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College of Engineering and Computing

X-Ray Photoelectron Spectroscopy

The College of Engineering and Computing maintains the X-ray Photoelectron Spectroscopy (XPS) Center which can be used by researchers at the University or customers from outside industry. The center also boasts SIMS, or secondary ion mass spectrometry, capabilities.

What Is XPS?

X-ray photoelectron spectroscopy, which is also known as Electron Spectroscopy for Chemical Analysis (ESCA), provides quantitative compositional information from the top atomic layers of a sample surface for the elements lithium to uranium. The center can also obtain information regarding the chemical states of any elements present.

XPS is used to analyze the change in surface chemistry of a material after certain chemical or physical treatments such as: 

  • leaching;
  • reduction;
  • fracture;
  • cutting or scraping in air or UHV to expose the bulk chemistry;
  • ion beam etching to clean off some of the surface contamination;
  • exposure to heat to study the changes due to heating;
  • exposure to reactive gases or solutions;
  • exposure to ion beam implant;
  • and, exposure to UV light.

How Measurements are Obtained

A sample is irradiated with a beam of monochromatic soft X-rays. Photoelectron emission results from the atoms in the specimen. The kinetic energies of these electrons relates to the atom and orbital from which they originated. The distribution of kinetic energies from a sample is then measured directly by the electron spectrometer.

Atomic orbitals from atoms of the same element in different chemical environments are found to possess slightly different (but measurable) binding energies. These "chemical shifts" arise because of the variations in electrostatic screening experienced by core electrons as the valence and conduction electrons are drawn towards or away from the specific atom. Differences in oxidation state, molecular environment and co-ordination number all provide different chemical shifts.

Photoelectron binding energy shifts are, therefore, the principal source of chemical information. It should be noted that these shifts can be very small and can only be detected using a high performance instrument with suitable software such as our Kratos Axis Ultra DLD instrument equipped with a monochromated Al Ka x-ray source and hemispherical analyzer.

SIMS Capability

SIMS stands for secondary ion mass spectrometry. A high energy primary ion beam is focused to bombard the surface of materials. The impact ruptures surface structures , producing neutral species, electrons, and ions (secondary ions) ejected from the surface. Analyzing the secondary ion provides surface chemical composition information. SIMS is the most sensitive material characterization technique, with a detection limit from parts per million to parts per billion. The most important application for SIMS is to perform a depth profiling (also called dynamic SIMS) of materials. At USC, our SIMS capability is directly attached to XPS system.