Assessment of the frequency and nature of erroneous x-ray photoelectron spectroscopy analyses in the scientific literature

Abstract

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.

Comments on the XPS Analysis of Carbon Materials

“…The C (1s) core-level spectrum of graphitic carbon, such as that of HOPG, is characterized by an asymmetric peak shape with a binding energy typically reported as 284.5 eV (although values between 284.3 and 284.6 eV have been reported [12,14]) and a characteristic π-π* shake-up structure with structure centered around 291 and 294.5 eV (Figure 1). The fitting of the graphitic C (1s) envelope is one of the most common errors encountered in the literature [15], and the asymmetric shape is something commonly ignored in The fitting of the graphitic C (1s) envelope is one of the most common errors encountered in the literature [15], and the asymmetric shape is something commonly ignored in may published papers. Recently, Linford et al [11] reported a method for fitting the C (1s) envelope of graphitic carbon materials, using a reference spectrum of clean graphite/HOPG as a model for the graphitic backbone.…”

Box plots: A simple graphical tool for visualizing overfitting in peak fitting as demonstrated with X-ray photoelectron spectroscopy data

“…In the first, all of the peaks, except the one modeling the shake-up signal, were constrained to have the same width. While this restriction should increase the likelihood that physically reasonable results will be obtained (widely varying peak widths in an XPS peak fit are often a sign of poor fitting [18], the box plot summarizing the results of performing this fit with 32 different/random initial conditions (Figure 3c), suggests that these constraints do not lead to a robust fit. That is, the widths of the boxes in Figure 3c indicate that multiple, local minima are present in this fit space.…”

“…40 % of the fitted XPS data in the literature is significantly flawed. [18] Other concerns regarding XPS include inappropriate data acquisition and archiving. [19] These issues are part of the so-called reproducibility crisis in science.…”

“…We illustrate our approach to identifying fits with multiple local minima using a C 1s spectrum from a complex sample (see Figure 1a). Our interest in C 1s narrow scans is a result of a recent report that showed that they are the most frequently presented and researched narrow scans in the scientific literature, [18] and that they are often fit incorrectly. [29,30] Calculations for the four-component peak fits.…”

Characterization of Charge States in Conducting Organic Nanoparticles by X-ray Photoemission Spectroscopy

“…% (composition and stoichiometry) can be estimated. This represents the most extended use of XPS, although too often, the popularity and accessibility of the technique leads to incorrect interpretations [34].…”