Considering complexities with insulated XPS samples

X-ray Photoelectron Spectroscopy (XPS) is a powerful technique for investigating chemical bonding in all sorts of materials. Originally, XPS was primarily done with samples electrically grounded, but problems with spectra distortion encountered in insulators and inhomogeneous specimens led to such samples being deliberately isolated from ground and their electrical charge controlled with a flood gun. While the underlying principles for both configurations are the same, there are key differences that researchers need to be aware of.

Grzegorz Greczynski compared XPS results from the same set of specimens analyzed with and without contact with the spectrometer to determine the effect of sample mounting on the resulting measured peak positions.

“XPS measurements with samples isolated from ground have become more common in recent years, predominantly because large fractions of novel materials fall under the category of electrical insulators,” said Greczynski. “What is, however, often not realized is that the energy reference levels are not the same as for analyses performed in a conventional way on conductors.”

The key difference Greczynski demonstrated is that while spectra from grounded conductors are referenced to the Fermi level, spectra from isolated samples align instead to the vacuum level — an energy reference level corresponding to an electron at rest and outside a solid. The result is that the sample work function needs to be considered for correct interpretation of peak positions. He provided multiple case studies, featuring common experimental setups, that highlighted this complication.

“This is a very important point to realize before making interpretations of any peak shifts — chemical shifts can be easily mixed up with shifts caused by changes in the sample work function, which is highly sensitive to many parameters and is often difficult to control,” said Greczynski.

He hopes that his work will serve as a guide to others looking to use XPS to analyze insulating materials.

Source: “Towards reliable x-ray photoelectron spectroscopy: Understanding fundamental differences between analyses performed on samples with and without electrical contact to spectrometer,” by Grzegorz Greczynski, Journal of Vacuum Science and Technology: A (2026). The article can be accessed at https://doi.org/10.1116/6.0005180 .

This paper is part of the Reproducibility Challenges and Solutions II with a Focus on Surface and Interface Analysis Collection, learn more here .