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. 3 1. 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.
7. X-ray Photoelectron Spectroscopy Small Area Detection X-ray Beam X-ray penetration depth ~1 m. Electrons can be excited in this entire volume. X-ray excitation area ~1x1 cm 2 . Electrons are emitted from this entire area Electrons are extracted only from a narrow solid angle. 1 mm 2 10 nm
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13. Fermi Level Referencing Free electrons (those giving rise to conductivity) find an equal potential which is constant throughout the material. Fermi-Dirac Statistics: f(E) = 1 exp[(E-E f )/kT] + 1 1.0 f(E) 0 0.5 E f 1. At T=0 K: f(E)=1 for E<E f f(E)=0 for E>E f 2. At kT<<E f (at room temperature kT=0.025 eV) f(E)=0.5 for E=E f T=0 K kT<<E f
15. Sample/Spectrometer Energy Level Diagram- Conducting Sample hv Because the Fermi levels of the sample and spectrometer are aligned, we only need to know the spectrometer work function, spec , to calculate BE(1s). E 1s Sample Spectrometer e - Free Electron Energy Fermi Level, E f Vacuum Level, E v sample KE(1s) KE(1s) spec BE(1s)
16. Sample/Spectrometer Energy Level Diagram- Insulating Sample hv A relative build-up of electrons at the spectrometer raises the Fermi level of the spectrometer relative to the sample. A potential E ch will develop. E 1s Sample Spectrometer e - Free Electron Energy BE(1s) Fermi Level, E f Vacuum Level, E v KE(1s) spec E ch
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18. Where do Binding Energy Shifts Come From? -or How Can We Identify Elements and Compounds? Electron-electron repulsion Electron-nucleus attraction Electron Nucleus Binding Energy Pure Element Electron-Nucleus Separation Fermi Level Look for changes here by observing electron binding energies
21. Binding Energy Determination The photoelectron’s binding energy will be based on the element’s final-state configuration. Conduction Band Valence Band Fermi Level Free Electon Level Conduction Band Valence Band 1s 2s 2p Initial State Final State
22. Chemical Shifts- Electronegativity Effects Carbon-Oxygen Bond Valence Level C 2p Core Level C 1s Carbon Nucleus Oxygen Atom C 1s Binding Energy Electron-oxygen atom attraction (Oxygen Electro-negativity) Electron-nucleus attraction (Loss of Electronic Screening) Shift to higher binding energy
29. Electron Scattering Effects Energy Loss Peaks Photoelectrons 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). e ph + e solid e* ph + e** solid
32. Quantitative Analysis by XPS For a Homogeneous sample: I = N DJL AT where: N = atoms/cm 3 = photoelectric cross-section, cm 2 D = detector efficiency J = X-ray flux, photon/cm 2 -sec L = orbital symmetry factor = inelastic electron mean-free path, cm A = analysis area, cm 2 T = analyzer transmission efficiency
33. Quantitative Analysis by XPS N = I/ DJL AT Let denominator = elemental sensitivity factor, S N = I / S Can describe Relative Concentration of observed elements as a number fraction by: C x = N x / N i C x = I x /S x / I i /S i The values of S are based on empirical data.
39. 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.
42. X-ray Photoelectron Spectrometer X-ray Source Electron Optics Hemispherical Energy Analyzer Position Sensitive Detector (PSD) Magnetic Shield Outer Sphere Inner Sphere Sample Computer System Analyzer Control Multi-Channel Plate Electron Multiplier Resistive Anode Encoder Lenses for Energy Adjustment (Retardation) Lenses for Analysis Area Definition Position Computer Position Address Converter
43. X-ray Generation Conduction Band Valence Band 1s 2s 2p Conduction Band Valence Band L2,L3 L1 K Fermi Level Free Electron Level 1s 2s 2p Secondary electron Incident electron X-ray Photon
44. Relative Probabilities of Relaxation of a K Shell Core Hole Note: The light elements have a low cross section for X-ray emission. Auger Electron Emission X-ray Photon Emission
45. Schematic of Dual Anode X-ray Source Anode Fence Anode 1 Anode 2 Filament 1 Filament 2 Fence Cooling Water Cooling Water Water Outlet Water Inlet Anode Assembly Filament 1 Anode 1 Fence Filament 2 Anode 2
46. Schematic of X-ray Monochromator Sample X-ray Anode Energy Analyzer Quartz Crystal Disperser Rowland Circle e -
48. XPS Analysis of Pigment from Mummy Artwork 150 145 140 135 130 Binding Energy (eV) PbO 2 Pb 3 O 4 500 400 300 200 100 0 Binding Energy (eV) O Pb Pb Pb N Ca C Na Cl XPS analysis showed that the pigment used on the mummy wrapping was Pb 3 O 4 rather than Fe 2 O 3 Egyptian Mummy 2nd Century AD World Heritage Museum University of Illinois
49. Analysis of Carbon Fiber- Polymer Composite Material by XPS Woven carbon fiber composite XPS analysis identifies the functional groups present on composite surface. Chemical nature of fiber-polymer interface will influence its properties. -C-C- -C-O -C=O
50. Analysis of Materials for Solar Energy Collection by XPS Depth Profiling- The amorphous-SiC/SnO 2 Interface The profile indicates a reduction of the SnO 2 occurred at the interface during deposition. Such a reduction would effect the collector’s efficiency. Photo-voltaic Collector Conductive Oxide- SnO 2 p-type a-SiC a-Si Solar Energy SnO 2 Sn Depth 500 496 492 488 484 480 Binding Energy, eV Data courtesy A. Nurrudin and J. Abelson, University of Illinois
51. Angle-resolved XPS =15° = 90° More Surface Sensitive Less Surface Sensitive Information depth = dsin d = Escape depth ~ 3 = Emission angle relative to surface = Inelastic Mean Free Path
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