Common XPS Questions - Insights from Workshop Participants

From a recent online workshop I had received 92 submitted questions from the participants prior to the date of the workshop. Using ChatGPT to summarize the top 10 themes related to the question reveals an interesting take on common XPS user struggles. In the main portion of the workshop I cover themes 1, 3 and 10 fairly completely, while touching on themes 2, 4 and 6 as well. I further expand on theme 6 (backgrounds) at 1:03:09 in the video, and cover theme 8 (O 1s) at 1:41:49.  Of particular interest is my take on themes 5 and 7 - charge correction, charging effects, insulating samples and, in particular, defending  the usage of adventitious carbon (AdC) in our charge correction methodologies. See 1:11:35 in the video.  

  
1️⃣ Reliable Peak Fitting & Deconvolution
How to perform defensible, physically meaningful peak fitting — avoiding overfitting while properly handling multiplets, satellites, asymmetry, and constraints.
2️⃣ Overlapping Peaks in Complex Systems
Strategies for separating overlapping core levels (e.g., Fe/Co, Ba–Co, Cr/Te, C 1s overlaps) and mixed-phase materials.
3️⃣ Oxidation State Identification
How to confidently distinguish oxidation states (e.g., Fe²⁺/Fe³⁺, Mn multivalency, Ag⁰ vs Ag⁺) and interpret satellite structures.
4️⃣ Quantitative Accuracy
How to correctly calculate atomic percentages, apply RSFs, account for transmission functions, and interpret stoichiometry mismatches.
5️⃣ Energy Referencing & Carbon Correction
Reliability of C 1s calibration, handling adventitious carbon, alternatives to carbon referencing, and the impact of improper calibration.
6️⃣ Background Selection & Fitting Parameters
Correct choice of inelastic background (Shirley vs Tougaard), FWHM constraints, peak shapes, spin–orbit rules, and acceptable χ² values.
7️⃣ Charging Effects (Especially Insulators & Operando Work)
How to detect, correct, and minimize charging in powders, polymers, biological materials, and electrochemical systems.
8️⃣ Oxygen Peak Interpretation
Deconvoluting O 1s spectra in mixed oxides, identifying oxygen vacancies, and resolving oxygen contributions in multi-metal systems.
9️⃣ Publication Standards & Reviewer Expectations
How many components are acceptable? Is peak fitting mandatory? What are common reviewer criticisms? How should survey and HR spectra be presented?
🔟 Surface Sensitivity & Depth Information
Understanding probing depth, interaction volume, oxide thickness estimation, surface vs subsurface contributions, and when XPS truly represents “surface-only” chemistry.

Using Adventitious Carbon for Charge Correcting

The C 1s spectrum for adventitious carbon can be fit as follows.  A single peak, ascribed to alkyl type carbon (C-C, C-H), is fit to the main peak of the C 1s spectrum.  A second peak is added that is constrained to be 1.5 eV above the main peak, of equal FWHM to the main peak (C-C, C-H). This higher binding energy peak is ascribed to alcohol and/or ester functionality (C-OH, C-O-C). Further high binding energy components can be added if required. For example: C=O at approximately 3 eV above the main peak and O-C=O at 3.8 to 4.3 eV above the main peak. One or both of these peaks may also have to be constrained to the FWHM of the main peak if they are poorly resolved.  Reference [1] and the table below outline standard starting fitting parameters for adventitious carbon. 

Adventitious carbon C 1s curve-fitting parameters [1].

Spectra from insulating samples can then be charge corrected by shifting all peaks to the adventitious C 1s spectral component (C-C, C-H) binding energy set to 284.8 eV. There is certainly error associated with this assignment. Swift [2] lists a number of studies showing errors ranging from ±0.1eV to ±0.4 eV.  “Newer” studies (late 1970's) range from ±0.1 to ±0.3 eV. “Older” studies (late 1960's to early 1970's) were in the ±0.4eV range - however, reproducibility and resolution of the spectrometers of the time may have played a role.  Barr's [3] work from 1995 states that error in using adventitious carbon is ±0.2 eV.  Our work [4] in 2002 also suggests error in the ±0.2eV to  ±0.3eV range.  Experience with numerous conducting samples (1995 to present) and a routinely calibrated instrument have shown that the C 1s signal generally ranges from 284.7 eV to as high as 285.2 eV [5].  Reference [1] presents a detailed assessment of the analysis of insulating samples from a multi-user facility from over a 5-year period that showed an adventitious C 1s (C-C, C-H) binding of 284.91 eV ±0.25eV.  A similar study confirming the utility of the adventitious carbon technique with a similar multi-user facility analysis has been published by Morgan [6].

For organic systems, especially polymers, it is convenient to charge correct to the C-C, C-H signal set to 285.0 eV. This makes for easier comparison to the polymer handbook [7] which uses this number for charge correction.

References:
[1] M.C. Biesinger, Appl. Surf. Sci, 597 (2022) 153681.

[2] T.L. Barr, S. Seal, J. Vac. Sci. Technol. A 13(3) (1995) 1239.
[3] P. Swift, Surf. Interface Anal. 4 (1982) 47.
[4] D.J. Miller, M.C. Biesinger, N.S. McIntyre, Surf. Interface Anal. 33 (2002) 299.
[5] M.C. Biesinger, unpublished results

[6] D.J. Morgan, Surf. Interface Anal. 57 (2025) 28.
[7] G. Beamson, D. Briggs, High Resolution XPS of Organic Polymers - The Scienta ESCA300 Database Wiley Interscience, 1992.

Video - XPS: The Basics, Curve-Fitting and Advanced Studies

A recent lecture given by Dr. Biesinger to the Canadian Biomaterials Society - hosted by the Quebec City Student Chapter of the Canadian Biomaterials Society (CBS-QCSC) at Laval University.

Video: Advanced Analysis of Copper XPS Spectra

2020 Kratos North American User Meeting talk by Dr. Mark Biesinger, Director of Surface Science Western at Western University, London, Ontario, Canada.  Various strategies for the analysis of Cu XPS (X-ray photoelectron spectroscopy) spectra.

Auger Parameter Video

Video explaining the use of the Auger parameter.

Oxygen 1s Curve-Fitting Video

Video showing curve-fitting of the oxygen 1s (O 1s) spectrum for metallic surfaces using CasaXPS.

Titanium 2p Curve-Fitting Video

Video showing curve-fitting of the titanium 2p (Ti 2p) XPS spectrum using CasaXPS.



Chromium 2p3/2 Curve-Fitting Video

Video showing curve-fitting of the chromium (Cr 2p3/2) XPS spectrum using CasaXPS.

Charge Correcting to C 1s In CasaXPS

1. Fit the C 1s peak (see this post for fitting the C 1s to adventitious carbon), obtain the binding energy for the main peak (C-C, C-H).
2. Select all peaks to be charge corrected.
3. Under Options, select Processing - Calibration tab.
4. Enter measured C 1s (C-C, C-H) peak binding energy.
5. Enter true C 1s binding energy (usually 284.8 eV for most work, 285.0 eV for polymer work).
6. Select (click boxes for) Regions and Components.
7. Press Apply to Selection (for multiple spectra) or Apply (for a single spectrum only).