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.

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).

Charge Compensation

Insulating samples pose a unique challenge for XPS analysis. As photoelectrons are lost during the photoemission process a positive charge will build up on the sample. As this occurs, the kinetic energy of the emitted photoelectrons will decrease resulting in a shift to higher binding energy of the observed peaks in the spectrum. A number of schemes have been developed to compensate for this. Most commonly a low energy electron flood gun is used to replace emitted electrons. A slight overcompensation is normally used [1], setting up an equilibrium state with peaks in the spectrum shifted a few eV to lower binding energy. In post processing the peaks are then shifted back by referencing to a set internal standard such as adventitious carbon or a known species in that particular sample (e.g. a metallic peak, a well characterized oxide peak, a graphitic species, see [2] for further examples).

In some cases only a portion of the sample is insulating with some discrete areas or layers of the sample being conducting or semi-conducting. In these cases a phenomenon known as differential charging can occur. It is possible to imagine a sample with insulating domains or island structures that have varying thicknesses on a conducting surface. The spectral features from these areas will be shifted to lower binding energies by the action of the charge neutralizer whereas the spectral features of the underlying material will not be shifted. This may be complicated if the islands or domain structures behave as semi-conductors and their conductivity varies with thickness. The resulting spectra may be broadened significantly as a result. Layered structures may also charge differentially. The thin oxide layer on a metallic (conducting) material can behave as a conductor, as a semi-conductor, or as an insulator as the film grows in thickness.  The position of the oxide peak in relation to the metallic peak can change depending on oxide thickness and/or changes in charge neutralizer settings. See examples of this for Al oxide on Al metal in references [3] and [4]. A multilayered system may include alternating layers of conducting, semi-conducting and insulating species in a variety of combinations. All of this can result in changes in selected peak positions that must be understood or erroneous assignment of chemical states may result. 

One excellent way to combat differential charging effects is to electrically isolate (or "float") the entire sample from the specimen holder. This "specimen isolation" technique is similar to that used in secondary ion mass spectrometry [5] and effectively makes the entire sample (both insulating, semi-conductive and conductive areas) behave non-conductively. Methods employed with good success here at Surface Science Western include mounting samples on non-conductive double sided tape or mounting on glass slides. Recent work [2] has highlighted the effectiveness of this technique in conjunction with charge correction procedures using adventitious carbon. 

The Kratos AXIS (Supra, Ultra and Nova) systems employ both a low electron flood gun as well as a set of sub-sample magnetic fields that return over-focused and under-focused photoelectrons (that are not being admitted through to the spectrometer) back to the surface of the sample. This system has had excellent success in analyzing a wide range of insulating samples.

Figure 1. Differential charging issues can be caused by both insulating to semi-conductive island structures (top) or by layered systems (bottom).

References:
[1] M.A. Kelly, Analysing Insulators with XPS and AES, in: D. Briggs, J.T. Grant (Eds.) Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, IM Publications, Chichester, 2003, pp. 191-210.

[2] M.C. Biesinger, Appl. Surf. Sci. 597 (2022) 153681.

[3] A.J. Lizarbe, G.H. Major, V. Fernandez, N. Fairley, M.R. Linford, Surf. Interface Anal. 55 (2023) 651-657.

[4] D.R. Baer, K. Artyushkova, J. Cohen, C.D. Easton, M. Engelhard, T.R. Gengenbach, G. Greczynski, P. Mack, D.J. Morgan, A. Roberts, J. Vac. Sci. Tech. A 38 (2020) 031204.

[5] J.B. Metson, G.M. Bancroft, N.S. McIntyre, W.J. Chauvin, Surf. Interface Anal. 5 (1983) 181-185.

Spectrometer Calibration

For the instruments at Surface Science Western using monochromatic Al k(alpha) X-ray sources, the instrument work function is calibrated to give an Au 4f7/2 metallic gold binding energy of 83.96 eV and the spectrometer dispersion is adjusted to give a binding energy of 932.62 eV for metallic Cu 2p3/2 (binding energy values are +/-0.025eV for the purpose of calibration).  Ag 3d5/2 is at 368.21 eV.  These values are in accordance with the latest (ISO) standards (ISO 1572:20001) [1,2]. Calibration of the instruments are carried out every six months, if spectral irregularities are noted, or if major repairs/maintenance on the instruments have occurred.

References

[1] M.P. Seah, I.S. Gilmore, G. Beamson, Surf. Interface Anal. 26 (1998) 642.

[2] M.P. Seah, Surf. Interface Anal. 31 (2001) 721.