X-ray photoelectron spectroscopy: Towards reliable binding energy referencing

TLDR

X‑ray photoelectron spectroscopy is a widely used surface‑analysis technique whose accuracy depends on reliable binding‑energy calibration, yet the common practice of referencing to adventitious carbon is criticized for its uncertain origin and composition. This review aims to evaluate the current state of XPS, clarify its operating principles, dispel myths about C 1s referencing, and provide a guide to best practices for the community. The authors critically assess XPS methodology, review historical developments, and synthesize recent literature to offer a comprehensive account of the technique’s principles and referencing challenges. Their survey shows that using the adventitious C 1s peak for calibration is highly arbitrary, leading to incorrect spectral interpretation, contradictory results, and a wide spread of reported binding‑energy values for the same chemical state.

Abstract

With more than 9000 papers published annually, X-ray photoelectron spectroscopy (XPS) is an indispensable technique in modern surface and materials science for the determination of chemical bonding. The accuracy of chemical-state determination relies, however, on a trustworthy calibration of the binding energy (BE) scale, which is a nontrivial task due to the lack of an internal BE reference. One approach, proposed in the early days of XPS, employs the C 1s spectra of an adventitious carbon layer, which is present on all surfaces exposed to air. Despite accumulating criticism, pointing to the unknown origin and composition of the adventitious carbon, this is by far the most commonly used method today for all types of samples, not necessarily electrically insulating. Alarmingly, as revealed by our survey of recent XPS literature, the calibration procedure based on the C 1s peak of adventitious carbon is highly arbitrary, which results in incorrect spectral interpretation, contradictory results, and generates a large spread in reported BE values for elements even present in the same chemical state. The purpose of this review is to critically evaluate the status quo of XPS with a historical perspective, provide the technique's operating principles, resolve myths associated with C 1s referencing, and offer a comprehensive account of recent findings. Owing to the huge volume of XPS literature produced each year, the consequences of improper referencing are dramatic. Our intention is to promote awareness within a growing XPS community as to the problems reported over the last six decades and present a guide with best practice for using the C 1s BE referencing method.