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Materials characterization (SEM, TEM, XRD, XPS)

Which correctly describes the information depth of XPS (X-ray photoelectron spectroscopy) and the information it provides?

  1. AX-rays penetrate several micrometers into the specimen, so the average composition to a depth of several micrometers is obtained
  2. BThe information depth is a few nanometers, and chemical shifts of the peaks reveal the bonding state of elements (oxidation state and so on)
  3. CThe information depth is a few nanometers, but only the kinds of elements are revealed, not their bonding state
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Answer: B. The information depth is a few nanometers, and chemical shifts of the peaks reveal the bonding state of elements (oxidation state and so on)

In XPS the incident X-rays themselves penetrate several micrometers into the specimen, but the photoelectrons produced readily lose energy by inelastic scattering, and only those generated within a few nanometers of the surface escape with their original energy. XPS is therefore a technique that probes the topmost few nanometers. The binding energy of photoelectrons shifts slightly with the chemical state of the atom (chemical shift); for Si 2p, for example, SiO₂ is about 4 eV higher than elemental Si, so the presence of oxidation and the oxidation state can be distinguished. Depth distributions are investigated by measuring while sputtering with Ar ions or by varying the photoelectron take-off angle. Determining the average composition to a depth of several micrometers is the job of techniques such as EDS in the SEM.

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