Getting it right: X-ray photoelectron spectroscopy

Irradiating a sample with X-rays ejects electrons via the photoelectric effect. The kinetic energy of those electrons correlates to their binding energy, which indicates what elements are present in the sample and, more importantly, in what bonding configurations. This technique, known as X-ray photoelectron spectroscopy (XPS), can be used to analyze thin films, solids, powders, liquids, and gases.

Last year, around 12,000 papers using XPS were published. However, many researchers have issues with correctly interpreting the resultant data. Greczynski and Hultman provided an entry level, step-by-step tutorial on XPS to improve the quality of experiments, data analysis, and results presentation.

“There are papers that did a survey of the problems that appear in XPS literature, and they concluded that about 30 percent of the papers had major errors, to the point that the conclusions could be seriously questioned,” said author Grzegorz Greczynski. “Another 30 percent contained significant errors that could compromise the message.”

The tutorial includes sections on common errors. One example is misinterpretation of spin-split peaks as two chemical states. Also, the peak fitting procedure to separate overlapping peaks is frequently performed incorrectly. Additionally, insulating samples require a charge reference to nail down the binding energy positions of the spectral peaks, which is often done incorrectly or not at all.

“Many systems are fully automated, and I think that’s a fast way of doing things, but you don’t get an understanding of what’s going on,” said Greczynski. “And then, you might have a problem interpreting the spectrum the correct way.”

The team plans to continue improving XPS techniques through development of reliable charge referencing methods and nondestructive surface analysis techniques.

Source: “A step-by-step guide to perform X-ray photoelectron spectroscopy,” by Grzegorz Greczynski and Lars Hultman, Journal of Applied Physics (2022). The article can be accessed at https://doi.org/10.1063/5.0086359 .