The use of peak fitting to extract information from x-ray photoelectron spectroscopy (XPS) data is of growing use and importance. Due to increased instrument accessibility and reliability, the use of XPS instrumentation has significantly increased around the world. However, the increased use has not been matched by the expertise of the new users, and the erroneous application of curve fitting has contributed to ambiguity and confusion in parts of the literature. This guide discusses the physics and chemistry involved in generating XPS spectra, describes good practices for peak fitting, and provides examples of appropriate use along with tools for avoiding mistakes.

The carbon 1s photoelectron spectrum is the most widely fit and analyzed narrow scan in the x-ray photoelectron spectroscopy (XPS) literature. It is, therefore, critically important to adopt well-established protocols based on best practices for its analysis, since results of these efforts affect research outcomes in a wide range of different application areas across materials science. Unfortunately, much XPS peak fitting in the scientific literature is inaccurate. In this guide, we describe and explain the most common problems associated with C 1s narrow scan analysis in the XPS literature. We then provide an overview of rules, principles, and considerations that, taken together, should guide the approach to the analysis of C 1s spectra. We propose that following this approach should result in (1) the avoidance of common problems and (2) the extraction of reliable, reproducible, and meaningful information from experimental data.

This study was undertaken to understand the extent and nature of problems in x-ray photoelectron spectroscopy (XPS) data reported in the literature. It first presents an assessment of the XPS data in three high-quality journals over a six-month period. This analysis of 409 publications showing XPS spectra provides insight into how XPS is being used, identifies the common mistakes or errors in XPS analysis, and reveals which elements are most commonly analyzed. More than 65% of the 409 papers showed fitting of XP spectra. An ad hoc group (herein identified as “the committee”) of experienced XPS analysts reviewed these spectra and found that peak fitting was a common source of significant errors. The papers were ranked based on the perceived seriousness of the errors, which ranged from minor to major. Major errors, which, in the opinion of the ad hoc committee, can render the interpretation of the data meaningless, occurred when fitting protocols ignored underlying physics and chemistry or contained major errors in the analysis. Consistent with other materials analysis data, ca. 30% of the XPS data or analysis was identified as having major errors. Out of the publications with fitted spectra, ca. 40% had major errors. The most common elements analyzed by XPS in the papers sampled and researched at an online database, include carbon, oxygen, nitrogen, sulfur, and titanium. A scrutiny of the papers showing carbon and oxygen XPS spectra revealed the classes of materials being studied and the extent of problems in these analyses. As might be expected, C 1s and O 1s analyses are most often performed on sp2-type materials and inorganic oxides, respectively. These findings have helped focus a series of XPS guides and tutorials that deal with common analysis issues. The extent of problematic data is larger than the authors had expected. Quantification of the problem, examination of some of the common problem areas, and the development of targeted guides and tutorials may provide both the motivation and resources that enable the community to improve the overall quality and reliability of XPS analysis reported in the literature.

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