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).

References:

[1] J.D. Henderson, B.P. Payne, N.S. McIntyre, M.C. Biesinger, Surf. Interface Anal. 57 (2025) 214-220.
[2] B.P. Payne, M.C. Biesinger, N.S. McIntyre, J. Electron Spectrosc. Relat. Phenom. 184 (2011) 29-37.

Inelastic Mean Free Path Calculations

Link to User's Guide to the NIST IMFP Calculator

Link to NIST IMFP Calculator at NIST website.

Data needed for inelastic mean free path (IMFP) calculations from predictive formula will include:
1) Density of the compound in g/cm-3 (the CRC Handbook is a good place to get this information)
2) Number of valence electrons in the compound - typically you will include electrons that have an excitation energy of less than 50 eV [1]
3) Band-Gap Energy (Eg) of the compound in eV - this is generally the hardest data to find and may not be available for all compounds

Reference:
[1] S. Tanuma, C.J. Powell, D.R. Penn, Surface and Interface Analysis, 17 (1991) 911-926.

p to p3/2 and d to d5/2 Binding Energy Converter

Sometimes you may have a 2p orbital binding energy reference but what you need is a 2p3/2 binding energy (or vice versa). Or perhaps you have a 3d orbital binding energy reference but what you need is a 3d5/2 binding energy (or vice versa).  This calculator allows you to easily switch between the two values.

(Note: you must download the file to Excel to use it - it is locked in Google Docs).

Aluminum Oxide Thickness Measurement

If the metal:oxide ratio can be determined for a thin film oxide sample (~0-9 nm) and if the inelastic mean free path (IMFP, λ) of the metal (λm) and oxide (λox) is known (or can be calculated), the oxide film thickness can be calculated using the calculations of the type developed by Strohmeier [1] and Carlson [2] defined as follows:

d=λoxsinθ ln(((NmλmIox)/(NoxλoxIm))+1) (Eq. 1)

where θ is the photoelectron take-off angle, Iox and Im are the area percentages of the oxide and metal peaks from the high-resolution spectrum, and Nm and Nox are the volume densities of the metal atoms in the metal and oxide, respectively.

For an aluminum oxide film on an aluminum substrate, the Al 2p spectrum has well separated oxide and metal peaks (example shown in Figure 1) and Io and Im values can be readily ascertained. Using the λ and N values proposed by Strohmeier [1], an Excel based aluminum oxide thickness calculator is presented. Just input the percentage of metal from the high-resolution spectrum to get a film thickness in Angstroms. (Note: you must download the file to Excel to use it - it is locked in Google Docs).

Figure 1. Al 2p XPS spectrum of a thin film Al oxide on Al metal with a calculated oxide thickness of 3.7 nm.

References:
1. B.R. Strohmeier, Surf. Interface Anal. 15 (1990) 51.
2. T.A. Carlson, G.E. McGuire, J. Electron Spectrosc. Relat. Phenom, 1 (1972/73) 161.

Cu(0):Cu(II) or Cu(I):Cu(II) Calculations

The presence of the well known shake-up satellite found in Cu 2p spectra as an indication of the presence of Cu(II) species is well known. Recently, a study of the surface chemistry of the flotation separation of chalcocite (Cu₂S) from heazelwoodite (Ni₃S₂) employed a fitting procedure and calculation that quantifies the amount of Cu(II) species present on the surface of Cu(I) sulfide[1] as first developed by Jasieniak and Gerson[2] and now described in reference [3] and [4]. The calculation takes into account the photoelectron yield from both the main 2p_3/2 peak and the shake-up peak and is based on main peak/shake-up peak ratios derived from Cu(OH)₂ standard spectra.

Quantification of the amount of Cu(II) species on a Cu(0) or Cu(I) containing surface does appear to be possible. If, for example, a Cu metal surface is oxidized to Cu(II), the shake-up structure associated with the Cu(II) species can be used for a Cu(0):Cu(II) quantification. Alternatively if Cu(II) species and Cu(I) species are present, the Cu(I):Cu(II) ratio can be determined. This method of Cu(0):Cu(II) (or Cu(I):Cu(II)) determination depends on shake-up peaks that are present in the spectra of d⁹ Cu(II) containing samples but are absent in d¹⁰ Cu(0) (or Cu(I)) spectra. Shake-up peaks may occur when the outgoing photoelectron simultaneously interacts with a valence electron and excites it to a higher-energy level. The kinetic energy of the shaken-up core electron is then slightly reduced giving a satellite structure a few eV below (higher on the calculated BE scale) the core level position[5]. Hence, these electrons are part of the total Cu 2p emission and should be included in both total Cu and relative chemical state speciation. For example, the main emission line (A) in Figure 1 contains both Cu(II) (A1) and Cu(0) (A2) contributions but the satellite intensity (B) is entirely from Cu(II). The total intensity from Cu(II) species is represented in the combination of the signals from the direct photoemission (A1) and the shaken-up photoemission (B).

Accurate Cu(0):Cu(II) ratios for samples containing a mixture of Cu(0) and Cu(II) rely on determining an accurate ratio of the main peak /shake-up peak areas (A1_s/B_s) for a 100% pure Cu(II) sample. With a reliable value of A1_s/B_s obtained for Cu(OH)₂ or CuO (where all copper present is in the Cu(II) state), the relative concentrations of Cu(0) and Cu(II) species present on a surface that contains both species can be obtained by the following simple equations[3,4]:

% Cu(0) = A2/(A+B)*100 = (A-A1)/(A+B)*100 = (A-(A1_s/B_s)B)/(A+B)*100

% Cu(II) = (B+A1)/(A+B)*100 = B(1+(A1_s/B_s))/(A+B)*100

where B is the area of the shake-up peak and A is the total area of the main peak.

In order to determine accurate values of A1_s/B_s, seven Cu 2p_3/2 analyses of pure Cu(OH)₂ were obtained. Analyses were carried out on the various Cu(OH)₂ samples at acquisition times of generally less than a few minutes as it has been shown that reduction of Cu(OH)₂ can occur after extended X-ray exposure[6]. Our studies suggest that after X-ray exposures of 3 h, up to 10% of Cu(OH)₂ has been reduced to Cu(I). At pass energies of 20 eV and 40 eV, A1_s/B_s values of 1.57±0.1 and 1.59±0.1 were found, respectively. A similar analysis of a pure CuO sample was also carried out and gave a A1_s/B_s value of 1.89±0.08 (20 eV pass energy). Figure 1 shows spectra for a sputter cleaned metal surface, CuO and Cu(OH)₂ standards used for A1_s/B_s determination and a spectrum of the native oxide on a pure metal surface with the amount of oxidation of the surface calculated[3,4].

It should be noted that the peak-shape and main peak to shake-up peak separation is quite different for Cu(OH)₂ and CuO (Figure 1). This is useful (along with the O 1s signal if only Cu species are present) in determining which A1_s/B_s value to use for Cu(0):Cu(II) (or Cu(I):Cu(II)) calculations. If the Cu(0) or Cu(I) signal is relatively strong, (and the sample is conducting) some assessment of which is present in the sample may be made based on the BE of the 2p_3/2 peak[3,4].

An excel spreadsheet calculator that uses the equations above for Cu(II) determinations can be found here. (Note: you must download the file to Excel to use it - it is locked in Google Docs).

Figure 1. Cu 2p spectra for a sputter cleaned Cu metal surface (bottom), Cu₂O standard (2^nd from bottom, a small amount of Cu(II) was found in this sample), CuO standard (3^rd from bottom), Cu(OH)₂ standard (4^th from bottom) used for A1_s/B_s determination and a spectrum of a native oxide on a metal surface (top) with the proportion of Cu(0) and Cu(II) calculated [3,4].

References:

[1] M.C. Biesinger, B.R. Hart, R. Polack, B.A. Kobe, R.St.C. Smart, Miner. Eng. 20 (2007) 152.

[2] A.R. Gerson, M. Jasieniak, The Effect of Surface Oxidation on the Cu Activation of Pentlandite and Pyrrhotite, in: W.D. Duo, S.C. Yao, W.F. Liang, Z.L. Cheng; Long H. (Eds.), Proceedings of the XXIV International Minerals Processing Congress, Science Press Beijing, Beijing, China, 2008, pp. 1054-1063.

[3] M.C. Biesinger, L.W.M. Lau, A.R. Gerson, R.St.C. Smart, Appl. Surf. Sci. 257 (2010) 887-898.

[4] M.C. Biesinger, Surf. Interface Anal. 49 (2017) 1325-1334.

[5] J.F. Watts, J. Wolstenholme, An Introduction to Surface Analysis by XPS and AES, Wiley, Rexdale (2003) 71.

[6] W.M. Skinner, C.A. Prestidge, R.St.C. Smart, Surf. Interface Anal. 24 (1996) 620.

Converting From Atomic Percent to Weight Percent and Vice Versa

Sometimes it is necessary to convert from atomic percent to weight percent (or vice versa) to be able to compare XPS data to data from other techniques such as EDX.  This converter (Excel based spreadsheet) allows you to easily convert between the two values. (Note: you must download the file to Excel to use it - it is locked in Google Docs).

The Thickogram

Peter Cumpson[1] at NPL in the United Kingdom has developed a useful graphical method for measuring overlayer thickness in samples where the overlayer has a different elemental chemistry than the substrate (for example: a niobium film over a silicon substrate). The method allows for the following:

1) Uniform surface contamination (e.g. adventitious carbon layer) is not important.
2) Unknown instrumental factors common to substrate and oxide cancel out.
3) Uses a simple equation.
4) Works well for large and small film thickness.

Some rules:
1) Emission angle must be between 0 and 60 degrees (ie. Take-off angle of 90 to 30 degrees). Emission angles around 45 degrees are the most accurate.
2) Applicable to a wide range of Kinetic Energies above ~500 eV.
3) Error in result is +/-10% based on accuracy of attenuation lengths obtained by calculations.

Values Needed: o = Overlayer, s= Substrate
Io = Intensity of Overlayer Peak (or Peak Area)
Is = Intensity of Substrate Peak (or Peak Area)
So = R.S.F. of Overlayer Peak
Ss = R.S.F. of Substrate
Eo = K.E. of Overlayer Peak
Es = K.E. of Substrate Peak
Theta = emission angle, (0 for 90 degree take-off)
Cos(Theta) (=1 for 90 degree take-off)
Lambda o = attenuation length of photoelectrons (from the overlayer) in the overlayer.

How to Use It: A printable version of the Thickogram (Figure 1) can be downloaded here. An Excel spreadsheet that is useful for multiple calculations is presented here. (Note: you must download the file to Excel to use it - it is locked in Google Docs).
1) Calculate A = Io/So / Is/So, add point on Thickogram
2) Calculate B = Eo/Es, add point on Thickogram
3) Draw a line from A to B. Point C is found on the curve.
4) Thickness (t) is calculated as t = C(Lambda o)cos(Theta)



Figure 1. The Thickogram showing points A, B and C.
If you wish to use this method I would highly recommend reading through the original paper first [1].

Reference:
[1] Peter J. Cumpson, Surf. Interface Anal. 29, 403-406 (2000).