X-ray Photoelectron Spectroscopy

X-ray Photoelectron Spectroscopy (XPS) is the grandaddy of electron spectroscopies, and one of many techniques that involves X-ray light. This is a technique in which X-rays hit a sample and interact with core-level electrons. Some of this electrons manage to escape, so it’s X-rays in and electrons out. For those keeping track the other big X-ray techniques out there are X-ray diffraction (XRD) which is the scattering of X-ray light (so X-rays in and X-rays out), and X-ray fluorescence where an atom is excited to a high energy level and it emits X-rays in falling back down to its ground state (usually electrons in and X-rays out).

There are a couple of features that make XPS so valuable as an analytical technique. The first thing is that it’s surface sensitive. This may be a feature or a bug depending on your point of view, but for a group like ours where we could make a “perfect” semiconductor right up until it reacts with oxygen in the air, surface sensitivity is absolutely critical for understanding what happened. The second aspect of XPS that is useful is that it reveals quantitative ratios of the atomic species at that surface. Materials rearrange at their surfaces and that makes surface quantification very important in material science. For instance, water adopts a particular structure in the liquid state so that it can form four hydrogen bonds, but at the surface, a water molecule wouldn’t be surrounded by water anymore! So, water molecules at the surface rearragnge to make the best of a bad situation, and this one reason why water has such a high surface tension! Even the atoms in a solid rearrange at a surface, and if that’s the case, being able to quantify exactly what atoms are where may be critically important to that material’s behavior!

For a detailed description of surface processes and measurement techniques, see John A. Venables lecture notes on Surfaces and Thin Films. Bruce Brunschwig at Caltech also maintains a series of tutorials as well.

Our group utilizes XPS to quantify the oxidation of unstable semiconductors and to establish the coverage of organic molecules on a semiconductor surface. Recently we utilized angle-resolved XPS to differentiate between different surface terminations on a layered semiconductor that change based on etching or tape exfoliation! If any of these experiments sound like something that you need and you’re in and around the New England area, contact Grimm right away!

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Ultraviolet photoelectron spectroscopy

In contrast to the interaction of X-rays with core-level electrons, ultraviolet light interacts with the valence electrons that are involved in chemical bonds. Thus, ultraviolet photoelectron spectroscopy (UPS) conveys information the density of states of particular valence levels as a function of energy, and can establish those energy levels on an absolute scale. For materials such as the semiconductors that we study, UPS provides the work function energy of the material (also called a Fermi-level energy or in E_f semiconductors). UPS further elucidates the energetic difference between the valence-band edge energy, E_vb, and the Fermi-level energy E_f. The difference between these two values establishes the dopant density of a semiconductor and whether it’s a p-type or an n-type material. Separately, a Tauc plot (it’s pronounced /taʊts/) of a UV-Vis absorbance spectrum of that semiconductor reveals the onset of absorption in a semiconductor that should be the difference in the valence-band-edge energy and the conduction-band-edge energy, E_cb. In concert, UPS and UV-Vis reveal semiconductor band edge energies on an absolute scale.

In our instrument, a helium gas-discharge “lamp” serves as the UV light source. Under normal operation, we tune for the emission of the He I spectroscopic line that yields 21.22 eV (58.2 nm) light. We recently utilized UPS to elucidate the band edge positions of oxide-free Cs₂TiBr₆, as well as to demonstrate how chemical etchants change surface chemical states that in turn shift the electronic properties of BiOI(001) single crystal surfaces. If any of these experiments sound like something that you need and you’re in and around the New England area, contact Grimm right away!

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Boilerplate text for manuscripts

The text below may serve as a good starting point for the XPS acquisition detail in a manuscript.  The specifics may change based on whether charge neutralization was or was not employed, whether non-standard acquisition steps/pass energies were employed, what specific regions were collected, and how the data in those regions were fit.  Contact Grimm if you have any questions.

A PHI5600 XPS system with a third-party data acquisition system (RBD Instruments, Bend Oregon) acquired all photoelectron spectra as detailed previously.# Analysis chamber base pressures were <1 × 10−9 Torr. A hemispherical energy analyzer that was positioned at 90° with respect to the incoming monochromated Al Kα X-ray flux and 45° with respect to standard sample positioning collected the photoelectrons. Survey spectra utilized a 117 eV pass energy, a 0.5 eV step size, and a 50-ms-per-step dwell time. High-resolution XP spectra employed a 23.5 eV pass energy, 0.025 eV step size, and a 50 ms dwell time per step. Based on a consistent 285 eV position of features ascribed to adventitious carbon, acquisitions did not necessitate charge neutralization.*  Post-acquisition data fitting employed W-Tougaard-style baselines and GL(70) peak shapes for features within the Cs 3d, I 3d, Sn 3d; a Shirley-style baseline and GL(70) peak shape within the Pb 4f region; a W-Tougaard baseline and GL(30) peak shape for Br 3d and N 1s regions; and linear baselines with GL(30) peak shapes for B 1s, C 1s, F 1s, and O 1s regions. Fits that employ multiple peaks within a spectral region utilized identical fwhm values for each peak to minimize mathematically optimized but possibly chemically unrealistic fits.

# Cite the first manuscript from the group that described our XPS instrument, 10.1021/acsami.7b07117.

* If charge neutralization was employed, talk to Grimm.