Asymmetric Peak Shapes

For conductive samples, such as metals and graphite, there is a distribution of unfilled one-electron levels (conduction electrons) that are available for shake-up like events following core electron photoemission. When this occurs, instead of a discrete structure like that seen for shake-up satellites, a tail on the higher binding energy side of the main peak – an asymmetric peak shape is evident[1]. An example of this is shown in Figure 1 for a sputter cleaned vanadium metal surface. It is clear from this figure that the asymmetric tail of the metal peak shape will overlap with higher oxidation state species. As such it is important that the total photoelectron yield contribution from the metal is captured during curve-fitting analysis. The use of standard spectra that is fit with mathematically derived asymmetric peak shapes allows for this.

Figure 1. Asymmetric peak shapes in the V 2p spectrum of an argon ion sputter cleaned surface of vanadium metal [2].

David Morgan at Cardiff University has recently published an excellent insight article [3] on asymmetric peak shapes in XPS.  This article goes into detail about the causes of asymmetry, curve-fitting of asymmetric peaks, implications of using hard X-ray sources (HAXPES), and asymmetry in other materials such as conductive metal oxides, graphitic materials, and polymeric materials. Well worth the read for a more in-depth look.

References: 

[1] D. Briggs, XPS: Basic Principles, Spectral Features and Qualitative Analysis, in: D. Briggs, J.T. Grant (Eds.), Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, IM Publications, Chichester, 2003, pp. 31-56.
[2] M.C. Biesinger, L.W.M. Lau, A.R. Gerson, R.St.C. Smart, Resolving Surface Chemical States in XPS Analysis of First Row Transition Metals, Oxides and Hydroxides: Sc, Ti, V,Cu and Zn, Applied Surface Science, 257 (2010) 887-898.

[3] D.J. Morgan, XPS insights: Asymmetric peak shapes in XPS, Surface and Interface Analysis, 55 (2023) 567-571.

Polyethylene Surfaces

The C 1s spectrum of polyethylene shows vibrational structure[1] that leads to an asymmetric peak-shape.  If this vibrational structure is not accounted for it can lead to an erroneous assignment of C-OH, C-O-C or an overestimate of contamination/oxidation species[2]. A fitting of a series of polyethylene standards (Figure 1) as well as a fitting of a C18 alkane (Figure 2) on a clean silicon wafer gave consistent peak-shape results and were fit with an asymmetric peak-shape defined in CasaXPS as LA(4.2,9,4) and with peak widths of 0.64-0.65 eV (at 10 eV pass energy) and 0.67-0.68 eV (at 20 eV pass energy). Peak fitting parameters for PE surfaces with small amounts of oxidation and/or contamination are presented in Table 1 and an example is shown in Figure 3. 

Table 1. Contaminated/oxidized polyethylene surface C 1s fitting parameters.

Figure 1. C 1s spectrum of a standard polyethylene sample (20 eV pass energy).

Figure 2. C 1s spectrum of C18 alkane (10 eV pass energy).

Figure 3. C 1s spectrum of a contaminated/oxidized PE surface using the fitting parameters from Table 1.

A similar analysis of polypropylene (Figure 4) gave a peak that can be fit with an asymmetric peak-shape defined in CasaXPS as LA(5.5,9,4) and with peak-widths of 0.82 eV for a 10 eV pass energy and 0.83 eV for a 20 eV pass energy.

Figure 4. C 1s spectrum of a standard polypropylene sample (20 eV pass energy).

References:
[1] G. Beamson, D. Briggs, High Resolution XPS of Organic Polymers - The Scienta ESCA300 Database Wiley Interscience, 1992.
[2] M.J. Walzak, pers. comm. 2015.

Graphite

Graphite and graphitic-like compounds (including carbon nanotubes) have an asymmetric C 1s peak-shape (as they are conductive) centered at 284.5 eV. Also present will be structure related to the pi to pi* transition (shake-up) at around 290.9 eV. Example shown below, click here for a CasaXPS ready file.