近代分析实验原理：第十一课-电子能谱

十一、电子能谱

（ElectronSpectroscopyforSurface

Analysis）近代分析实验原理（Introductionofmodernanalyticalmethods）121.BasicPrinciplesElectronSpectroscopyelementalanalysisEmitcharacteristicelectronsPhotoelectronsAugerelectronsAugerelectronspectroscopy(AES)X-rayphotoelectronspectroscopy(XPS)surfacechemicalanalysis(generally20–2000eV)(adepthof10nmorless)3Emissionprocessesofcharacteristicelectrons:(a)a1sphotoelectron;and(b)aKL1L2,3Augerelectron41.1X-rayPhotoelectronSpectroscopyTheX-rayphotoelectronisanelectronejectedfromanelectronshellofanatomwhentheatomabsorbsanX-rayphoton.thebindingenergyoftheatom’sphotoelectron(EB):aphotoelectronwithkineticenergyEKtheenergyrequiredforanelectrontoescapefromamaterial’ssurfacecharacteristicvaluesidentifieschemicalelementsthebindingenergyIncidentX-rayphoton5XPSspectrumofanoxidizedaluminumsurface.peaksfromAugerelectronselementsymbolplusashellsymbol6Thephotoelectronsemittedbysubshellsp,dandfarecommonlymarkedwithanadditionalfractionnumberJ,71.2AugerElectronSpectroscopyAugerelectronswerenamedafterPierreAugerwho,togetherwithLiseMeitner,discoveredAugerelectronemissionin1920s.anincidentelectronknocksoutaKshellelectron,aL1shellelectronrefillstheKshellvacancy,andaL2,3shellelectronisejectedastheAugerelectron.thebindingenergyofelectronshell-Φ8AESspectraofanoxidizedaluminumsurface:(a)directspectrumofintensityversuskineticenergyofAugerelectrons;and(b)differentialspectrumofintensityversuskineticenergyofAugerelectrons.9SchematiccomparisonofAugerpeakintensitywithotherelectronsescapedfromasolidsurface.Eoindicatesenergyofincidentelectrons.ThekineticenergyofelectronscanbedividedintothreeregionsI,IIandIIIfromlowtohigh.TheprimaryelectronsejectedfromasolidsurfacebyinelasticscatteringcomprisethebackgroundofanAESspectrumintheregionofhighkineticenergieswhilethesecondaryelectronscomprisethebackgroundintheregionoflowkineticenergies.102.InstrumentationStructureofanelectronspectrometer（combiningXPSandAES）10−8–10−10mbarPreventthescatteringKeepthesurfacecleanstainlesssteelcrushedcoppergasketsanelectrongun,anX-raygunandasharedanalyzerofelectronenergy.magneticshielding112.1SourceGuns2.1.1X-rayGunNon-monochromaticX-rayradiationfromanX-raygunwithanAltarget.ThecharacteristicAlKαlineisatabout1.5keV.commonlyAlorMgLowerenergyX-raysnarrowlinewidthXPSrequiresalinewidthlessthan1.0eVtoensuregoodenergyresolution.BothAlKαandMgKαexhibitlinewidthslessthan1.0eVandalsohavesufficientenergies(>1000eV)forphotoelectronexcitation.usesbothnon-monochromaticandmonochromaticX-raysourcesAlKαandMgKα1.4866and1.2536keVCuKαandMoKα8.04keVand17.44keV12AnX-raygunwithtwo-anodes.Twotaperedanodefaces(oneisAlandtheotherisMg)havesemi-circularfilaments,whichareneargroundpotential.Anaccelerationvoltageofabout15kVbetweenafilamentandanodegeneratesX-raysthatexitthroughanAlwindow.switchingbetweenMgKαandAlKα，AlKαandMgKαare1.4866and1.2536keVlinewidthlessthan1.0eV132.1.2ElectronGunsimilartothoseusedinelectronmicroscopy（LaB6andfieldemissionguns）2.1.3IonGunThefunctionsofaniongunaretwofold.First,itprovidesahighenergyionfluxtocleansamplesurfacesbeforeexamination.Thesecondfunctionoftheiongunistosputteroutsampleatomslayerbylayersothatanelementaldepthprofilecanberevealed.（argonion）Energy：0.5to5.0keVfocusedtoadiameterdowntoseveraltensofmicrometers.scanasurfaceareaaslargeas10×10mm142.2ElectronEnergyAnalyzersWorkingprinciplesofaconcentrichemisphericalanalyzer.concentrichemisphericalanalyzer(CHA)(hemisphericalsectoranalyzer(HSA))NegativeTheCHAonlyallowstheelectronswithenergyE=eVo,whichareinjectedtangentiallytothemediansurface,topassthroughitschannelandreachthedetector.V0passenergy15constantanalyzerenergy(CAE)modeXPSconstantretardingratio(CRR)modeAESelectronretardationElectronenergyreductionCHAXPSrequireshighabsoluteresolutionofabout0.5eVinthewholerangeofaspectrum.CHAhasarelativeresolutionlimit.ForE=200eV,aCHArequiresarelativeresolutionof0.025tosatisfytheXPSabsoluteresolutionof0.5eV.However,forE=1500eV,aCHArequiresarelativeresolutionof0.003todoso,whichisnotpractical.lowCHApassenergy:10–100eVAugeranalysisrequiressuppressingtheelectronsignalatthelowenergyendofitsspectrum.CHA:LowtransmissionratewithlowpassenergyWhenaconstantretardationratioisapplied,alowAugerelectronenergygenerateslowCHApassenergy.163.CharacteristicsofElectronSpectra3.1PhotoelectronSpectraAnXPSspectrumofsilverexcitedMgKαwithpassenergyof100eV.thevalence-levelpeaktheAugerpeakscore-levelphotoelectronpeaksElementalanalysisPrimarilyusefulinstudiesoftheelectronicstructureofmaterials.Thevalence-levelpeaksarethoseatlowbindingenergy(0–20eV)17ExamplesofseveraltypesofsatellitepeaksinXPSspectra:(a)shake-uppeaksinaCuOspectrum;(b)shake-uppeaksandmultipletsplittinginaNiOspectrum;and(c)plasmonlosspeakinacleanAlspectrum.resultfrominteractionbetweenaphotoelectronandavalenceelectron.hasunpairedelectronsinitsvalencelevelexcitescollectivevibrationsinconductionelectronsinametalNousefulinformation183.2AugerElectronSpectraAugerspectraofacontaminatedtungstenfoilacquiredinafixedretardingratiomodewith0.6%relativeresolution:(a)directspectrum;and(b)differentialspectrum.ElementsP,N,O,W,Careindicated.thefirstderivativeofthecurvePeak

positionslightlydifferent.19ChartofprincipalAugerelectronenergiesofKLL,LMMandMNNlinesAlightelementisoftenidentifiedfromitsKLLAugerlines,whichdominateintheAugerspectrumrange.However,foranelementwithatomicnumberhigherthan15,eitherLMMorMNNAugerlinesaredominant.TheLMMlinesforanelementaredividedintothree,astriplets.TheLMMtripletfeatureresultsfromthedifferenceinsubshellsinvolvedintheAugerprocess.20PrincipalAugerKLLpeaksoflightelements,Be,B,C,N,O,FandNa.KL23L23isthemostvisibleKLLpeakforeachelement;forexample,OKL1L1(468eV),OKL1L23(483eV)andOKL23L23(503eV).21TripletpeaksofAugerspectraforCr,MnandFe.TheLMMtripletsoccurintransitionmetals.ThelowkineticenergypeaksareofL2,3VVwhereVrepresentsthelevelofvalenceelectrons.224QualitativeandQuantitativeAnalysisChemicalanalysisidentifychemicalelementschemicalstatusthespatialdistributionsofelements4.1QualitativeAnalysis23PeakIdentificationThepeaksinanAESspectrumcanbeidentifiedbycomparingtheexperimentalpeakswithstandardpeaksfoundinreferencebooksorcomputerdatabases.PeakidentificationsinXPSspectra,however,aremorecomplicatedbecauseAugerpeaksmaybepresent.distinguishtheAugerpeaksfromphotoelectronpeaksAnAugerpeakwillshiftinapparentbindingenergyinanXPSspectrumwhenwechangetheX-raysource.Forexample,anAugerpeakshiftsby233eVintheXPSspectrumwhenwechangetheradiationfromMgKα(1253.6eV)toAlKα(1486.6eV).CalibrationC1speakat284.8eVFixedPeakpositionsinanXPSspectrumarelikelytobeaffectedbyspectrometerconditionsandthesamplesurface.24ChemicalShiftsChemicalshiftsofbindingenergypeaksforanelementarecausedbythesurroundingchemicalstateoftheelement.XPSspectrumofpoly(vinyltrifluoroacetate):(a)C1s;and(b)O1swithmonochromaticAlKαexcitation.聚（乙烯基三氟乙酸）carefullyresolvetheoverlappedpeakswithassistanceofcomputersoftware.25ChartofcarbonchemicalshiftinXPSspectra.Thelargerthenumberofelectronstransferred,thehigherthechemicalshift.26ComparisonofpositionsandshapesofOKLLAugerpeaksinseveralsolidoxides.ChemicalshiftsalsooccurinAESspectra,andthechemicalshiftscanbesignificantlylargerthantheshiftsinXPS.Forexample,theshiftbetweenmetallicandoxideAlpeaksofAlKL2L3ismorethan5eVwhilethecorrespondingshiftofAl2pbindingenergyisonlyabout1eVComparisonofpositionsandshapesofOKLLAugerpeaksinseveralsolidoxides.27InsulatingSample:ChargeaccumulationonsurfaceUncertainΦchargeneutralizationForAES,thissurfacechargeproblemwithinsulatingsamplesismoredifficulttoovercomebecausetheelectronshavetoberemovedfromtheinsulatingsurface,insteadofcompensatingforelectronloss.AESdoesnotworkwellwithtotallyinsulatingmaterials.XPS28CompositionImagingsimilartotheEDSmappingResolution:10μm(XPS);10nm(AES)Comparisonbetweenimagesofgold-coatedstainlesssteel:(a)ascanningelectronmicroscope(SEM)secondaryelectronimage;(b)ironAugerimage;(c)oxygenAugerimage;(d)goldAugerimage;and(e)nickelAugerimage.29XPSimagesofaTiAlNthinfilmonastainlesssteelsubstrate:(a)Ti2pphotoelectronimage;and(b)Fe2pphotoelectronimage.Theoxidizedfilmcontainsironthathasmigratedfromthesubstrate.304.2QuantitativeAnalysisAESsensitivityfactorsnormalizedtotheCuLMMlinefor10keVelectronradiation.Sensitivityfactorsarecalculatedfromthepeak