200nm and Range 150nm. Check that the "Amplitude Specforward"optionisactivated. Measure amplitudedistancecurves by decreasing the "Start value" step by step (for example in 10 nm steps)untilyouobserveachangeintheamplitude(i.e.achangeoftheslopeoftheamplitudedistance curve). Always confirm each change with the return key, otherwise the change is not applied to the measurement.ifthemeanvalueoftheamplitudeissmallerthan50mv,donotfurtherdecreasethe"start value,butaskyoursupervisorforhelp. Saveonemeasurementandretractthecantileverfromthesamplebyclicking"Withdraw".

15 ExperimentB47:AtomicForceMicroscopy 15 Tocalibratethefreevibrationamplitude,measuretheslope(=sensitivity)ofthecurvewiththe"Measure Length"toolfromthe"Analysis"panel. Openthe"Scanheadcalibrationeditor"(Settings>Calibration>Edit)andgotothe"I/OSignals"tab. Calculatefromthemeasuredslopethevalueforthedeflection,whichwouldbeexpectedforanamplitude of10vandinsertitinthecorrespondingfield. Tasks: DeterminethecalibrationfactorforthevibrationamplitudeinVoltperNanometer. Howlargeisafreeamplitudeof200mV(asusedinthepreviousmeasurements)innm?

16 16 3BExperiment equencysweepis Click"Capture"in dtohaveafree uencyandone frequencyinthe quencysweep"window (see approximatelythesameas before. Amplitudedistanceandresonancecurves thefrequencybelowtheresonancefrequency.youcanchangetheexcitation "Vibrationfrequencysearchdialog"byshiftingthemarkerinthe"AmplitudeFre Figure9).Selectthenewfrequencysuch,thattheresultingamplitudeis Acquisitionofresonancecurves: Approachthecantileveragaintothesurface.Whenpressingthe approach button,afr doneautomaticallybythesoftware. Usethe"Freq.Sweep"buttoninthe"Acquisition"paneltodisplaytheresonancecurve. ordertosavetheresonancecurve.howlargeistheexcitationamplitude,whichisneede vibrationamplitudeof200nm? Measuretwoamplitudedistancecurves,oneatafrequencyabovetheresonancefreq with Figure9:Resonancecurvewithchosenexcitationfrequencyabove(left)andbelow(right)theresonancefrequency.Theamplitudeisin Tasks: Examinethedependenceoftheamplitudedistance thereforetwoamplitudedistancecurves,onewith withthefrequencybelowtheresonancefrequency(u SaveatleastoneresonancecurvetoestimatetheQ factordependon? Whichexcitationamplitudeofthecantileverisnecessar bothcasesapproximatelythesame(bluehorizontalline). Measure andone doestheq 0mV? curvesonthechosenexcitationfrequency. afrequencyabovetheresonancefrequency setheprocedureexplainedabove). factorinyourreport.onwhichvariables ytorealizeafreevibrationamplitudeof20

17 ExperimentB47:AtomicForceMicroscopy 17

18 18 3BExperiment CONTACTMODEMEASUREMENTS Exchangingthecantilever Exchange the cantilever in presence of the supervisor. Insert a contact mode cantilever (ContAlG, f R =25kHz,k=0.2N/m). Changethefollowingparametersinthesoftware: Figure10:"Acquisition"und"ZController"parameterforcontactmodeAFM Measurementofanopticalstoragedisc Weprovide3samplesofdifferentopticalstoragediscs(CD,DVD,BluRay).Youcanchooseoneofthethreeto determinewhichkindofopticalstoragediscitis. Chooseasampleandputitintothemicroscope. Approachthecantilevercarefullytothesamplesurface.IMPORTANT:Theshadowofthe notbevisibleasclearlyasonthecalibrationgridbefore. cantilevermight Tasks: Searchaniceareatomeasurethestructureoftheopticaldisc.Firstperformanoverviewscanandapplya slopecorrection.zoomstepwiseintoagoodspottodeterminetheminimalbitsizeandthetrackpitchin yourreport(donotforgettosaveyourmeasurements).optimizethefeedbackparameterstogetagood image.theoptimalparametersvarystronglyforeachsample,cantilever,...whichkindofopticalstorage discdoyouhave? Conductaforcedistancecurvewiththesameprocedureasexplainedinchapter4.2.4.Saveatleastone successfulforcedistancecurveanddiscussitinyourreport.areyouabletocalibratetheamplitudefrom mvtonanometerfromthiscurve(analogoustochapter4.2.4)?

19 ExperimentB47:AtomicForceMicroscopy MeasurementofCollagen InsertthecollagensampleintotheeasyScan2AFM. Carefully approach the cantilever to the sample surface. You can switch the camera view in the video optiontoidentifyagoodplacetoapproachonacollagenfibrilbeforehand. Tasks: Measure the collagen sample analogous to the measurement on the optical disc: Perform an overview scanandzoomstepwisetoareasonablespotuntilyoucanseethetypicalbandpattern(seefigure11as anexample).youmayneedtotrydifferentfibrestofindasuitableone.whyisthebandpatternofthe fibresoftenbettervisibleinthedeflectionsignalthaninthetopographysignal? Bonus:Conductoneforcedistancecurvemeasurementonthefibrilandoneonthesiliconsubstrate. Figure11:TopographyanddeflectionsignalofaCollagensample. Check that you have really done all mentioned tasks and that you have saved all your files. For further analyzingyourdataathomeyouwillneedtheeasyscan2software.acopyofthesoftwareisavailableon

20 20 4BDataAnalysis StudOn, or ask your supervisor for a copy. Further analysis software can be downloaded without charge fromthefollowinghomepages: GWYDDION: WSxM: thelatestversion,notthe"stable"one,isrecommended) 5. DATAANALYSIS Writeareportwhichincludesadescriptionofthetasksfromtheexperimentalpart.UseyourmeasuredAFM topographiesandcurvesforthedocumentation.inaddition,answerthefollowingquestionsinyourreport, andprovidearoughdeterminationofthemeasurementerrors. 5.1.DYNAMICMODE a) Describe what you see on the measurement of the calibration grid. What is the reason for the lines appearing while the "linefitfilter" is activated? How can these stripes be removed? Describe your observationsfortoolowandtoohighiandpgain. Figure12:Calibrationgridwith(left)andwithout(right)"linefitfilter"(Figureadaptedfrom[2]) b) Indicateyourcalibrationfactorsforthexyscanner(pluserror).Howlargeistheresidualxyerrorafter thecalibration? c) IndicatethecalibrationfactoroftheamplitudeinmV/nm(pluserror).Whatkindofsampledoyouneed togetameaningfulcalibrationwiththismethod? d) Provideaninterpretationoftheamplitudedistancecurvesaboveandbelowtheresonancefrequency: Compare their shape with the theoretical expected shape (as shown in the literature). What is the

21 ExperimentB47:AtomicForceMicroscopy 21 differenceforanexcitationbelowandabovetheresonancefrequencyofthecantilever?estimatethe sizeoftheattractiveinteractionregimebetweentipandsamplefromtheamplitudedistancecurves. e) DeterminetheQfactoroftheresonancecurve. 5.2.CONTACTMODE a) Discusstheshapeoftheforcedistancecurve.Explainthehysteresisbetweenapproachingandretracting the probe to/from the sample. Determine a suitable conversion factor for the force from the force distancecurves.calculatethemaximalrepulsiveforceandthemaximaladhesionforce(erroranalysis). b) Briefly describe the mean differences between CD, DVD, and BlueRay discs. Determine with the "MeasureLength"tooltheminimalbitsizeandthepitchtrack.Whatkindofopticalstoragedischave youmeasured? c) Determinethecharacteristicperiodicityofthecollagenfibrilsfromyourmeasurements.Usethecross sectiontooltogetacrosssectionacrossthecollagenfibrils. d) Bonus:Forcedistancecurves:Whichissofter,theSisubstrateorthecollagenfibrils? 6. REFERENCES [1] [2] B.Bhushan,SpringerHandbookofNanotechnology(Springer,2010). Nanosurf,NanosurfeasyScan2AFMOperatingInstructions2011).