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.
[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.