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.

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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 Western University (London, 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 Supra, Ultra and Nova), and CasaXPS tips pertaining to questions we normally get from our students and clients, and other odd bits of information are presented.




 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.
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Magnesium 2p and Auger Parameter Values

Magnesium 2p binding energy and modified Auger parameter values are shown in Table 1. It is worth noting that accounting for the difference between charge referencing procedures is vital for correct analysis of magnesium compounds (especially for MgO and Mg(OH)2) [1]. For MgO, the peak position for the Mg 2p transition is 49.4 eV when charge referenced to adventitious carbon at 284.8 eV, and 50.8 when referenced to the Mg 2p metal peak at 49.73 eV (or grounded). This has led to a lot of confusion regarding the Mg 2p peak positions to use for analysis. The results from a consistently analyzed dataset are shown in Table 1 together with compiled literature values [1]. Since the chemical sensitivity of the Mg 2p transition is low for magnesium the Auger parameter and anion signals are particularly important to consider for improved speciation.

Table 1: Experimental and literature values for the Mg 2p binding energy and the modified Auger parameter for different magnesium compounds. Subscripts and superscripts: “sc” denotes single crystal; “g” denotes grounded and no charge correction was performed; “Mg” denotes floated and the Mg 2p metal peak was used as a charge reference at 49.73 eV; “O 1s” denotes floated and the O 1s lattice peak of MgO or Mg(OH)2 was used as a charge reference at 531.1 eV and 532.7 eV, respectively. Samples with no superscript were floated, and adventitious carbon was used as a charge reference at 284.8 eV. 

The anion fitting parameters—binding energy, peak width, and line shape—obtained from fitting reference samples are reported in Table 2 from a consistently analyzed data set [1]. The O 1s signal can be used to separate MgO and Mg(OH)2 despite their overlapping Mg 2p signals. The O 1s peak position of Mg(OH)2 is +1.6 eV with respect to the main lattice peak of MgO. However, the peak position for Mg(OH)2 can overlap with other environments such as the MgO defective oxide and adventitious carbon, which may need to be accounted for in the analysis.

Table 2: Overview of anion fitting parameters, binding energy, peak width, and line shape, from reference data. Subscripts and superscripts: “sc” denotes single crystal;  “g” denotes grounded and no charge correction was performed; “Mg” denotes floated and the Mg 2p metal peak was used as a charge reference at 49.73 eV; “ref” denotes reference values when charge referencing to the O 1s lattice peak. Samples with no superscript were floated, and adventitious carbon was used as a charge reference at 284.8 eV. 

Reference:
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Magnesium Induced Ghost Peaks

Ghost peaks occur when internal X-rays are produced within the sample, ejecting a detectable number of core-level photoelectrons. Since these internally produced X-rays have lower energy than the source X-rays, they eject photoelectrons with a lower kinetic energy (higher apparent binding energy). While ghost peaks in many cases appear with low intensity and therefore do not generally interfere with XPS analysis, they may cause confusion during peak assignment.

Ghost peaks are often observed for magnesium-rich samples from the Mg Kα emission lines (X-rays produced when a valence electron fills the Mg 1s core hole). The average photon energy of these lines is 1253.6 eV, and the position of the ghost peaks on the binding energy scale is dependent on the X-ray source. For example, the Mg Kα induced O 1s ghost peak is seen in the XPS spectra of Mg(OH)2 at 765.7 eV, when using an Al Kα source (1486.7 eV). This peak position is shifted by 233.1 eV, which is the energy difference between internal and source X-rays (Figure 1). 

Alternatively, this can also be calculated as follows: the Mg K X-ray induced peak for O 1s is found at a binding energy of ~ 765.7 eV, as the kinetic energy of these photoelectrons would be 1253.6 eV (Mg Kα X-rays) - 532.6 eV (O 1s B.E.) = 721.0 eV. Thus, they would then be seen as a peak at a binding energy of 1486.7 ( Al K(α) X-rays) - 721.0 eV = 765.7 eV. Similarly, the C 1s Mg X-ray induced peak would be at a binding energy of ~517 eV.

Also of note for spectra of magnesium-rich samples: Mg Auger structure can be found at ~242 eV, which is not a ghost peak and often not noted in most libraries (Figure 1, Mg KLL-2) and may cause confusion during survey scan peak assignment.  

Figure 1. Survey spectrum of Mg(OH)2 powder where the O 1s ghost peak is clearly visible at 765.7 eV. The figure is reproduced from reference [1]. 

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XPS Detection Limits

In general, detection limits for XPS range from 0.1 to 1 atomic percent.  However there are cases where limits could be much better or much worse.  An article from Alexander Shard [1] gives an excellent look at detection limits in over 6000 binary systems for both Al and Mg X-ray sources.  In such systems a heavy element, such as gold, in a light element matrix, say carbon, would have detection limits closer to 0.01 atomic percent.  For the opposite situation, carbon in a gold matrix, detection limits for carbon would be around 3 atomic percent.  Spectral overlaps can also degrade detection limits significantly and are incorporated into the tables presented.  Overlaps with Auger peaks can be overcome by changing X-ray sources used (i.e. using Mg K instead of Al K).  One can also work with advanced curve-fitting techniques to overcome some of these overlap issues.

Download printable PDF of detection limits for Mg X-ray spectra.

XPS detection limits using Al K(alpha) radiation [1].
Reference:
[1] A.G. Shard, Surf. Interface Anal. 46 (2014) 175-185.
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Carbon 1s for Organic Compounds

The seminal work of Graham Beamson and Dave Briggs in their "High Resolution XPS of Organic Polymers – The Scienta ESCA300 Database" has been utilized since 1992 as an invaluable resource for the XPS analysis of polymers and organic materials.  A summary of carbon 1s binding energies for organic functional groups from this work are presented here. The original work calibrates the binding energy scale to 285.0 eV for aliphatic carbon C 1s. The values presented here are now calibrated to 284.8 eV for aliphatic carbon, in line with recent results [2].  

Figure 1. Summary of the mean, maximum, and minimum carbon 1s binding energies for different organic functionalities according to the work of Beamson and Briggs [1].  Binding energy calibration presented here have been adjusted to the main aliphatic C 1s peak at 284.8 eV.

Table 1. Summary of the mean, maximum, and minimum carbon 1s binding energies for different organic functionalities according to the work of Beamson and Briggs [1].  Binding energy calibration presented here have been adjusted to the main aliphatic C 1s peak at 284.8 eV.


Table 2. Summary of beta shift effects from Beamson and Briggs (Appendix 2) [1].

The effects of various functional groups on beta carbon binding energies can be significant (Table 2).  Note that, in this context, the alpha carbon is the carbon directly attached to the functional group, and the beta carbon is attached to the alpha carbon. These effects have been included in the refinement of the binding energy value for the aliphatic carbon component in adventitious carbon [2]. 

References:
[1] G. Beamson, D. Briggs, High Resolution XPS of Organic Polymers - The Scienta ESCA300 Database, Wiley Interscience, 1992, Appendices 1 and 2.
[2] L.H. Grey, H.-Y. Nie, M.C. Biesinger, Appl. Surf. Sci. 653 (2024) 159319.
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Oxygen 1s for Organic Compounds

Figure 1. Summary of the mean, maximum, and minimum oxygen 1s binding energies for different organic functionalities according to the work of Beamson and Briggs [1].  Binding energy calibration presented here have been adjusted to the main aliphatic C 1s peak at 284.8 eV [2].

Figure 2. Oxygen 1s binding energy means and ranges for various organic compound types. Plotted from data supplied in Beamson and Briggs [1]. Referenced to main aliphatic C 1s peak at 285.0 eV as in the original source data.

Additional Notes: 
C-OH (aliphatic) Ref to C 1s at 284.8 eV: Average 532.7 eV, Min. 532.5 eV, Max. 532.9 eV
C-OH (aliphatic) Ref to C 1s at 285.0 eV: Average 532.9 eV, Min. 532.7 eV, Max. 533.1 eV
C-OH (aromatic) Ref to C 1s at 284.8 eV: 533.4 eV 
C-OH (aromatic) Ref to C 1s at 285.0 eV: 533.6 eV 

Also note that Si 2p3/2 for PDMS (silicone) is at 101.79 eV (Si 2p = 102.0 eV) with the C 1s at 284.38 eV and O 1s at 532.00 eV (referenced to aliphatic C at 285.0 eV).  If we shift the C 1s to 285.0 eV then Si 2p3/2 is at 102.41 eV (Si 2p = 102.6 eV) and O 1s is at 532.62 eV for silicone. If we then shift the C 1s to 284.8 eV then Si 2p3/2 is at 102.21 eV (Si 2p = 102.4 eV) and O 1s is at 532.42 eV for silicone.

References:
[1] G. Beamson, D. Briggs, High Resolution XPS of Organic Polymers - The Scienta ESCA300 Database, Wiley Interscience, 1992, Appendices 3.1 and 3.2.
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Calculating Oxygen Content from Adventitious Carbon 1s Spectra

Adventitious carbon (AdC) is commonly detected in X-ray photoelectron spectroscopy (XPS) analyses of most samples. While AdC can be beneficial in some cases, such as for charge correction purposes during the analysis of insulators, its associated C–O functionalities can complicate the interpretation of O 1s spectra. Accurately accounting for AdC’s contribution within the O 1s spectrum is essential but challenging due to significant spectral overlap and poorly resolved components in the high-resolution O 1s spectrum.

Rather than assigning multiple components without clear physical meaning—potentially leading to misinterpretations—incorporating stoichiometry offers a more reliable approach to improving data accuracy. However, applying stoichiometry can be tedious and challenging, particularly for novice users.

A recently published article [1] describes an approximation to enhance oxygen spectra interpretation by estimating oxygen linked to AdC. This publication provides background information, key assumptions, and an easy-to-use Excel calculator to assist XPS researchers in analyzing their own O 1s spectra.

This approach is particularly useful for accurately quantifying survey spectra when AdC influence must be minimized and for modelling high-binding-energy components in the oxygen 1s spectrum. The latter example is important to many transition metal oxides which have overlapping hydroxide and/or defect oxide components in the same binding energy window. Detailed examples of these applications are presented and discussed in reference [1]. These types of calculations were originally introduced in [2].

This Excel based calculator (also available at supplementary material in [1]) takes information from the survey and high resolution carbon 1s spectra and determines the amount of oxygen that is present from adventitious carbon species. This amount can then be deducted from the overall oxygen concentration.
(Note: you must download the file to Excel to use it - it is locked in Google Docs).

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