Sulphur

Table 1. S 2p3/2 binding energies compiled from the NIST database [1] and other sources.

Notes: 2p3/2 - 2p1/2 doublet separation = 1.18eV, peaks constrained to a 2:1 area ratio (2p3/2 : 2p1/2), generally one sets both peaks to an equal FWHM for ease of use although in pure samples this may not be the case.

Smart et al. [9] and Pratt et al. [10] give an excellent overview of binding energy ranges for the study of mineral surfaces. These ranges can be used with other sulphur containing systems as well. Of particular interest is the assignment for polysulphides eg. (S4)2- = 162.0-163.0 eV, (S5)2- = 161.9 - 163.2 eV, (Sx)2-) = 163.7 eV. Surface species can also play a role in XPS, especially for in-situ fractured sulphide mineral species [11].

[a] Nesbitt et al. [12] give a value of 162.2 eV for the disulphide in arsenopyrite.

[b] A more detailed look at organic sulphur species can be found here.

In a recent paper from Sarah Harmer's group at Flinders University, synchrotron XPS is used to convincingly elucidate surface 3-coordinate, bulk and surface 4-coordinate and bulk 5-coordinate sulfur in the chalcogenide (Fe,Ni)9S8.  This work shows that sulfide coordination changes can be seen by XPS [13].

A Na2S2O3.5H2O (sodium thiosulphite cooled to -130C during analysis) reference sample gave S 2p3/2 peak positions of 162.1 eV and 168.1 eV for S*SO3 and SS*O3 moieties, respectively.

There is a lot of confusion in the literature when presenting the data for sulphur. Some papers mention S 2p when they really mean S 2p3/2, these are not interchangeable! Please remember to be specific about the exact peak you are referring to.  

A recent paper from Clark et al. [14] highlights how widespread the issue of erroneous peak fitting of S 2p is.  Section B within this paper is worth a look as it highlights some of the common errors that should be avoided, these include:

1) Lack of spin–orbit splitting. Doublets (2p3/2 and 2p1/2 peaks) in their appropriate 2:1 ratios, respectively, should be used to represent each chemical state in the material.

2) Inconsistent and widely varying peak widths/full widths at half maximum (FWHMs).

3) Questionable assignments of the peaks to chemical species or oxidation states.

4) Backgrounds that cut through and then extend above the data on the high and low binding energy sides of the peak envelopes. 

5) Relatively large range of peak binding energy positions or fit components that are assigned as the same chemical states and should have well defined positions.

6) Noisy spectra, insufficient S/N.

References:

[1] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R.Jr. Rumble, NIST Standard Reference Database 20, Version 3.4 (web version) (http:/srdata.nist.gov/xps/) 2003.
[2][2] Z.E. Pettifer, J.S. Quinton, S.L. Harmer, Minerals Engineering, 184 (2022) 107666.

[3] A.N. Buckley, W.M. Skinner, S.L. Harmer, A. Pring, L-J. Fan, Geochimica et Cosmochimica Acta, 73 (2009) 4452-4467.

[4] A.N. Buckley, W.M. Skinner, S.L. Harmer, A. Pring, R.N. Lamb, L.J. Fan, Y. Yang, Canadian Journal of Chemistry, 85 (2007) 767- 781.

[5] S.L. Harmer, A.R. Pratt, H.W. Nesbitt, M.E. Fleet, Canadian Mineralogist, 43 (5) (2005) 1619-1630.

[6] M.E. Fleet, X. Liu, S.L. Harmer, H.W. Nesbitt, Surface Science, 584 (2005) 133-145.

[7] V.P. Zakaznova-Iakovleva, S.L. Harmer, H.W. Nesbitt, G.M. Bancroft, A.R. Pratt, R. Flemming, Surface Science, 600(2) (2006) 348-356.

[8] A.R. Pratt, H.W. Nesbitt, American Mineralogist, 85 (2000) 619-622.

[9] R.St.C. Smart, W.M. Skinner and A.R. Gerson, Surface and Interface Analysis, 28 (1999) 101-105.

[10] A.R. Pratt, I.J. Muir and H.W. Nesbitt, Geochimica et Cosmochimica Acta, 58 (2) (1994) 827-841.

[11] H.W. Nesbitt, M. Scaini, H. Hochst, G.M. Bancroft, A.G. Schaufuss and R. Szargan, American Mineralogist, 85 (2000) 850-857, 

[12] H.W. Nesbitt, I.J. Muir, A.R. Pratt, Geochimica et Cosmochimica Acta, 59 (9) (1995) 1773-1786,

[13] Z.E. Pettifer, J.S. Quinton, W.M. Skinner, S.L. Harmer, Applied Surface Science, 504 (2020) 144458. 

[14] B.M. Clark, G.H. Major, J.W. Pinder, D.E. Austin, D.R. Baer, M.C. Biesinger, C.D. Easton, S.L. Harmer, A. Herrera-Gomez, A.E. Hughes, W.M. Skinner, M.R. Linford, Journal of Vacuum Science and Technology A, 42 (2024) 063213.

Advanced Analysis of Indium

Analysing indium and indium-based compounds using X-ray Photoelectron Spectroscopy is challenging due to only slight shifts in the binding energies of the commonly used In 3d5/2 core line. A recent paper shares a comprehensive set of reference data for indium and its compounds, covering the In 3d, 3p, and 4d core lines, the In MNN Auger signal, as well as relevant counter ion signals [1]. Valuable tools, such as the modified Auger parameter and chemical state (Wagner) plots, which aid in differentiating indium species are also discussed.

Figure 1 and Table 1 present average literature values for the In 3d5/2 core line, highlighting both the average and standard deviations. These values illustrate the apparent challenge in distinguishing between various indium compounds. Factors like natural line widths, line shapes, and potential errors in charge correction add further complexity to accurate speciation.

Figure 1. Average In 3d5/2 literature values for indium compounds. 

Table 1. Average In 3d5/2 literature values for indium compounds.

Experimental data (Figure 2 and Table 2) presents a similar trend, underscoring that the In 3d5/2 core line alone is not enough to reliably distinguish between indium species. Notably, the In 3d5/2 core line of indium oxides shows variable asymmetry in line shape, which has led to differing interpretations in the literature. Some researchers attribute the high binding energy component to hydroxide or oxy-hydroxide species, while others suggest that it reflects electronic properties (screening effects). The current experiments [1] support the view that screening effects play an important role in this asymmetry. Excellent studies on these screening effects have been conducted by Körber [2] and Harvey [3].

Figure 2 In 3d spectra from [1].

Table 2. Experimental In 3d5/2 values from [1].

The In M4,5N4,5N4,5 transitions show a broader range of binding energy than the In 3d5/2 core level, making it better suited for accurate speciation, particularly by making use of the modified Auger parameter (Figure 3 and Table 3). In mixed-system analysis, i.e., a system containing multiple indium species, both the position and shape of the In M4,5N4,5N4,5 Auger electron signal can useful for speciation. Béchu and Fairley have provided an excellent discussion on the application of nonlinear and linear least-squares fitting methods to the In M4,5N4,5N4,5 signal, specifically for the oxidation of InSb [4]. Table 4 presents the fitting parameters needed to reproduce the M4,5N4,5N4,5 line shapes in order to fit complex experimental envelopes.

Figure 3. In MNN spectra for various indium compounds [1]. For reference, vertical lines indicating the kinetic energy (MNN) for metallic indium have been overlaid in each tile. Note that the additional signal present for InPO4 at 414.7 eV was due to Na contamination.

Table 3. In M4N4,5N4,5 and modified Auger parameter values [1]. 

Table 4. In MNN Auger peak fitting parameters [1].

Considering the information presented above, a comprehensive interpretation of XPS data involving indium and its compounds should involve a combination of the available data, including survey spectra (i.e., stoichiometry), the In 3d5/2 and In M4N4,5N4,5 Auger spectra, as well as the relevant counterion spectra (see [1]). For systems containing multiple indium compounds, the position and shape of the M4,5N4,5N4,5 transition can offer a more accurate approach than using the 3d5/2 core line alone.

References:

[1] J.D. Henderson, L.P. Pearson, H-Y. Nie, M.C. Biesinger, Surf. Interface Anal. 57 (2024) 81. https://doi.org/10.1002/sia.7356 

[2] C. Körber (et al.), Phys. Rev. B, 81 (2010) 165207

[3] S.P. Harvey (et al.), J. Phys. D Appl. Phys., 39 (2006) 3959.

[4] S. Béchu and N. Fairley, J. Vac. Sci. Technol. A, 42 (2024) 013202.

Iron

For the analysis of photoelectron spectra of relatively pure iron oxides, one can use peak shape and peak binding energy comparisons to standard compounds to derive oxide composition. McIntyre and Zetaruk’s [1] paper is widely cited and is still an excellent starting point for qualitative iron oxide determination. Pratt et al. [2] used a series of multiplet peaks to curve fit oxidized iron sulfide (pyrrhotite) surfaces.

Grosvenor et al.[3] fitted the various iron oxide, hydroxide and halide peak shapes with a close approximation of the Gupta and Sen[4] multiplet structure. Multiplet FWHM, splittings and weightings are presented. An analysis of satellite to main peak separation is also given. All Fe(II) (high spin only as low spin Fe(II) does not exhibit multiplet splitting) and Fe(III) species can be fitted with Gupta and Sen multiplet structure. Variation in peak spacing and intensity occur for different ligands. Broad satellite peaks of varying intensities at binding energies above the main Fe 2p3/2 structure are present in the spectra for all high spin compounds. However paper [3] only presents the main multiplet lines, excluding the details needed to fit the broader higher binding energy satellite structures.

Table 1 [5] presents full fitting parameters including the multiplet and satellite structure. FWHM values are reported for 10 eV pass energy only. To accommodate lower resolution settings slightly broader peaks would be necessary for best fit values. For these fits a Shirley background encompassing only the 2p3/2 portion of the spectrum is used. Also included in this Table are new spectral fitting parameters for FeCr2O4 and NiFe2O4, species that are important for the examination of oxide films on Fe-Cr-Ni alloys, as well as data for new analyses of α-Fe2O3 and γ-Fe2O3[5]. Fitting parameters for FeCO3, which has been noted in certain corrosion products, are also presented in Table 1. These analyses were collected from a mineral sample of siderite (cleaved in vacuum). Carbon 1s binding energy for FeCO3 is at 290.1 eV. The many spectra are best viewed in the original papers - see links in reference section.

New! Modified/updated fitting parameters for FeO, FeOOH and Fe3O4 are now included in Table 1 from work presented in reference [6]. In particular the analysis and fitting of FeO is improved substantially as the new FeO standard, sputter cleaned with a GCIS (not available during the original work in [3,5]), is free of low levels of surface Fe2O3/Fe3O4 contamination.

Table 1. Fe 2p3/2 spectral fitting parameters: binding energy (eV), percentage of total area, FWHM value (eV) and spectral component separation (eV) [5,6].

While these values [5] and reference spectra [1,3,5] will be useful for identification of pure oxide or oxy-hydroxide species, curve fitting of mixed systems quickly becomes complicated due to spectral overlaps. For example, it can be seen that various Fe(III) compounds have a similar range of Fe 2p binding energies and vary mostly in peak shape and satellite intensities. Any attempt at fitting two or more Fe(III) species to a spectrum will consequently contain an inherent degree of error. As well, overlap of the Fe(III) satellite structure with the Fe(0) and Fe(II) Fe 2p1/2 portion of the spectrum will result in setting the higher binding energy background endpoint placement at a point that will not cover the satellite structure of the Fe(III) species. This will require any fitting of mixed chemical state systems containing Fe(III) species to omit the higher binding energy Fe(III) satellite (e.g. Fe2O3, FeOOH) from the envelope of peaks. This will again increase the error associated with the curve fitting.  Finally, determination of the Fe species present, especially in a mix of Fe(III) species, should include corroborating evidence from O 1s analysis and even other analytical techniques such as Raman spectroscopy or, for thin crystalline films, grazing angle XRD. Some examples of fittings in mixed species samples are presented in [5].

Compared to the other transition metal species, the complex multiple species fitting of Fe is the most problematic. With so many possible species having overlapping binding energies erroneous interpretation can result. A sample with two distinct species can likely be fitted accurately, three species much less so, while four or more species must be looked at as indicative but unreliable. 

New! Recent work [6,7] has demonstrated the utility of these methodologies, with extremely good chemical state speciation achieved for oxide/hydroxide mixtures.  For metal/oxide/hydroxide mixtures, good success was found for low levels of metal content.  As the amount of metal grows (particularly above 25%) the amount of Fe(II) species tends to be underestimated. Results from the original curve-fittings from [5] were improved, particularly for FeO.  This work again emphasizes that the better the pure compound spectrum is, the better the final curve-fitting results!

References:
[1] N.S. McIntyre, D.G. Zetaruk, Anal. Chem. 49 (1977) 1521.

[2] A.R. Pratt, I.J. Muir, H.W. Nesbitt, Geochim. Cosmochim. Acta 58 (1994) 827.
[3] A.P. Grosvenor, B.A. Kobe, M.C. Biesinger, N.S. McIntyre, Surf. Interface Anal. 36 (2004) 1564.
[4] R.P. Gupta, S.K. Sen, Phys Rev. B 12 (1975) 15.
[5] M.C. Biesinger, B.P. Payne, A.P. Grosvenor, L.W.M. Lau, A.R. Gerson, R.St.C. Smart, Appl. Surf. Sci. 257 (2011) 2717.

[6] A.E. Hughes, C.D. Easton, T.R. Gengenbach, M.C. Biesinger, M. Laleh, (2024) JVST A, 42 (2024) 053205.

[7] A.E. Hughes, C.D. Easton, T.R. Gengenbach, M.C. Biesinger, M. Laleh, (2024) JVST A, 42 (2024) 053206.

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.

XPS Details Needed for Publications

Below is a list of some of the XPS experimental details that may be needed for most scientific publications. Beside each detail are some examples of what could be used or stated.

1) Instrument Name: Kratos Axis Supra, Kratos Axis Nova, Surface Science Laboratories SSX-100, PHI Quantera, PHI VersaProbe, VG ESCALAB 250, type of energy analyzer etc.

2) X-ray Source: monochromatic Al K(alpha) at 1486.6 eV, non-monochromatic Mg K(alpha), Zr L, He(I), He(II), synchrotron tunable source set at ‘x’ kV

3) X-ray Power: 15 mA and 15 kV, 225 W (= 15mA x 15 kV)

4) Spectrometer Calibration Details: The instrument work function was calibrated to give a binding energy (BE) of 83.96 eV for the Au 4f7/2 line for metallic gold and the spectrometer dispersion was adjusted to give a BE of 932.62 eV for the Cu 2p3/2 line of metallic copper.

5) Spot Size, Area of Analysis, Angle: 300x700 micron area of analysis, 300 micron spot, small spot analysis using a 55 micron spot size, 90 degree take-off angle / 0 degree of sample tilt, angle resolved analysis

6) Charge Neutralizer Use: The Kratos magnetic confinement charge compensation system was used on all samples (One may also want to include charge neutralizer settings although this can vary depending on the age of the filament, contamination of the charge plates etc. A note on the how good charge neutralization was deemed to have occurred may suffice). An electron flood gun at ‘x’ settings.

7) Effectiveness of Neutralizer: Charge neutralization was deemed to have been fully achieved by monitoring the C 1s signal for adventitious carbon. A sharp main peak with no lower binding energy structure is generally expected.

8) Charge Correction Procedures: Spectra have been charge corrected to the main line of the carbon 1s spectrum (adventitious carbon) set to 284.8 eV (or 285.0 eV for polymer samples), Au 4f7/2 at 83.96 eV, no charge correction (conductive samples)

9) Instrument Base Pressure: 8 x 10^-10 Torr.

10) Scan Details, Pass Energy, Number of Sweeps, Step Size, Scan Window: The C 1s spectra were taken with a minimum of 10 - 60s scans with a scan window of 278-295 eV using a 0.05 eV step and 20 eV pass energy.

11) Spectrometer Resolution Details: Ag 3d5/2 line FWHM at 10 eV pass energy was 0.48 eV. Source resolution for monochromatic Al K(alpha) X-rays is ~0.3 eV.  The instrumental resolution was determined to be 0.35 eV at 10 eV pass energy using the Fermi edge of the valence band for metallic silver. Resolution with charge compensation system on <0.68 eV FWHM on PET.

12) Sample Mounting Details: held by metal clips and grounded to the holder, mounted on double sided adhesive tape, electrically isolated from the sample holder 

13) Software Used for Curve-Fitting: CasaXPS version (2.3.26), XPSPeak, Vision 2 Processing Software, Avantage

14) Line-shape Details: 50% Gaussian/50% Lorentzian, Asymmetric line-shape defined by…,

15) Background Used for Curve-Fitting: Shirley, Linear, Tougaard

16) Other Curve-Fitting Details as Needed: FWHM, constraints, doublet separations

A recent article from Pinder et al.[1] discusses a similar list as above for reporting instrument parameters. They also discuss common errors seen in XPS publications, well worth a read as a quality check prior to publishing your own data. 

Reference:

[1] J.W. Pinder, G.H. Major, D.R. Baer, J. Terry, J.E. Whitten, J. Cechal, J.D. Crossman, A.J. Lizarbe, S. Jafari, C.D. Easton, J.Baltrusaitis, M.A. van Spronse, M.R. Linford, Appl. Surf. Sci. Adv., 19 (2024) 100534.

What is Adventitious Carbon?

A thin layer of carbonaceous material is usually found on the surface of most air exposed samples, this layer is generally known as adventitious carbon. Even small exposures to atmosphere can produce these films. Adventitious carbon is generally comprised of a variety of (relatively short chain [1]) hydrocarbons species with small amounts of both singly and doubly bound oxygen functionality. The source of this carbon has been debated over the years. It does not appear to be graphitic in nature and in most modern high vacuum systems vacuum oils are not readily present (as they have been in the past) [1,2,3,4]. There may be some evidence that CO or CO2 species may play a role in the gradual appearance of carbon on pristine surfaces within the vacuum of the XPS chamber [3].

It’s presence on insulating surfaces provides for a convenient charge reference by setting the main line of the C 1s spectrum to 284.8 eV (although values ranging from 285.0 eV to 284.5 eV have been used in some cases, remember to check for this value when looking for binding energy references in the literature). The error in this value (284.8 eV) is, for most systems, on the order of +/-0.2 eV to 0.3 eV.  An in-depth look at the effectiveness of using AdC for charge correction purposes, including standardized fitting procedures, is presented in [5].

  

Work by Grey et al. [6] has explored the nature of adventitious carbon by XPS and time-of-flight secondary ion mass spectrometry (ToF-SIMS).  XPS D-parameter and ToF-SIMS analyses confirms that AdC is not graphitic in nature. An average C 1s spectrum for AdC (Figure 1, Table 1) was derived and shows that, on average, ~ 25 % of the carbon species in AdC is directly associated with oxygen functionality.  Similarly, ToF-SIMS analyses show that AdC is comprised of mainly short chain hydrocarbons with some oxygen functionality.

An advanced method for curve-fitting of the C 1s envelope for AdC (Table 2) was developed that included the effects of beta carbons (in this context, the alpha carbon is the carbon directly attached to the oxygen, and the beta carbon is attached to the alpha carbon) and were informed by the configurations of possible volatile organic compounds (VOC) that are the source of most AdC [6]. Using this method in combination with the dataset from [5], the average C–C/C–H AdC aliphatic peak position was shown to be 284.81 eV (+/- 0.25 eV) via verification with a secondary internal reference.

Figure 1. Average of 80 adventitious carbon C 1s XPS spectra.

Table 1. Average adventitious carbon C 1s fitting parameters from an average of 80 AdC spectra.

Table 2. Curve-fitting parameters for AdC C 1s including shifted beta peaks (*) (peaks E, F and G). Areas for peaks A, B, C, and D should be left unconstrained. # If peak-shape for peak D is well-defined the FWHM constraint can be removed.

References:

[1] T.L. Barr, S. Seal, J. Vac. Sci. Technol. A 13(3) (1995) 1239.

[2] P. Swift, Surf. Interface Anal. 4 (1982) 47.
[3] D.J. Miller, M.C. Biesinger, N.S. McIntyre, Surf. Interface Anal. 33 (2002) 299.
[4] H. Piao, N.S. McIntyre, Surf. Interface Anal. 33 (2002) 591.

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

[6] L.H. Grey, H.-Y. Nie, M.C. Biesinger, Appl. Surf. Sci. 653 (2024) 159319.

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. (2024) https://doi.org/10.1002/sia.7360. 
[7] G. Beamson, D. Briggs, High Resolution XPS of Organic Polymers - The Scienta ESCA300 Database Wiley Interscience, 1992.