Systematic and Collaborative Approach to Problem Solving using X-ray Photoelectron Spectroscopy

Our recent article [1] in Applied Surface Science Advances highlights methodology developed as a result of years of interactions between many junior and senior X-ray Photoelectron Spectroscopy (XPS) users operating within the CasaXPS spectral processing and interpretation program framework. In particular, discussions arising from a series of workshops have been a significant source for developing the overall XPS data processing concept and are the motivation for creating this work. These workshops organized by the Institut des Matériaux Jean Rouxel (IMN), Nantes gather both experienced and novice users of XPS for a week of discourse in conceptual experiment design and the resulting data processing. However, the framework constructed and utilized within these workshops encouraged the dissemination of knowledge beyond XPS data analysis and emphasized the importance of a multi-disciplinary collaborative approach to surface analysis problem-solving. The material presented here embodies data treatment originating from data made available to the first CNRS Thematic Workshop presented at Roscoff 2013. The methodology described here has evolved over the subsequent workshops in 2016 and 2019 and currently represents the philosophy used in CasaXPS spectral data processing paradigm.

This article also serves as a useful reference descriptor of the CasaXPS software program. 

Reference:

[1] N. Fairley, V. Fernandez, M. Richard‐Plouet, C. Guillot-Deudon, J.Walton, E. Smith, D. Flahaut, M. Greiner, M. Biesinger, S. Tougaard, D. Morgan, J. Baltrusaitis, Systematic and collaborative approach to problem solving using X-ray photoelectron spectroscopy, Applied Surface Science Advances, 5 (2021) 100112.

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.

Exporting Survey Scan Quantification Data to ASCII

To export survey scan quantification data to an ASCII file format for importing to Excel or other spreadsheet software.

1. Select all quantified survey spectra
2. Select Quantification Parameters -> Report Spec. tab
3) Under Custom Report -> press Regions button, then Area Report button
4) Save file

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).

Line-Shape Definitions

Some common line-shape definitions used in CasaXPS are:

GL(p) = Gaussian/Lorentzian product formula where the mixing is determined by m = p/100, GL(100) is a pure Lorentzian while GL(0) is pure Gaussian[1]. e.g. GL(30) is 30% Lorentzian and 70% Gaussian.
SGL(p) = Gaussian/Lorentzian sum formula where the mixing is determined by m = p/100, SGL(100) is a pure Lorentzian while SGL(0) is pure Gaussian [1].

Note: The contribution of the spectrometer is Gaussian in shape while the natural core level line-shape is Lorentzian (with a small degree of tailing or asymmetry to higher binding energy). The K(alpha) X-ray source line-shape is also Lorentzian in shape [2].

A(a,b,n)GL(p) = Gaussian/Lorentzian product modified by an asymmetric form. Parameters a and b change the shape of the asymmetry while n defines the width of the Gaussian component used to convolute the shape of the profile [1].

LA(α, β, m) = Asymmetric line-shape where α and β define the spread of the tail on either side of the Lorentzian component. The parameter m specifies the width of the Gaussian used to convolute the Lorentzian curve. If values of α and β greater than unity are used this line-shape will correct a problem with previous asymmetric line-shapes (like A(a,b,n)GL(p) above) that tend to incorrectly estimate the peak area by incorporating area under the curve from binding energies well above the peak profile [3,4].

Reference:
1) N. Fairley, http://www.casaxps.com, © Casa software Ltd. 2005
2) D. Briggs "Surface Analysis of Polymers by XPS and Static SIMS" Cambridge University Press, Cambridge (1998) 53.
3) M.C. Biesinger, B.P. Payne, L.W.M. Lau, A. Gerson, R.St.C. Smart, Surf. Interface Anal. 41 (2009) 324-332.
4) N. Fairley, personal communication.

Converting XPS Data to ASCII

To convert fitted (or unfitted) XPS spectra to ASCII data for use in Excel, SigmaPlot or other graphing programs:
1. In CasaXPS, open your file and click on the spectrum of interest in the blocks area (right side of screen). You can also do multiple spectra at once if desired. This is useful if you are comparing various spectra.
2. Click on the button "Save Tab ASCII"
3. Save the file with an appropriate name.

The ASCII data contains the data in both Kinetic Energy (first set of data) and Binding Energy sets (second set of data) and includes data points for CPS, peak fits, background line and the resulting fitted envelope.

Fitting High-Resolution Spectra: CasaXPS Basics

1. Click on the Quantification Parameters button and go to the Regions panel.
2. Click on Create. This will make a peak region. Drag the endpoints to the desired location. To change the type of background type S for a Shirley background in the BG Type box, type L for a linear background, or T for a Tougaard background. For other available backgrounds see the CasaXPS manual. The starting and endpoints of the region can also have an offset applied to them by changing the St. Offset and End Offset values.
3. Under the Components panel click on Create to create a peak. You can drag and move the peak using the mouse if desired or you can position and change the parameters of the peak by adjusting the Area, fwhm and Position values.
4. Add more peaks as needed using the Create button.
5. You can add peak constraints by entering values into the Area Constr., fwhm Constr. or Pos. Constr. boxes. These can either be a range of values (fwhm Constr.example:1, 2.5; Pos. Constr. example: 285.00, 285.45) or a simple mathematical formula (fwhm Constr. example: A*1 (peak will be the same width as peak A); Pos. Constr. example: A+1.5 (peak will be 1.5 eV above peak A); Area Constr. example A*0.5 (peak will be half the area of peak A – useful for constraining spin orbit doublets).
6. The label for each peak can be changed in the Name box.
7. Click on Fit Components to have the program fit the peaks.
8. If desired, the residual fit can be viewed using the Residual On <-> Off button.
9. In the Annotation, Components panel, click on Apply. This will bring up the fitting components on the spectrum.

Quantification From Survey Scans

1. Click on Library button, choose element from the periodic table or from the element table by clicking on the peak of interest.
2. After finding elements for each peak present in your spectrum click on Annotation button, Peak Labels panel. Hold down CTRL and click on the appropriate peaks labels. Press Apply.
3. Click on the Quantification Parameters button and go to the Regions panel. Click on Create from Labels. This will make a peak region for all labels used. Delete duplicate element labels.
4. Zoom in on the spectrum and adjust the backgrounds for each region.
5. In the Annotation, Regions panel, click on Apply. This will bring up the quantification on the spectrum.
6. If you have missed some peaks, go to Library, Element Table panel, click on the peak (eg. Fe 2p), then in the Quantification Parameters, Regions panel click on the create button, adjust the peak background. Add the peak label from the Annotation, Peak Labels panel.

Points to remember.
A) For most elements the R.S.F. value will be for the entire peak-shape. E.g. For Fe 2p, both the Fe 2p3/2 and Fe 2p1/2 peaks must be included, for Au 4f, both the Au 4f7/2 and Au 4f5/2 peaks must be included.

B) If you need to use only one peak of the doublet (e.g. Fe 2p3/2) you must change the R.S.F. value to reflect this. See the RSF (Relative Sensitivity Factors) values table on this site.

Converting ASCII Files for Use in CasaXPS

1. Put ASCII data in an Excel spreadsheet, BE then CPS
2. Format cells to 8 decimal points
3. Save as tab delimited file (.txt), put it into a directory by itself
4. In CasaXPS, in Convert dialog, enter a new file name with the extension as follows:
filename.arx -xy
5. Press Open