Advanced Quantification Combining Survey Scan and High Resolution Data

Using a combination of the results from survey scans and fitted high resolution data one can, using relatively simple math, develop a clear picture of the different species on a surface.  Recent work [1,2] highlights this and will be used as an example below.

A series of oxide films of Ni-Cr-Mo alloys (corrosion resistant alloys in both oxidizing and reducing environments) where analysed by XPS.  Survey scans (how to quantify here) and high resolution scans of the Ni 2p, Cr 2p, Mo 3d, O 1s and C 1s peaks were taken and the various chemical states determined and quantified using the curve-fitting procedures from [3-5] and [6].


Figure 1. XPS survey spectrum recorded on C22 after polarization at 0 V (+) in a pH = 7 solution.

Figure 2. Surface composition (normalized) obtained from the survey spectra of (a) C22 and (b) BC1 alloy after polarization at, 0 V and 0.5 V (positive scan) and 0 V and -0.4 V (negative scan) at pH = 7 solution. (The small amount of adventitious carbon has been factored out of these quantifications.)  

Carbon, which was mainly present as a small amount of adventitious carbon, has been simply factored out of the quantification in this case.  In cases where carbon plays a bigger role or is part of the species of interest one must include it in the calculations.  For example, if you want to calculate the contribution of carbonaceous oxygen in the O 1s spectrum you can use this calculator.


Figure 3. High-resolution deconvoluted XPS spectra for (a) O 1s, (b) Ni 2p, (c) Cr 2p and (d) Mo 3d collected on C22 at 0 V (+) and pH = 7.

These deconvoluted spectra will give us the percentage of each species as a function of each element. For example, 70% of the total Cr is present as Cr(OH)3, 20 % as Cr2O3 and 10 % as Cr metal.


Figure 4. Normalized relative film composition (%) of Ni, Cr and Mo and their relative metal, oxide, hydroxide components present in films polarized at specific potentials for (a) C22 and (b) BC1 alloy.

In Figure 4 the percentages of each oxide and metal species, from the high resolution data, has been combined with the amounts of each element in the survey scan data to give us a full picture of the amount of each species present at the surface of these films.

Caveat: Depth effects have not been accounted for here. Further work using Auger and/or XPS depth profiling, angle resolved analysis or QUASES analysis can help to further clarify the positions of various species. It is of course assumed here that the metal detected is from the underlying alloy.

References:
[1] Ebrahimi, Nafiseh, "The Influence of Alloying Elements on The Crevice Corrosion Behaviour of Ni-Cr-Mo Alloys" (2015). Electronic Thesis and Dissertation Repository. 3316.  http://ir.lib.uwo.ca/etd/3316
[2] N. Ebrahami, M.C. Biesinger,D.W. Shoesmith, J.J. Noel, The Influence of Chromium and Molybdenum on the Repassivationof Nickel-Chromium-Molybdenum Alloys in Saline Solutions, Surface and Interface Analysis, 49 (2017) 1359.
[3] M.C. Biesinger, B.P. Payne, A.P. Grosvenor, L.W. Lau, A.R. Gerson, R.St.C. Smart, Appl. Surf. Sci. 257 (2011) 2717.
[4] M.C. Biesinger, B.P. Payne, L.W. Lau, A.R. Gerson, R.St.C. Smart, Surf. Interface Anal. 41 (2009) 324.
[5] M.C. Biesinger, C. Brown, J.R. Mycroft, R.D. Davidson, N.S. McIntyre, Surf. Interface Anal. 36 (2004) 1550.
[6] P. Spevack, N.S. McIntyre, J. Phys. Chem. 96 (1992) 9029.

Quantitative Analysis

Semi-quantitative analysis is possible by measuring the peak areas of specific elemental core lines (I) and by applying appropriate atomic sensitivity factors (S), also known as relative sensitivity factors (RSF), using the general equation:

C_x = (I_x/S_x) / (∑I_i/S_i) 

where C_x is the atomic fraction of element x in a sample[1]. The sensitivity factors can be calculated from theory or derived empirically from the analysis of standard samples. The use of standard samples is the preferred method (and is the method applied in the Kratos line of spectrometers). Peak areas are defined by applying an appropriate background correction across the binding energy range of the peaks of interest. In general, three types of backgrounds are used: 1) a simple straight line or linear background, 2) the Shirley background in which the background intensity at any given binding energy is proportional to the intensity of the total peak area above the background in the lower binding energy peak range[2] (i.e. the background goes up in proportion to the total number of secondary photoelectrons below its binding energy position) and 3) the Tougaard background (or Tougaard universal cross-section approach) which offers practical background computation (based on electron energy losses) with more control over the background shape then the Shirley procedure[3]. The simple linear background suffers from large peak area changes depending on the position of the chosen end points and is the least accurate. The Tougaard background is the most accurate but suffers from complications in practical use, particularly if there are peak overlaps at binding energies above the integrated peak. The Shirley background is reasonably accurate and its ease of use has resulted in its widespread adoption.

References:
[1] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp, Eden Prairie, MN, 1992.
[2] M.P. Seah, Quantification of AES and XPS, in: D. Briggs, M.P.Seah (Eds.), Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, John Wiley & Sons, Chichester UK, 1983, p. 204.
[3] Neal Fairley, XPS lineshapes and Curve Fitting, in: D. Briggs, J.T. Grant (Eds.), Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, IM Publications, Chichester UK, 2003, p. 398.

Relative Sensitivity Factors (R.S.F.)

In some instances, different portions of the various core levels (other than those normally used) need to be used for quantification. For example: when overlaps of lines from different species are present it may be necessary to quantify using only the 2p3/2 line instead of the total 2p lineshape. Table 1 presents common relative sensitivity factor (R.S.F.) values derived for CasaXPS based on R.S.F. values used for the Kratos AXIS Supra, Ultra and Nova (this may differ from your instrument - check your library values versus the standard values listed (example: 2p, 3d, 4f, F 1s = 1) before using the derived values (example: 2p3/2, 3d5/2, 4f7/2)). Values that have been derived for specific cases based on standard samples are also presented (example: Mg KLL, Ba for BaTiO3, Eu 3d and U 4f).

To calculate a new R.S.F. for other elements/core lines (for the R.S.F. table type used by the CasaXPS/ Kratos Supra/Ultra/Nova combination) the following formula are used.

RSFp3/2 = RSFp * 2/3

RSFp1/2 = RSFp * 1/3

RSFd5/2 = RSFd * 3/5

RSFd3/2 = RSFd * 2/5

RSFf7/2 = RSFf * 4/7

RSFf5/2 = RSFf * 3/7

Table 1. Relative sensitivity factors for CasaXPS / Kratos Supra  / Ultra / Nova.

a) Use a window of 298-300eV (see Mg link for more details)

b) Calculated from BaTiO3 standard sample

c) New value from standard sample

d) Calculated values based on RSF's from SSX-100 XPS (U 4f = 48.86, Th 4f= 42.75) ratiod to Th 4f from Kratos instruments (Th 4f = 12.75). 48.86/42.75 = RSF(U 4f Kratos)/12.75; RSF(U 4f Kratos) = 14.544. It would be useful if this value could be confirmed with a well characterized UO2 sample.