X-Ray Photoelectron Spectroscopy天津大学研究生仪器分析课程 英文版·.pdf

Prof.Jinlong Gong(巩金龙巩金龙)Sept.24,2010X-Ray Photoelectron SpectroscopyCharacterization of Materials SurfaceChemical EngineeringMyselfTianjinAu Model CatalystsYourselves?SurfaceElectric FieldElectronHeatPhotonsNeutralsIonsSurface AnalysisThe Study of the Outer-Most Layers of Materials(100).Electron SpectroscopiesXPS:X-ray Photoelectron SpectroscopyAES:Auger Electron SpectroscopyEELS:Electron Energy Loss Spectroscopy Ion SpectroscopiesSIMS:Secondary Ion Mass SpectrometrySNMS:Sputtered Neutral Mass SpectrometryISS:Ion Scattering SpectroscopyOutline of Lecture Principles of XPS The photoelectron effect Instrumentation-how measurements are made Analysis Capabilities Elemental analysis Chemical state analysis(core level shifts)Surface Sensitivity Applications Comparisons with other techniques AESLight can take on many forms.Radio waves,microwaves,infrared,visible,ultraviolet,X-ray and gamma radiation are all different forms of light.The energy of the photon tells what kind of light it is.Radio waves are composed of low energy photons.Optical photons-the only photons perceived by the human eye-are a million times more energetic than the typical radio photon.The energies of X-ray photons range from hundreds to thousands of times higher than that of optical photons.Very low temperatures(hundreds of degrees below zero Celsius)produce low energy radio and microwave photons,whereas cool bodies like ours(about 30 degrees Celsius)produce infrared radiation.Very high temperatures(millions of degrees Celsius)produce X-rays.Basics of Light,EM Spectrum,and X-raysX X-ray Photoelectron spectroscopy,also ray Photoelectron spectroscopy,also know as Electron Spectroscopy for know as Electron Spectroscopy for Chemical Analysis(ESCA),is based on Chemical Analysis(ESCA),is based on the photoelectric effect.the photoelectric effect.1,21,2It was It was developed in the middeveloped in the mid-19601960 s by Kai s by Kai SiegbahnSiegbahn and his research group at the and his research group at the University of Uppsala,Sweden.University of Uppsala,Sweden.3 31.H.Hertz,Ann.Physik 31,983(1887).2.A.Einstein,Ann.Physik 17,132(1905).1921 Nobel Prize in Physics.3.K.Siegbahn,Et.Al.,Nova Acta Regiae Soc.Sci.,Ser.IV,Vol.20(1967).1981 Nobel Prize in Physics.What is XPS?Introduction-XPS provides unique information about chemical compositionand chemical state of a surface-useful for biomaterials-advantages-surface sensitive(top few monolayers)-wide range of solids-relatively non-destructive-disadvantages-expensive,slow,poor spatial resolution,requires highvacuumXPS used to measure:*all elements with an atomic number of 3(lithium)and above.*elemental composition of the surface(top 110 nm usually)*empirical formula of pure materials*elements that contaminate a surface*chemical or electronic state of each element in the surface*uniformity of elemental composition across the top surface(or line profiling or mapping)*uniformity of elemental composition as a function of ion beam etching(or depth profiling)Instrumentation for XPSSurface analysis by XPS requires Surface analysis by XPS requires irradiating a solid in an Ultrairradiating a solid in an Ultra-high Vacuum high Vacuum(UHV)chamber with(UHV)chamber with monoenergeticmonoenergetic soft Xsoft X-rays and analyzing the energies of the rays and analyzing the energies of the emitted electrons.emitted electrons.?Remove adsorbed gases from Remove adsorbed gases from the sample.the sample.?Eliminate adsorption of Eliminate adsorption of contaminants on the sample.contaminants on the sample.?Prevent arcing and high Prevent arcing and high voltage breakdown.voltage breakdown.?Increase the mean free path for Increase the mean free path for electrons,ions and photons.electrons,ions and photons.Degree of VacuumDegree of Vacuum101010101010101010102 2-1 1-4 4-8 8-1111Low VacuumLow VacuumMedium VacuumMedium VacuumHigh VacuumHigh VacuumUltraUltra-High VacuumHigh VacuumPressurePressureTorrTorrWhy UHV for Surface Analysis?How to Achieve UHV?Extraordinary steps are required to reach UHV:*High pumping speed possibly multiple vacuum pumps in series and/or parallel*Minimize surface area in the chamber*High conductance tubing to pumps short and fat,without obstruction*Use low-outgassing materials such as certain stainless steels*Avoid creating pits of trapped gas behind bolts,welding voids,etc.*Electropolish all metal parts after machining or welding*Use low vapor pressure materials(ceramics,glass,metals,teflon if unbaked)*Bake the system to remove water or hydrocarbons adsorbed to the walls*Chill chamber walls to cryogenic temperatures during use*Avoid all traces of hydrocarbons,including skin oils in a fingerprint always use glovesHow Does XPS Work?A monoenergetic x-ray beam emits photoelectrons from the from the surface of the sample.The X-Rays either of two energies:Al Ka(1486.6eV)Mg Ka(1253.6 eV)The XPS spectrum contains information only about the top 10-100 of the sample.Ultrahigh vacuum environment to eliminate excessive surface contamination.Cylindrical Mirror Analyzer(CMA)measures the KE of emitted e-s.The spectrum plotted by the computer from the analyzer signal.The binding energies can be determined from the peak positions and the elements present in the sample identified.?XPS spectral lines are XPS spectral lines are identified by the shell from identified by the shell from which the electron was which the electron was ejected(1s,2s,2p,etc.).ejected(1s,2s,2p,etc.).?The ejected photoelectron has The ejected photoelectron has kinetic energy:kinetic energy:KE=KE=hvhv-BEBE-?Following this process,the Following this process,the atom will release energy by atom will release energy by the emission of an Auger the emission of an Auger Electron.Electron.Conduction BandConduction BandValence BandValence BandL2,L3L2,L3L1L1K KFermiFermiLevelLevelFree Free Electron Electron LevelLevelIncident XIncident X-rayrayEjected PhotoelectronEjected Photoelectron1s1s2s2s2p2pThe Photoelectric ProcessBEBEThe Atom and the X-RayCore electronsValence electronsX-RayFree electronprotonneutronelectronelectron vacancyThe core electrons respond very well to the X-Ray energy?L electron falls to fill core level L electron falls to fill core level vacancy(step 1).vacancy(step 1).?KLL Auger electron emitted to KLL Auger electron emitted to conserve energy released in conserve energy released in step 1.step 1.?The kinetic energy of the The kinetic energy of the emitted Auger electron is:emitted Auger electron is:KE=E(K)KE=E(K)-E(L2)E(L2)-E(L3).E(L3).Conduction BandConduction BandValence BandValence BandL2,L3L2,L3L1L1K KFermiFermiLevelLevelFree Free Electron Electron LevelLevelEmitted Auger ElectronEmitted Auger Electron1s1s2s2s2p2pAuger Relation of Core HoleKE=KE=hvhv-BEBE-X-Rays on the SurfaceThe X-Rays will penetrate to the core e-of the atoms in the sample.Some e-s are going to be released without any problem giving the KineticEnergies(KE)characteristic of their elements.Other e-s will come from inner layers and collide with other e-s of upper layers These e-will be lower in lower energy.They will contribute to the noise signal of the spectrum.Apparatus SetupX-ray Photoelectron SpectrometerNote:There are many different designs of electron energy analyzer but the preferred option for photoemission experiments is a concentric hemispherical analyzer(CHA)which uses an electric field between two hemispherical surfaces to disperse the electrons according to their kinetic energy.X-ray Photoelectron Spectrometer5 4.7X X-rayraySourceSourceElectronElectronOpticsOpticsHemispherical Energy AnalyzerHemispherical Energy AnalyzerPosition Sensitive Position Sensitive Detector(PSD)Detector(PSD)Magnetic ShieldShieldOuter SphereOuter SphereInner SphereInner SphereSampleSampleComputer Computer SystemSystemAnalyzer ControlAnalyzer ControlMultiMulti-Channel Plate Channel Plate Electron MultiplierElectron MultiplierResistive Anode Resistive Anode EncoderEncoderLenses for Energy Lenses for Energy Adjustment Adjustment(Retardation)(Retardation)Lenses for Analysis Lenses for Analysis Area DefinitionArea DefinitionPosition ComputerPosition ComputerPosition Address Position Address ConverterConverterInstrumentation:How are measurements made?Essential components:Sample:usually 1 cm2X-ray source:Al:1486.6 eV;Mg 1256.6 eVElectron Energy Analyzer:100 mm radius concentric hemispherical analyzer;vary voltages to vary pass energy.Detector:electron multiplier(channeltron)Electronics,ComputerNote:All in ultrahigh vacuum(10-8Torr)(Si4+)BE increases.When atom gains valence charge(O-O-)BE decreases.(Si2p BE increases)E q/r BE-(BE)oCarbonCarbon-Oxygen BondOxygen BondValence LevelValence LevelC 2pC 2pCore LevelCore LevelC 1sC 1sCarbon NucleusCarbon NucleusOxygen AtomOxygen AtomC 1s C 1s BindingBindingEnergyEnergyElectronElectron-oxygen oxygen atom attractionatom attraction(Oxygen Electro(Oxygen Electro-negativity)negativity)ElectronElectron-nucleus nucleus attraction(Loss of attraction(Loss of Electronic Screening)Electronic Screening)Shift to higher Shift to higher binding energybinding energyChemical Shifts-Electronegativity EffectsChemical Shifts-Electronegativity EffectsFunctionalGroupBinding Energy(eV)hydrocarbonC-H,C -C 285.0amineC-N 286.0alcohol,etherC-O-H,C -O-C 286.5Cl bound to CC-Cl 286.5F bound to CC-F 287.8carbonylC=O 288.0Polymethylmethacralate(PMMA)A deconvolution procedure may be needed when the components are not resolved.To obtain unique decomposition of spectrum,additional information may be required,including estimates from ab initio simulations.?Binding energy increases with increasing oxidation state of the cations.Si 2p Core Levelhttp:/www.emsl.pnl.gov/new/emsl2002/tutorials/engelhard_xps.pdfChemical Shifts of Si oxideRelative Sensitivities of the Elements024681012Elemental SymbolRelative SensitivityLiBeBCNOFNeNaMAlSiPSClArKCaScTiVCrMFeCoNiCuZnGGAsSeBrKrRbSrYZrNbMTcRuRhPdAgCdInSnSbTeIXeCsBaLaCePrNdPSEuGTbDyHoErTYbLuHfTaWReOsIrPtAuHgTlPbBi1s2p3d4d4fPeak FittingBefore starting any peak-fit effort,the analyst performing the peak-fit needs to know if the topmost 15 nm of the sample is expected to be a homogeneous material or is expected to be a mixture of materials.If the top 15 nm is a homogeneous material with only very minor amounts of adventitious carbon and adsorbed gases,then the analyst can usetheoretical peak area ratios to enhance the peak-fitting process.Variables that affect or define peak-fit results include:*FWHMs*Chemical Shifts*Peak shapes*Instrument design factors*Experimental settings*Sample factorsFWHMsWhen using high energy resolution experiment settings on an XPS equipped with a monochromatic Al K-alpha X-ray source,the FWHM of the major XPS peaks range from 0.3 eV to 1.7 eV.The following is a simple summary of FWHM from major XPS signals:*Main metal peaks(e.g.1s,2p3,3d5,4f7)from pure metals have FWHMs that range from 0.30 eVto 1.0 eV*Main metal peaks(e.g.1s,2p3,3d5,4f7)from binary metal oxides have FWHMs that range from 0.9 eV to 1.7 eV*The O(1s)peak from binary metal oxides have FWHMs that,in general,range from 1.0 eV to 1.4 eV*The C(1s)peak from adventitious hydrocarbons have FWHMs that,in general,range from 1.0 eVto 1.4 eVChemical ShiftsChemical shift values depend on the degree of electron bond polarization between nearest neighbor atoms.A specific chemical shift is the difference in BE values of one specific chemical state versus the BE of the pure element.Peaks derived from peak-fitting a raw chemical state spectrum are due to the presence of different chemical states.Peak shapesDepends on instrument parameters,experimental parameters and sample characteristicsInstrument design factorsFWHM and purity of X-rays used(monochromatic Al,non-monochromatic Mg,Synchrotron,Ag,Zr.)Design of electron analyzer(CMA,HSA,retarding field.)Experiment settingsSettings of the electron analyzer(e.g.pass energy,step size)Sample factorsPhysical form of the sample(single crystal,polished,powder,corroded.)Number of physical defects within the analysis volume(from Argon ion etching,from laser cleaning.)Applications of X-ray Photoelectron Spectroscopy(XPS)XPS of Copper-Nickel alloy-1100-900-700-500-300-100020406080100120ThousandsBinding Energy(eV)N(E)/EPeakAreaMct-eV/secRel.Sens.AtomicConc%Ni2.654.04449Cu3.655.32151Cu 2pCu 3pNi 2pNi 3pNi LMM Ni LMMNi LMMCuLMMCuLMMCuLMMAnalysis of Carbon Fiber-Polymer Composite Material by XPSWoven carbon Woven carbon fiber compositefiber compositeXPS analysis identifies the functional XPS analysis identifies the functional groups present on composite surface.groups present on composite surface.Chemical nature of fiberChemical nature of fiber-polymer polymer interface will influence its properties.interface will influence its properties.-C C-C C-C C-OO-C=OC=O-300-295-290-285-280Binding energy(eV)N(E)/EAnalysis of Materials for Solar Energy Collection by XPS Depth Profiling-The amorphous-SiC/SnO2InterfaceThe profile indicates a reduction of the SnOThe profile indicates a reduction of the SnO2 2occurred at the interface during deposition.occurred at the interface during deposition.Such a reduction would effect the collectorSuch a reduction would effect the collector s s efficiency.efficiency.PhotoPhoto-voltaic Collectorvoltaic CollectorConductive OxideConductive Oxide-SnOSnO2 2p p-type atype a-SiCSiCa a-SiSiSolar EnergySolar EnergySnOSnO2 2SnSnDepth500496492488484480Binding Energy,eVData courtesy A.Nurrudin and J.Abelson,University of IllinoisAngle-resolved XPS =15 =90More Surface More Surface SensitiveSensitiveLess Surface Less Surface SensitiveSensitiveInformation depth=Information depth=dsindsin d=Escape depth 3 d=Escape depth 3 =Emission angle relative to surface=Emission angle relative to surface =Inelastic Mean Free PathInelastic Mean Free PathAngle-resolved XPS Analysis of Self-Assembling MonolayersAngle Resolved XPS Can Angle Resolved XPS Can DetermineDetermine?OverOver-layer Thicknesslayer Thickness?OverOver-layer Coveragelayer CoverageData courtesy L.Data courtesy L.GeGe,R.Haasch and A.,R.Haasch and A.GewirthGewirth,University of Illinois,University of Illinois0204060801000.10.20.30.40.50.6C(W)C(Au)AuSiWO1240dElectron Emission Angle,degreesExpt.DataModelXPS studies of polyimidePyromellitic dianhydride-oxydianilinePMDA-ODAC KLL AugerApplications to biomaterials and biointerfaces Biological interfaces have a limited number of elements(C,H,O,N,S,P,Si)Extracting useful surface information is challenging.XPS can be used to(a)detect the presence of adsorbed proteins.(b)estimate the amount of protein present(c)resolution of one protein from another is difficult since many proteins share chemical features.When spectra are taken as a function of take-off angle,Useful information can be obtained,for example,for the uniformity of an overlayer;fraction covered;protein film thickness;and orientation of protein in the film.AUGER ELECTRON SPECTROSCOPY PRINCIPLES AND APPLICATIONSAuger Electron Spectroscopy Auger Electron Spectroscopy(AES),is a widel