The Use and Misuse of Curve Fitting in the Analysis of Core X-ray Photoelectron Spectroscopic Data

Peter Sherwood at the University of Washington has recently published a very good review article [1] that goes through many of the issues and pitfalls associated with achieving meaningful results from curve fitting XPS spectra.  It also includes a useful tutorial section that uses the W 4f spectrum of oxidized tungsten to illustrate the methods outlined in the paper.  A good starting point read for those who are getting into curve-fitting of XPS spectra (and for those needing a refresher!). 

[1] Peter M.A. Sherwood, The use and misuse of curve fitting in the analysis of core X‐ray photoelectron spectroscopic data, Surf. Interface Anal. (2019).

Advanced Analysis of Copper X-ray Photoelectron Spectra

This recent paper [1] builds upon and extends previously published X-ray photoelectron spectroscopy (XPS) curve-fitting and data analysis procedures [2,3] for a wide range of copper containing species. Steps undertaken include: 1) an examination of existing Cu 2p3/2 main peak and Cu 2p3/2 - Cu L3M4,5M4,5 Auger parameter literature data, 2) analysis of a series of quality standard samples, 3) curve-fitting procedures for both the Cu 2p3/2 and the Cu L3M4,5M4,5 spectra (as well as associated anions), 4) calculations that determine the amount of Cu(II) species in a mixed oxidation state system, 5) calculations and necessary data for thin film mixed oxide/hydroxide thickness measurements and 6) a presentation of literature and standard sample values in a Wagner (chemical state) plot.

Some examples of the extensive datasets and spectra available in this paper are presented below.
Cu 2p3/2 spectra of various Cu(II) species.

Cu L3M4,5M4,5 spectra for (left) Cu(0), Cu(I) species, mineral samples, and (right) Cu(II) species.

Curve-fitted Cu L3M4,5M4,5 spectrum of wrought copper sample submerged in an Ar aerated 3M NaCl solution for 30 days.  Curve-fitting results suggest 22 % Cu(0), 65 % Cu2O and 13% Cu2S.

Cu 2p3/2 - Cu L3M4,5M4,5 Wagner (chemical state) plot with literature and standard sample data.
Cu 2p3/2 and Cu 2p3/2 - Cu L3M4,5M4,5 Auger parameter literature values from Cu species

Cu 2p3/2 (main peak only) and Cu 2p3/2 - Cu L3M4,5M4,5 Auger parameter values from standard samples (20 eV pass energy). b) Note that this value is specified for the Kratos instruments used in this work. Other instruments may not have this accuracy. This will also not apply for Cu(0) in a non-conductive environment where calibration to other peaks are needed (e.g. adventitious carbon C 1s which has an error of 0.1 to 0.2 eV associated with it).
[1] M.C. Biesinger, Advanced Analysis of Copper X-ray Photoelectron (XPS) Spectra, Surface and Interface Analysis, 49 (2017) 1325-1334.
[2] M.C. Biesinger, L.W.M. Lau, A.R. Gerson, R.St.C. Smart, Appl. Surf. Sci. 257 (2010) 887-898.
[3] M.C. Biesinger, B.R. Hart, R. Polack, B.A. Kobe, R.St.C. Smart, Miner. Eng. 20 (2007) 152.

XPS Reference Pages

This site contains information gained from decades of X-ray photoelectron spectroscopy (XPS) analyses of an enormous variety of samples analyzed at Surface Science Western laboratories located at the University of Western Ontario. Originally this site was designed as a place for students and our clients to access valuable tips and information. It has since been opened to all those interested in the XPS technique. Summaries of literature data, relevant references and unpublished data taken of well characterized standard samples are presented. Also curve-fitting tips, instrument set-up tips (specifically for the Kratos AXIS Ultra and Kratos AXIS Nova), and CasaXPS tips pertaining to questions we normally get from our students and clients, and other odd bits of information are presented.

XPS and Surface Science Western

The fine print:
Surface Science Western and the University of Western Ontario does not warranty any of the information shown at this site. Any use of this data in scientific publications or other forms should include referencing to the originally published data referenced herein.