Introduction to X-Ray Photoelectron Spectroscopy

Aug 31, 2024

Lecture 31: X-Ray Photoelectron Spectroscopy (XPS)

Introduction

  • Transition from Ultraviolet Photoelectron Spectroscopy (UPS) to X-ray Photoelectron Spectroscopy (XPS).
  • Focus on applications of XPS in surface science.

Basic Principles of XPS

  • XPS uses high-energy X-ray photons to eject core electrons from materials.
  • Energy range for XPS is typically in the order of hundreds of electron volts (eV), compared to a few electron volts for UPS.
  • Core level binding energies are sensitive to the chemical environment, allowing for chemical analysis.

Example: Polyurethane Film

  • Polyurethane Composition: Carbon, Nitrogen, Oxygen.
  • Survey Scan:
    • Energy scanned from 0 to 100 eV.
    • Peaks observed for Carbon (most intense), Nitrogen (intermediate), and Oxygen (lowest intensity).

Detailed Analysis of Elements

  • Zoom in on individual elements:
    • Carbon 1s Level: Multiple peaks corresponding to different types of carbon (alkyl, aromatic).
    • Oxygen 1s Level: Observed types of oxygen (two types).
    • Nitrogen 1s Level: One peak corresponding to aminic nitrogen.
    • Peak intensity indicates relative amounts of the different elements present.

Binding Energy and Chemical Environment

  • Higher binding energy indicates a more oxidized state due to electronegative elements (e.g., oxygen).
  • Table Reference:
    • Hydrocarbon carbon ~ 285 eV.
    • Binding energy shifts when carbon is connected to more electronegative elements.

Spin-Orbit Coupling in XPS

  • Spin-orbit coupling affects higher elements:
    • Example: 2p states split into 2p1/2 and 2p3/2.
    • Indicates varying intensity due to degeneracy.

Surface Sensitivity of XPS

  • XPS is a surface-sensitive technique measuring a few nanometers from the surface.
  • Angle Resolved XPS: Technique to analyze thin films stacked together, improving surface sensitivity.
    • By adjusting the takeoff angle, only surface materials are detected, isolating information from layers below.

Example of Angle Resolved XPS

  • Case study: Silicon dioxide on silicon:
    • As the takeoff angle decreases, the silicon peak diminishes while the silicon dioxide peak becomes prominent.

Conclusion

  • XPS is a powerful tool in surface science for elemental and oxidation state analysis.
  • Future classes will cover more examples and applications of XPS.

Note: Keep in mind the sensitivity and resolution of the XPS technique when interpreting spectra.