Presentation on theme: "X-ray Photoelectron Spectroscopy (XPS)"— Presentation transcript:
1 X-ray Photoelectron Spectroscopy (XPS) Center for Microanalysis of MaterialsFrederick Seitz Materials Research LaboratoryUniversity of Illinois at Urbana-Champaign
2 Surface Analysis The Study of the Outer-Most Layers of Materials (<100 ). Electron SpectroscopiesXPS: X-ray Photoelectron SpectroscopyAES: Auger Electron SpectroscopyEELS: Electron Energy Loss SpectroscopyIon SpectroscopiesSIMS: Secondary Ion Mass SpectrometrySNMS: Sputtered Neutral Mass SpectrometryISS: Ion Scattering Spectroscopy
3 Introduction to X-ray Photoelectron Spectroscopy (XPS)
4 Introduction to X-ray Photoelectron Spectroscopy (XPS) What is XPS?- General TheoryHow can we identify elements and compounds?Instrumentation for XPSExamples of materials analysis with XPS
5 What is XPS?X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA) is a widely used technique to investigate the chemical composition of surfaces.
6 What is XPS?X-ray Photoelectron spectroscopy, based on the photoelectric effect,1,2 was developed in the mid-1960’s by Kai Siegbahn and his research group at the University of Uppsala, Sweden.31. H. Hertz, Ann. Physik 31,983 (1887).2. A. Einstein, Ann. Physik 17,132 (1905) Nobel Prize in Physics.3. K. Siegbahn, Et. Al.,Nova Acta Regiae Soc.Sci., Ser. IV, Vol. 20 (1967) Nobel Prize in Physics.
7 X-ray Photoelectron Spectroscopy Small Area Detection Electrons are extracted only from a narrow solid angle.X-ray BeamX-ray penetration depth ~1mm.Electrons can be excited in this entire volume.10 nm1 mm2X-ray excitation area ~1x1 cm2. Electrons are emitted from this entire area
8 The Photoelectric Process Ejected PhotoelectronIncident X-rayFreeElectronLevelXPS spectral lines are identified by the shell from which the electron was ejected (1s, 2s, 2p, etc.).The ejected photoelectron has kinetic energy:KE=hv-BE-Following this process, the atom will release energy by the emission of an Auger Electron.Conduction BandFermiLevelValence Band2pL2,L32sL11sK
9 Auger Relation of Core Hole Emitted Auger ElectronFreeElectronLevelL electron falls to fill core level vacancy (step 1).KLL Auger electron emitted to conserve energy released in step 1.The kinetic energy of the emitted Auger electron is:KE=E(K)-E(L2)-E(L3).Conduction BandFermiLevelValence Band2pL2,L32sL11sK
10 XPS Energy ScaleThe XPS instrument measures the kinetic energy of all collected electrons. The electron signal includes contributions from both photoelectron and Auger electron lines.
11 XPS Energy Scale- Kinetic energy KE = hv - BE - specWhere: BE= Electron Binding EnergyKE= Electron Kinetic Energyspec= Spectrometer Work FunctionPhotoelectron line energies: Dependent on photon energy.Auger electron line energies: Not Dependent on photon energy.If XPS spectra were presented on a kinetic energy scale, one would need to know the X-ray source energy used to collect the data in order to compare the chemical states in the sample with data collected using another source.
12 XPS Energy Scale- Binding energy BE = hv - KE - specWhere: BE= Electron Binding EnergyKE= Electron Kinetic Energyspec= Spectrometer Work FunctionPhotoelectron line energies: Not Dependent on photon energy.Auger electron line energies: Dependent on photon energy.The binding energy scale was derived to make uniform comparisons of chemical states straight forward.
13 Fermi Level Referencing Free electrons (those giving rise to conductivity) find an equal potential which is constant throughout the material.Fermi-Dirac Statistics:T=0 KkT<<Eff(E)f(E) =exp[(E-Ef)/kT] + 11.00.5Ef1. At T=0 K: f(E)=1 for E<Eff(E)=0 for E>Ef2. At kT<<Ef (at room temperature kT=0.025 eV)f(E)=0.5 for E=Ef
15 Sample/Spectrometer Energy Level Diagram- Conducting Sample Free Electron EnergyKE(1s)KE(1s)Vacuum Level, EvsamplespechvFermi Level, EfBE(1s)E1sBecause the Fermi levels of the sample and spectrometer are aligned, we only need to know the spectrometer work function, spec, to calculate BE(1s).
16 Sample/Spectrometer Energy Level Diagram- Insulating Sample Free Electron EnergyKE(1s)specVacuum Level, EvhvEchFermi Level, EfBE(1s)E1sA relative build-up of electrons at the spectrometer raises the Fermi level of the spectrometer relative to the sample. A potential Ech will develop.
17 Binding Energy Referencing BE = hv - KE - spec- EchWhere: BE= Electron Binding EnergyKE= Electron Kinetic Energyspec= Spectrometer Work FunctionEch= Surface Charge EnergyEch can be determined by electrically calibrating the instrument to a spectral feature.C1s at eVAu4f7/2 at 84.0 eV
18 Where do Binding Energy Shifts Come From Where do Binding Energy Shifts Come From? -or How Can We Identify Elements and Compounds?Pure ElementFermi LevelBindingEnergyElectron-electronrepulsionLook for changes here by observing electron binding energiesElectronElectron-nucleusattractionElectron-Nucleus SeparationNucleus
21 Binding Energy Determination The photoelectron’s binding energy will be based on the element’s final-state configuration.Initial StateFinal StateFreeElectonLevelConduction BandConduction BandFermiLevelValence BandValence Band2p2s1s
22 The Sudden Approximation Assumes the remaining orbitals (often called the passive orbitals) are the same in the final state as they were in the initial state (also called the frozen-orbital approximation). Under this assumption, the XPS experiment measures the negative Hartree-Fock orbital energy:Koopman’s Binding EnergyEB,K -B,KActual binding energy will represent the readjustment of the N-1 charges to minimize energy (relaxation):EB = Ef N-1 - Ei N
23 Binding Energy Shifts (Chemical Shifts) Point Charge Model:Ei = Ei kqi qi/rijEB in atom i in given refernce stateWeighted charge of iPotential at i due to surrounding charges
24 Chemical Shifts- Electronegativity Effects Carbon-Oxygen BondOxygen AtomElectron-oxygen atom attraction(Oxygen Electro-negativity)Valence LevelC 2pC 1sBindingEnergyCore LevelC 1sShift to higher binding energyElectron-nucleusattraction (Loss ofElectronic Screening)Carbon Nucleus
30 Electronic Effects- Spin-Orbit Coupling Ti MetalTi Oxide
31 Final State Effects- Shake-up/ Shake-off Results from energy made available in the relaxation of the final state configuration (due to a loss of the screening effect of the core level electron which underwent photoemission).L(2p) -> Cu(3d)Monopole transition: Only the principle quantum number changes. Spin and angular momentum cannot change.Shake-up: Relaxation energy used to excite electrons in valence levels to bound states (monopole excitation).Shake-off: Relaxation energy used to excite electrons in valence levels to unbound states (monopole ionization).
32 Final State Effects- Shake-up/ Shake-off Ni MetalNi Oxide
33 Final State Effects- Multiplet Splitting Following photoelectron emission, the remaining unpaired electron may couple with other unpaired electrons in the atom, resulting in an ion with several possible final state configurations with as many different energies. This produces a line which is split asymmetrically into several components.
34 Electron Scattering Effects Energy Loss Peaks eph + esolide*ph + e**solidPhotoelectrons travelling through the solid can interact with other electrons in the material. These interactions can result in the photoelectron exciting an electronic transition, thus losing some of its energy (inelastic scattering).
37 Quantitative Analysis by XPS For a Homogeneous sample:I = NsDJLlATwhere: N = atoms/cm3s = photoelectric cross-section, cm2D = detector efficiencyJ = X-ray flux, photon/cm2-secL = orbital symmetry factorl = inelastic electron mean-free path, cmA = analysis area, cm2T = analyzer transmission efficiency
38 Quantitative Analysis by XPS N = I/sDJLlATLet denominator = elemental sensitivity factor, SN = I / SCan describe Relative Concentration of observed elements as a number fraction by:Cx = Nx / SNiCx = Ix/Sx / S Ii/SiThe values of S are based on empirical data.
39 Relative Sensitivities of the Elements 3d4f2p4d1s
42 Instrumentation for X-ray Photoelectron Spectroscopy
43 Introduction to X-ray Photoelectron Spectroscopy (XPS) What is XPS?- General TheoryHow can we identify elements and compounds?Instrumentation for XPSExamples of materials analysis with XPS
44 Instrumentation for XPS Surface analysis by XPS requires irradiating a solid in an Ultra-high Vacuum (UHV) chamber with monoenergetic soft X-rays and analyzing the energies of the emitted electrons.
45 Why UHV for Surface Analysis? PressureTorrDegree of VacuumRemove adsorbed gases from the sample.Eliminate adsorption of contaminants on the sample.Prevent arcing and high voltage breakdown.Increase the mean free path for electrons, ions and photons.102Low Vacuum10-1Medium Vacuum10-4High Vacuum10-8Ultra-High Vacuum10-11
47 X-ray Photoelectron Spectrometer Computer SystemHemispherical Energy AnalyzerOuter SphereMagnetic ShieldAnalyzer ControlInner SphereElectronOpticsMulti-Channel Plate Electron MultiplierLenses for Energy Adjustment (Retardation)Resistive Anode EncoderX-raySourcePosition ComputerLenses for Analysis Area DefinitionPosition Address ConverterPosition Sensitive Detector (PSD)Sample
48 XPS at the ‘Magic Angle’ Orbital Angular Symmetry FactorLA (g) = 1 + bA (3sin2g/2 - 1)/2where: g = source-detector angleb = constant for a given sub-shell and X-ray photonAt 54.7º the ‘magic angle’LA = 1
49 Electron Detection Single Channel Detector Step 123Electron distribution on analyzer detection planeStep 123E1E2E3E1E2E3E1E2E3Counts in spectral memory
50 Electron Detection Multi-channel Position Sensitive Detector (PSD) Step 12345Electron distribution on analyzer detection planeStep 12345E1E2E3E1E2E3E1E2E3E1E2E3E1E2E3Counts in spectral memory
51 X-ray Generation X-ray Photon Secondary electron Incident electron FreeElectronLevelConduction BandConduction BandFermiLevelValence BandValence Band2p2pL2,L32s2sL11s1sK
52 Relative Probabilities of Relaxation of a K Shell Core Hole Auger Electron EmissionNote: The light elements have a low cross section for X-ray emission.X-ray Photon Emission
53 Schematic of Dual Anode X-ray Source Water OutletAnode AssemblyWater InletFenceFenceAnode 1Anode 2AnodeAnode 1Anode 2Filament 1Filament 2FenceCooling WaterFilament 1Filament 2Cooling Water
54 Schematic of X-ray Monochromator EnergyAnalyzerQuartzCrystal Dispersere-SampleRowland CircleX-ray Anode
55 Applications of X-ray Photoelectron Spectroscopy (XPS)
56 XPS Analysis of Pigment from Mummy Artwork Pb3O4Egyptian Mummy2nd Century ADWorld Heritage MuseumUniversity of IllinoisPbO2CO150145140135130Binding Energy (eV)PbPbNCaXPS analysis showed that the pigment used on the mummy wrapping was Pb3O4 rather than Fe2O3NaClPb500400300200100Binding Energy (eV)
57 Analysis of Carbon Fiber- Polymer Composite Material by XPS XPS analysis identifies the functional groups present on composite surface. Chemical nature of fiber-polymer interface will influence its properties.-C-C-Woven carbon fiber composite-C-O-C=O
58 Analysis of Materials for Solar Energy Collection by XPS Depth Profiling- The amorphous-SiC/SnO2 InterfaceThe profile indicates a reduction of the SnO2 occurred at the interface during deposition. Such a reduction would effect the collector’s efficiency.Photo-voltaic CollectorSnO2SnSolar EnergyConductive Oxide- SnO2Depthp-type a-SiC500496492488484480a-SiBinding Energy, eVData courtesy A. Nurrudin and J. Abelson, University of Illinois
59 Angle-resolved XPS q =15° q = 90° q q More Surface Sensitive Less Surface SensitiveqInformation depth = dsinqd = Escape depth ~ 3 lq = Emission angle relative to surfacel = Inelastic Mean Free Path
60 Angle-resolved XPS Analysis of Self-Assembling Monolayers Angle Resolved XPS Can DetermineOver-layer ThicknessOver-layer CoverageData courtesy L. Ge, R. Haasch and A. Gewirth, University of Illinois