XPS

X-ray PhotoelectronSpectroscopy (XPS) „ No cell phone use during the lecture Surface Analysis The Study of the Outer-Most Layers of Materials (<100 Α). „ Electron Spectroscopies XPS: X-ray Photoelectron Spectroscopy AES: Auger Electron Spectroscopy EELS: Electron Energy Loss Spectroscopy „ Ion Spectroscopies SIMS: Secondary Ion Mass Spectrometry SNMS: Sputtered Neutral Mass Spectrometry ISS: Ion Scattering Spectroscopy Introduction to X-ray Photoelectron Spectroscopy (XPS) . General Theory How can we identify elements and compounds? Instrumentation for XPS Examples of materials analysis with XPS .Introduction to X-ray Photoelectron Spectroscopy (XPS) „ „ „ „ What is XPS?. 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. . Ann. Hertz. Al. Physik 17.What is XPS? X-ray Photoelectron spectroscopy..Nova Acta Regiae Soc. Et. Physik 31. 1921 Nobel Prize in Physics. 2. based on the photoelectric effect. Vol. H.1.2 was developed in the mid-1960’s by Kai Siegbahn and his research group at the University of Uppsala. K. Sweden.Sci. A.132 (1905). IV. Ann.983 (1887).3 1. 1981 Nobel Prize in Physics. 3. Einstein. Siegbahn.. 20 (1967). . Ser. X-ray penetration depth ~1μm.X-ray Photoelectron Spectroscopy Small Area Detection X-ray Beam Electrons are extracted only from a narrow solid angle. Electrons can be excited in this entire volume. 10 nm 1 mm2 X-ray excitation area ~1x1 cm2. Electrons are emitted from this entire area . The Photoelectric Process Incident X-ray Conduction Band Ejected Photoelectron Free Electron Level Fermi Level „ „ Valence Band 2p L2. . 2s. 2p.). the atom will release energy by the emission of an Auger Electron. etc. The ejected photoelectron has kinetic energy: KE=hv-BE-Φ Following this process.L3 2s L1 1s K „ XPS spectral lines are identified by the shell from which the electron was ejected (1s. . The kinetic energy of the emitted Auger electron is: KE=E(K)-E(L2)-E(L3). KLL Auger electron emitted to conserve energy released in step 1.Auger Relation of Core Hole Emitted Auger Electron Conduction Band Free Electron Level Fermi Level Valence Band „ „ „ 2p L2.L3 2s L1 1s K L electron falls to fill core level vacancy (step 1). The electron signal includes contributions from both photoelectron and Auger electron lines. .XPS Energy Scale The XPS instrument measures the kinetic energy of all collected electrons. Kinetic energy KE = hv . . If XPS spectra were presented on a kinetic energy scale. Auger electron line energies: Not Dependent on photon energy.XPS Energy Scale.Φspec Where: BE= Electron Binding Energy KE= Electron Kinetic Energy Φspec= Spectrometer Work Function Photoelectron line energies: Dependent on photon energy.BE . 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. . Auger electron line energies: Dependent on photon energy. The binding energy scale was derived to make uniform comparisons of chemical states straight forward.XPS Energy Scale.Φspec Where: BE= Electron Binding Energy KE= Electron Kinetic Energy Φspec= Spectrometer Work Function Photoelectron line energies: Not Dependent on photon energy.KE .Binding energy BE = hv . At kT<<Ef (at room temperature kT=0.5 0 1.5 for E=Ef Ef .0 0. At T=0 K: f(E)=1 for E<Ef f(E)=0 for E>Ef 2. Fermi-Dirac Statistics: T=0 K kT<<Ef f(E) f(E) = 1 exp[(E-Ef)/kT] + 1 1.025 eV) f(E)=0.Fermi Level Referencing Free electrons (those giving rise to conductivity) find an equal potential which is constant throughout the material. Fermi Level Referencing . Conducting Sample e- Sample Spectrometer Free Electron Energy Vacuum Level. . Ev hv KE(1s) KE(1s) Φsample Φspec Fermi Level. Ef BE(1s) E1s Because the Fermi levels of the sample and spectrometer are aligned. Φspec.Sample/Spectrometer Energy Level Diagram. we only need to know the spectrometer work function. to calculate BE(1s). A potential Ech will develop. Ef E1s Ech hv BE(1s) A relative build-up of electrons at the spectrometer raises the Fermi level of the spectrometer relative to the sample. .Insulating Sample eSample Spectrometer Free Electron Energy KE(1s) Φspec Vacuum Level.Sample/Spectrometer Energy Level Diagram. Ev Fermi Level. Φspec. C1s at 285.KE .Ech Where: BE= Electron Binding Energy KE= Electron Kinetic Energy Φspec= Spectrometer Work Function Ech= Surface Charge Energy Ech can be determined by electrically calibrating the instrument to a spectral feature.Binding Energy Referencing BE = hv .0 eV .0 eV Au4f7/2 at 84. Where do Binding Energy Shifts Come From? -or How Can We Identify Elements and Compounds? Fermi Level Pure Element Electron-electron repulsion Look for changes here by observing electron binding energies Electron Electron-nucleus attraction Nucleus Binding Energy ElectronNucleus Separation . Elemental Shifts Binding Energy (eV) Element 2p3/2 3p Δ Fe 707 53 654 Co 778 60 718 Ni 853 67 786 Cu 933 75 858 Zn 1022 89 933 Electron-nucleus attraction helps us identify the elements . Elemental Shifts . Binding Energy Determination The photoelectron’s binding energy will be based on the element’s final-state configuration. Initial State Conduction Band Free Electon Level Fermi Level Valence Band Final State Conduction Band Valence Band 2p 2s 1s . Under this assumption. the XPS experiment measures the negative Hartree-Fock orbital energy: Koopman’s Binding Energy EB.K ≅ -εB.Ei N .K Actual binding energy will represent the readjustment of the N-1 charges to minimize energy (relaxation): EB = Ef N-1 .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). Binding Energy Shifts (Chemical Shifts) Point Charge Model: Ei = Ei0 EB in atom i in given refernce state + kqi + Weighted charge of i Σ qi/rij Potential at i due to surrounding charges . Chemical ShiftsElectronegativity Effects Carbon-Oxygen Bond Oxygen Atom Valence Level C 2p Electron-oxygen atom attraction (Oxygen Electronegativity) Core Level C 1s C 1s Binding Energy Shift to higher binding energy Electron-nucleus attraction (Loss of Electronic Screening) Carbon Nucleus . 0 alcohol. C-C Binding Energy (eV) 1s 285.0 amine C-N 286.5 F bound to C C-F 287.8 carbonyl C=O 288.Chemical ShiftsElectronegativity Effects Functional Group hydrocarbon C-H. C-O-C 286.5 Cl bound to C C-Cl 286.0 . ether C-O-H. Electronic Effects Spin-Orbit Coupling C 1s Orbital=s l=0 s=+/-1/2 ls=1/2 290 288 284 280 Binding Energy (eV) 276 . 3/2 2p1/2 Peak Area 965 1 19.8 : 2 955 945 935 Binding Energy (eV) 925 .Electronic Effects Spin-Orbit Coupling 2p3/2 Cu 2p Orbital=p l=1 s=+/-1/2 ls=1/2. 5/2 6.0 Peak Area 2 378 : 3 374 370 366 Binding Energy (eV) 362 .Electronic Effects Spin-Orbit Coupling 3d5/2 Ag 3d 3d3/2 Orbital=d l=2 s=+/-1/2 ls=3/2. 65 3 : 4 87 83 Binding Energy (eV) 79 .7/2 3.Electronic Effects Spin-OrbitCoupling Au 4f 4f7/2 4f5/2 Peak Area 91 Orbital=f l=3 s=+/-1/2 ls=5/2. Electronic Effects.Spin-Orbit Coupling Ti Metal Ti Oxide . Shake-off: Relaxation energy used to excite electrons in valence levels to unbound states (monopole ionization).Final State EffectsShake-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. Shake-up: Relaxation energy used to excite electrons in valence levels to bound states (monopole excitation). Spin and angular momentum cannot change. Final State EffectsShake-up/ Shake-off Ni Metal Ni Oxide . resulting in an ion with several possible final state configurations with as many different energies. the remaining unpaired electron may couple with other unpaired electrons in the atom.Multiplet Splitting Following photoelectron emission.Final State Effects. This produces a line which is split asymmetrically into several components. . Electron Scattering Effects Energy Loss Peaks eph + esolid e*ph + e**solid Photoelectrons travelling through the solid can interact with other electrons in the material. . thus losing some of its energy (inelastic scattering). These interactions can result in the photoelectron exciting an electronic transition. 3 eV a a a .Electron Scattering Effects Plasmon Loss Peak Al 2s a Metal A=15. Electron Scattering Effects Plasmon Loss Peak O 1s 21 eV Insulating Material x4 . cm A = analysis area. cm2 D = detector efficiency J = X-ray flux. cm2 T = analyzer transmission efficiency .Quantitative Analysis by XPS For a Homogeneous sample: I = NσDJLλAT where: N = atoms/cm3 σ = photoelectric cross-section. photon/cm2-sec L = orbital symmetry factor λ = inelastic electron mean-free path. Quantitative Analysis by XPS N = I/σDJLλAT Let denominator = elemental sensitivity factor, S N=I/S Can describe Relative Concentration of observed elements as a number fraction by: Cx = Nx / ΣNi Cx = Ix/Sx / Σ Ii/Si The values of S are based on empirical data. Relative Sensitivities of the Elements 12 3d Relative Sensitivity 10 8 4f 6 2p 4 2 4d 1s 0 Li B N F Na Al P Cl K Sc V M Co Cu G As Br Rb Y Nb Tc Rh Ag In Sb I Cs La Pr P Eu Tb Ho T Lu Ta Re Ir Au Tl Bi Be C O Ne M Si S Ar Ca Ti Cr Fe Ni Zn G Se Kr Sr Zr M Ru Pd Cd Sn Te Xe Ba Ce Nd S G Dy Er Yb Hf W Os Pt Hg Pb Elemental Symbol XPS of Copper-Nickel alloy Comparison of Sensitivities H Ne Co Zn Zr Sn Nd Yb Hg Th AESa nd XPS 1% 5E19 RBS PIXE PIXE 1ppm 5E16 SIMS 1ppb 0 20 40 60 ATOMIC NUMBER 80 5E13 100 . Instrumentation for X-ray Photoelectron Spectroscopy . General Theory How can we identify elements and compounds? Instrumentation for XPS Examples of materials analysis with XPS .Introduction to X-ray Photoelectron Spectroscopy (XPS) „ „ „ „ What is XPS?. Instrumentation for XPS Surface analysis by XPS requires irradiating a solid in an Ultra-high Vacuum (UHV) chamber with monoenergetic soft Xrays and analyzing the energies of the emitted electrons. . Increase the mean free path for electrons. Prevent arcing and high voltage breakdown. Eliminate adsorption of contaminants on the sample.Why UHV for Surface Analysis? Degree of Vacuum Pressure Torr 102 Low Vacuum Medium Vacuum High Vacuum „ 10-1 „ 10-4 „ 10-8 „ Ultra-High Vacuum 10-11 Remove adsorbed gases from the sample. . ions and photons. X-ray Photoelectron Spectrometer . 7 .X-ray Photoelectron Spectrometer Computer System Hemispherical Energy Analyzer Outer Sphere Magnetic Shield Analyzer Control Inner Sphere Electron Optics Lenses for Energy Adjustment (Retardation) Multi-Channel Plate Electron Multiplier Lenses for Analysis Area Definition Position Address Converter Resistive Anode Encoder Position Computer X-ray Source Position Sensitive Detector (PSD) Sample 5 4 . 7º the ‘magic angle’ LA = 1 .1)/2 where: γ = source-detector angle β = constant for a given sub-shell and X-ray photon At 54.XPS at the ‘Magic Angle’ Orbital Angular Symmetry Factor LA (γ) = 1 + βA (3sin2γ/2 . Electron Detection Single Channel Detector Step 1 2 3 Electron distribution on analyzer detection plane Step 1 2 E1 E2 E3 E1 E2 Counts in spectral memory 3 E3 E1 E2 E3 . Electron Detection Multi-channel Position Sensitive Detector (PSD) Step 1 2 3 4 5 Electron distribution on analyzer detection plane Step 1 E1 2 E2 3 E3 E1 E2 E3 E1 E2 Counts in spectral memory 4 E3 5 E1 E2 E3 E1 E2 E3 . L3 2s 2s L1 1s 1s K .X-ray Generation X-ray Photon Incident electron Secondary electron Conduction Band Conduction Band Valence Band Valence Band Free Electron Level Fermi Level 2p 2p L2. 2 Note: The light elements have a low cross section for X-ray emission. X-ray Photon Emission 0 5 10 15 20 25 30 35 40 Atomic Number B Ne P Ca Mn Zn Br Zr Elemental Symbol .0 Probability 0.8 Auger Electron Emission 0.4 0.6 0.Relative Probabilities of Relaxation of a K Shell Core Hole 1. Schematic of Dual Anode X-ray Source Water Outlet Anode Assembly Water Inlet Fence Anode 1 Anode Filament 1 Fence Fence Anode 2 Filament 2 Anode 1 Anode 2 Cooling Water Cooling Water Filament 1 Filament 2 . Schematic of X-ray Monochromator Energy Analyzer Quartz Crystal Disperser e- Sample Rowland Circle X-ray Anode . Applications of X-ray Photoelectron Spectroscopy (XPS) . XPS Analysis of Pigment from Mummy Artwork Pb3O4 Egyptian Mummy 2nd Century AD World Heritage Museum University of Illinois PbO2 C O 150 145 140 135 130 Binding Energy (eV) Pb Pb N Ca Na Cl 500 400 300 200 Binding Energy (eV) Pb 100 0 XPS analysis showed that the pigment used on the mummy wrapping was Pb3O4 rather than Fe2O3 . Analysis of Carbon Fiber. Chemical nature of fiber-polymer interface will influence its properties.Polymer Composite Material by XPS XPS analysis identifies the functional groups present on composite surface. -C-C- Woven carbon fiber composite -C-O -C=O . Analysis of Materials for Solar Energy Collection by XPS Depth ProfilingThe amorphous-SiC/SnO2 Interface Photo-voltaic Collector The profile indicates a reduction of the SnO2 occurred at the interface during deposition. SnO2 Sn Solar Energy Conductive Oxide. Such a reduction would effect the collector’s efficiency. Nurrudin and J.SnO2 p-type a-SiC a-Si Depth 500 496 492 488 484 480 Binding Energy. Abelson. eV Data courtesy A. University of Illinois . Angle-resolved XPS θ =15° More Surface Sensitive θ θ = 90° Less Surface Sensitive θ Information depth = dsinθ d = Escape depth ~ 3 λ θ = Emission angle relative to surface λ = Inelastic Mean Free Path . Haasch and A. Gewirth. R. University of Illinois .Angle-resolved XPS Analysis of SelfAssembling Monolayers Angle Resolved XPS Can Determine „Over-layer Thickness „Over-layer Coverage Data courtesy L. Ge.