Essential Principles and Practices in X-ray Photoelectron Spectroscopy

TL;DR

X-ray Photoelectron Spectroscopy (XPS) addresses near-surface chemical analysis by exploiting binding energy shifts in core levels. The paper delivers a concise, principle-based overview of photoemission, BE referencing, peak fitting, and quantitative strategies, with practical cautions to avoid common misinterpretations. It emphasizes consistency across spectra, chemical states, and samples, and discusses additional information carried in the full spectrum, such as shake-up satellites, multiplet splitting, and Auger processes. Overall, the work serves as an accessible signpost to foundational resources and provides guidelines to achieve reliable, quantitative surface characterization using XPS.

Abstract

X-ray Photoelectron Spectroscopy (XPS) is a widely utilized technique for chemical analysis of solid surfaces, sensitive to the chemical environment of atoms via core-level binding energy shifts. While modern instruments allow obtaining the experimental data with ease, their evaluation and interpretation is challenging to newcomers to the field as a profound knowledge of the method is required for a correct analysis. Here we present a concise yet comprehensive introduction to the fundamental principles and methodologies of XPS, covering photoemission processes, chemical shifts, charge referencing, peak fitting, and quantification strategies. This overview aims to bridge the gap between data collection and reliable analysis, providing essential knowledge for correct interpretation. By clarifying key concepts and common practices, this work supports improved accuracy in surface chemical characterization using XPS.

Figures (1)

• Figure 1: Summary of XPS. (A) Typical experimental setup. (B) X-rays penetrate $\upmu$m into the sample, whereas the electrons can escape only from a few nm according to the depth distribution function (DDF) given in (N) for two distinct emission angles defined in (M). (C) Formation of the spectrum by excitation of electrons with radiation of constant energy of $E_\mathrm{X\text{-}ray}$. (D) Photoelectron spectrum of Si wafer; vertical lines highlight plasmon loss peaks. (E) Energy diagram defining energy conservation rules. Eq. (1) is obtained by combining energy conservation for sample–spectrometer contact and photoemission events. (F) Schematics of the probed atom with marking of inner shells in X-ray and spectroscopic notations. (G) Detailed Si 2p spectra of two distinct thicknesses of oxide layer on bulk Si. Both peaks are doublets; their fitting is detailed in the bottom part. (H) Charged shell model for chemical shifts. Changing either valence charge density by $\Delta q$ of the mean radius of the valence level $\langle r\rangle$ changes the inner potential within the shell, which changes the BE of electrons on core levels. The potential is given as a function of $\langle r\rangle$; inside the shell, the potential is constant. (J) Detailed C 1s spectrum of graphite showing its inherent asymmetry and shake-up satellite. (K) Detailed Fe 2p spectra of pure Fe oxides (magnetite and hematite) showing broad peaks of complicated shape due to multiplet splitting. (L) Photoelectron spectrum of graphite showing with marked background and straight line fit of pre-peak intensity for C KLL.