1 / 60

Energy of Light

X-ray Photoelectron Spectroscopy ( XPS ) Text Book1: X-ray Photoelectron Spectroscopy: An introduction to Principles and Practices Paul van der Heide , Wiley 2011 Text book 2 : Photoelectron Spectroscopy Principles and Applications: Hüfner , Stephan , Springer 2011. Energy of Light.

lchandler
Download Presentation

Energy of Light

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. X-ray Photoelectron Spectroscopy (XPS)Text Book1: X-ray Photoelectron Spectroscopy: An introduction to Principles and PracticesPaul van der Heide, Wiley 2011Text book 2: Photoelectron Spectroscopy Principles and Applications: Hüfner, Stephan , Springer 2011

  2. Energy of Light Wavelength () 103m 1 m 10-6m 10-3m 106m Energy (E) 10-6eV 10-3eV 1 eV 1 KeV 1 MeV Short wave radio Infrared Broad-cast Gamma Ray X-ray UV Visible

  3. X-ray Photoelectron Spectroscopy (XPS)

  4. 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 Surface AnalysisThe Study of the Outer-Most Layers of Materials (<100 ).

  5. Introduction to X-ray Photoelectron Spectroscopy (XPS)

  6. Introduction to X-ray Photoelectron Spectroscopy (XPS) • What is XPS?- General Theory • How can we identify elements and compounds? • Instrumentation for XPS • Examples of materials analysis with XPS

  7. 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.

  8. 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.

  9. X-ray Photoelectron SpectroscopySmall Area Detection Electrons are extracted only from a narrow solid angle. X-ray Beam X-ray penetration depth ~1mm. 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

  10. The Photoelectric Process Ejected Photoelectron Incident X-ray • XPS 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. Free Electron Level Conduction Band Fermi Level Valence Band 2p L2,L3 2s L1 1s K

  11. Auger Relation of Core Hole Emitted Auger Electron Free Electron Level • L 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 Band Fermi Level Valence Band 2p L2,L3 2s L1 1s K

  12. XPS Energy Scale • The XPS instrument measures the kinetic energy of all collected electrons. The electron signal includes contributions from both photoelectron and Auger electron lines.

  13. XPS Energy Scale- Kinetic energy • KE= hv - BE - spec • Where: BE= Electron Binding Energy • KE= Electron Kinetic Energy • spec= Spectrometer Work Function • Photoelectron line energies: Dependenton photon energy. • Auger electron line energies: Not Dependenton 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.

  14. XPS Energy Scale- Binding energy • BE = hv - KE - spec • Where: BE= Electron Binding Energy • KE= Electron Kinetic Energy • spec= Spectrometer Work Function • Photoelectron line energies: Not Dependent on photon energy. • Auger electron line energies: Dependenton photon energy. • The binding energy scale was derived to make uniform comparisons of chemical states straight forward.

  15. 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 K kT<<Ef f(E) f(E) = 1 exp[(E-Ef)/kT] + 1 1.0 0.5 0 Ef 1. At T=0 K: f(E)=1 for E<Ef f(E)=0 for E>Ef 2. At kT<<Ef (at room temperature kT=0.025 eV) f(E)=0.5 for E=Ef

  16. Binding Energy Referencing • BE = hv - KE - spec- Ech • Where: BE= Electron Binding Energy • KE= Electron Kinetic Energy • spec= Spectrometer Work Function • Ech= Surface Charge Energy • Ech can be determined by electrically calibrating the instrument to a spectral feature. • C1s at 285.0 eV • Au4f7/2 at 84.0 eV

  17. Elemental Shifts

  18. Elemental Shifts

  19. Chemical Shifts- Electronegativity Effects

  20. Electronic EffectsSpin-Orbit Coupling

  21. Electronic EffectsSpin-Orbit Coupling

  22. Electronic EffectsSpin-Orbit Coupling

  23. Electronic EffectsSpin-OrbitCoupling

  24. Electronic Effects- Spin-Orbit Coupling Ti Metal Ti Oxide

  25. Ni Oxide Ni Metal

  26. Quantitative Analysis by XPS For a Homogeneous sample: I = NsDJLlAT where: N = atoms/cm3 s = photoelectric cross-section, cm2 D = detector efficiency J = X-ray flux, photon/cm2-sec L = orbital symmetry factor l = inelastic electron mean-free path, cm A = analysis area, cm2 T = analyzer transmission efficiency

  27. Relative Sensitivities of the Elements 3d 4f 2p 4d 1s

  28. XPS of Copper-Nickel alloy

  29. Instrumentation for X-ray Photoelectron Spectroscopy 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.

  30. Introduction to X-ray Photoelectron Spectroscopy (XPS) • What is XPS?- General Theory • How can we identify elements and compounds? • Instrumentation for XPS • Examples of materials analysis with XPS

  31. Early x-ray source X-ray tube Standard lab X-ray source is by very high energy e beam hitting the anode.

  32. A common Dual anode X-ray tube

  33. Why UHV for Surface Analysis? Pressure Torr Degree of Vacuum • Remove 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. 2 10 Low Vacuum -1 10 Medium Vacuum -4 10 High Vacuum -8 10 Ultra-High Vacuum -11 10

  34. X-ray Photoelectron Spectrometer

  35. X-ray Photoelectron Spectrometer Computer System Hemispherical Energy Analyzer Outer Sphere Magnetic Shield Analyzer Control Inner Sphere Electron Optics Multi-Channel Plate Electron Multiplier Lenses for Energy Adjustment (Retardation) Resistive Anode Encoder X-ray Source Position Computer Lenses for Analysis Area Definition Position Address Converter Position Sensitive Detector (PSD) Sample

  36. the radial electric field in the gap of the hemispheres is given by

  37. X-ray Generation X-ray Photon Secondary electron Incident electron Free Electron Level Conduction Band Conduction Band Fermi Level Valence Band Valence Band 2p 2p L2,L3 2s 2s L1 1s 1s K

  38. Relative Probabilities of Relaxation of a K Shell Core Hole Auger Electron Emission Note: The light elements have a low cross section for X-ray emission. X-ray Photon Emission

  39. Schematic of Dual Anode X-ray Source Water Outlet Anode Assembly Water Inlet Fence Anode 1 Anode 2 Anode Filament 1 Filament 2 Cooling Water Fence Cooling Water

  40. Schematic of X-ray Monochromator Energy Analyzer Quartz Crystal Disperser e- Sample Rowland Circle X-ray Anode

  41. Applications of X-ray Photoelectron Spectroscopy (XPS)

More Related