Table 1 lists Cu 2p3/2 BE and modified Auger parameter values from a survey of literature sources compiled in the NIST Database[1]. Of note here is the statistically similar BE values for the Cu metal and Cu(I) oxide species. The use of the modified Auger parameter (2p3/2, L3M45M45) as well as an inspection of the Auger peak-shape do allow for a more accurate assignment for these species and has been used effectively. Goh et al.[2] have shown (in their Figure 8) the distinctly different peak shapes of the X-ray generated Auger LMM spectra for copper as the metal, Cu2S and CuS. They also note the distinctive Cu L3M4,5M4,5 peak at 916.5 eV for Cu2O. Poulston et al.[3], in their study of surface oxidation and reduction of Cu2O and CuO, have used both the Cu LMM and the Auger parameter to distinguish Cu(0), Cu(I) and Cu(II). These parameters are very useful for identification of the different states present in the surface but they are difficult to quantify as relative amounts of each species. The Cu 2p XPS spectrum is still the signal most used for this purpose.
Table 2 shows similar results to those shown in Table 1 from our work [4] for a series of standard samples. In this analysis, a statistical separation of the Cu 2p3/2 peak position for Cu(0) and Cu2O is achieved. This should be expected, as most spectrometer calibration procedures include referencing to the ISO standard Cu metal line at 932.63 eV with deviation of this line set at ±0.025 eV. Curve-fitting of the Cu 2p3/2 line for both Cu metal and Cu2O employed Gaussian (10 %) – Lorentzian (90 %) p and Gaussian (20 %) – Lorentzian (80 %) peak-shapes, respectively (defined in CasaXPS as GL(90) and GL(80)). Peak-shapes for these species are shown in Figure 1.
In practice, quantifying a mix of Cu(0), Cu(I) and Cu(II) species would require precise constraints on BE, FWHM, and peak-shape parameters. Resolution of these components will be difficult with larger amounts of Cu(II) compounds present due to the overlap of peaks for these three components. It may be possible to fit the 2p3/2 spectrum using a set of constrained peaks that simulate the entire peak-shape (including the shake-up components) for the Cu(II) species present (Table 3) [4].
Table 1. Cu 2p3/2 and modified Auger parameter literature values for Cu species (compiled from reference [1]).
Table 2. Cu 2p3/2 and modified Auger parameter values for Cu species from [4]. [a) 932.63 eV for non-monochromatic Al X-ray source, 932.62 eV for monochromatic Al K(alpha) X-ray source]
Table 3. Cu 2p3/2 fitting parameters for Cu(II) species [4].
Figure 1. Cu 2p spectra for a sputter cleaned Cu metal surface (bottom), Cu2O standard (2nd from bottom, a small amount of Cu(II) was found in this sample), CuO standard (3rd from bottom) and Cu(OH)2 standard (top) [4].
An advanced analysis of copper XPS spectra is now available here and in reference [5].
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] S.W. Goh, A.N. Buckley, R.N. Lamb, R.A. Rosenberg, D. Moran, Geochim. Cosmochim. Acta 70 (2006) 2210.
[3] S. Poulston, P.M. Parlett, P. Stone, M. Bowker, Surf. Interface Anal. 24 (1996) 811.
