The Chemical Significance of XPS BE Shifts: A Perspective

A recent publication from Paul Bagus (University of North Texas), Connie Nelin, and Dick Brundle [1] discusses the chemical significance of XPS BE shifts. Paul, Connie and Dick have made many outstanding contributions to the field of XPS, in particular by using computational chemistry approaches to model various XPS phenomena and spectral shapes - especially of transition metal species with complex multiplet splitting and satellite structures. 

Dr. Bagus describes this perspective below. This will be a good starting point for researchers interested in applying MO theory to XPS measurements. 

An all too common interpretation of the shifts of XPS BEs, Delta BE, for a given element in different compounds and in different environments is to relate the sign of the BE shift to the change in the effective charge, Q, of the core ionized atom. Thus, a shift to lower BE from sample 1 to sample 2 is interpreted as meaning that the atom in sample 2 has a smaller positive Q or a larger negative Q than the same atom in sample 1. Similarly, a shift to a larger BE is taken to mean that the atom in sample 2 has a larger Q. This paper shows that this simple interpretation of BE shifts is incomplete and that it is likely to be misleading.

While the effective charge Q does contribute to BE shifts, it is not the only physical or chemical mechanism that can contribute to XPS BE shifts. Two other mechanisms are the environment of the ionized atom that can lead to electrostatic potential that are different at different sites in a given sample and are different for different samples. Another mechanism is the degree of hybridization of an atom again at different sites and different compounds. An important objective of this perspective is to examine the mechanisms that lead to BE shifts. The chemical and physical content of these different mechanisms is first examined for a model system. With this model system, the different mechanisms can be separated and the magnitudes of the XPS BE shifts due to the different mechanisms can be understood directly in terms of the electronic charge distribution. Then five specific examples of XPS BEs measured for real systems are discussed and the observed BEs related to the physical and chemical mechanisms which are the origin of the BE shifts. The paper also considers the initial and final state contributions to the BE shifts and identifies when it is likely that initial state effects will dominate.

An important goal of the paper is that the principles and mechanisms for BE shifts can be applied, not only to the specific systems discussed in the paper but also to the understanding of the Delta BE for systems in general. It can lead readers to make suggestions for theoretical studies to help explain specific observations of BE shifts.

Reference:

[1] P.S. Bagus, C.J. Nelin, C.R. Brundle, J. Vac. Sci. Technol. A 41 068501 (2023).

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.

Video - XPS: The Basics, Curve-Fitting and Advanced Studies

A recent lecture given by Dr. Biesinger to the Canadian Biomaterials Society - hosted by the Quebec City Student Chapter of the Canadian Biomaterials Society (CBS-QCSC) at Laval University.

Resources

The NIST XPS database is an absolutely essential tool for tracking down binding energy references and a wealth of other valuable data.

NIST X-ray Photoelectron Spectroscopy Database  

SESSA software is a convenient tool for simulating XPS intensities for simple/complex samples and experimental configurations (such as e.g. synchrotrons).

NIST Database for the Simulation of Electron Spectra for Surface Analysis (SESSA)

FEFF is an automated program for ab initio multiple scattering calculations of X-ray Absorption Fine Structure (XAFS), X-ray Absorption Near-Edge Structure (XANES) and various other spectra for clusters of atoms. The code yields scattering amplitudes and phases used in many modern XAFS analysis codes, as well as various other properties.

The FEFF Code

XPS Prediction Server - Caro Research Group (Aalto University): Accurate computational prediction of core-electron binding energies in carbon-based materials: A machine-learning model combining DFT and GW.

XPS Prediction Server

Other online XPS Databases

ThermoFisher Table of Elements: X-ray photoelectron spectroscopy of atomic elements

The International XPS Database of Monochromatic Reference Spectra

The XPS Library

HarwellXPS GURU

Most Intense/Most Used XPS Core Line Periodic Table

Periodic table of most intense/most used XPS core lines[1].



A printable pdf version of this chart can be downloaded here.

This table is useful to XPS users for the following reasons:
1. Most elements produce more than one peak in XPS. Thus, it is nice to have a convenient tabulation of the peaks that are most intense/most used in XPS analyses – this information provides a good starting point for an analysis.
2. An obvious, but still quite important, effect shown in the table is that the binding energy for a peak from a given orbital always increases as Z increases. This result would be expected from Coulomb’s law – the more protons in the nucleus, the more strongly the electrons are held in the atom.
3. As an important practical consideration, all of the peak energies listed in the table have binding energies less than 1200 eV. This choice allows the table to be used by analysts using both Al Kα and Mg Kα X-rays, which have energies of 1487 and 1254 eV, respectively. That is, this table is specifically targeted to those doing conventional XPS.
4. The table helps organize our thinking about XPS analysis. That is, one might regularly analyze a subset of the elements by XPS and memorize the best peaks for analyzing each of them without seeing how the preferred peaks for the different elements are related. This periodic table shows that the peaks that are most often used for XPS analysis are not randomly chosen among the possible signals produced by the atoms. Indeed, elements with about the same Z value generally have the same recommended/preferred peak. For example, beginning with lithium, which is the first element that gives an XPS signal, the 1s orbital produces the best signal for analyzing all of the subsequent elements up to Na. Then, from Mg to Si, the 2p orbital peak is preferred. After that, from P to Ga, the 2p3/2 peak is the preferred one, and so on. One of the reasons for the shifts we see here from one orbital to another as Z increases is that at some point, the binding energy of the orbital exceeds the energy of the probing X-ray, so a different orbital has to be considered.
5. We emphasize that the table lists ‘nominal’ binding energies, not exact ones. Clearly, it would not be possible for the table to list the exact positions of the peaks one may find in one’s XPS analysis because (i) elements in XPS undergo chemical shifts in response to the chemical environments they are in, and (ii) insulating samples often require charge compensation, which, in practice, can move peaks to higher or lower binding energies by a few eV.
6. As a follow up to the previous comment, we note that even though the table gives the best peak for an element for an analysis, e.g., for quantitative work, this does not mean that one should ignore the other peaks from that element in the spectrum. It is always a good idea to confirm the presence of an element in an analysis by making sure that the multiple signals expected from it are present in about the ratios expected for them.7
7. The recommended peak for some of the elements is an entire orbital that undergoes spin-orbit splitting, e.g., the recommended orbital for silicon is the 2p orbital, while for other elements the recommended peak is one of the two spin-orbit peaks, e.g., for the element after silicon, phosphorus, the recommended peak is the 2p3/2 signal. These recommendations are simply based on the energy difference between the spin-orbit peaks for a given element – when there is a large energy difference between the spin-orbit components, only one of them is listed, but when the peaks are too close to resolve well, the pair of them is recommended. For example, the recommended signal for Mg is the 2p peak, which only has a separation of 0.28 eV between its 2p3/2 and 2p1/2 components – these two peaks will show significant overlap by conventional XPS. Similarly, the 2p peak is recommended for Al, which has closely spaced spin-orbit components (0.44 eV energy difference), and the 3d peak is recommended for Ge (0.58 eV difference between its spin-orbit components).
8. In every case where one of the two spin-orbit peaks for an element is recommended, the peak with the higher j value is listed, i.e., the j = 3/2 state is recommended instead of the j = 1/2 state for p orbitals, the j = 5/2 state is recommended instead of the j = 3/2 state for d orbitals, and the j = 7/2 state is recommended instead of the j = 5/2 state for f orbitals. In each of these cases, the recommended peak corresponds to the larger of the two spin-orbit peaks, which should give a better signal-to-noise ratio for the measurement. In addition, the peak with the higher j value always comes at lower binding energy, which means that its background can overlap with the peak with the smaller j value, potentially complicating its analysis.
9. As a final note, we emphasize that the table here contains recommendations, not requirements. While XPS is convenient because most elements only show a few peaks, so overlaps between them are not extremely common, overlaps between peaks do occur. If the peak recommended in this table overlaps with a signal from another element, it may be best to consider a different peak from the element in question.

Reference:

[1] G.H. Major, G. Pinto, M.C. Biesinger, M.R. Linford, Vacuum Technology and Coating, July (2020) 34-36. 

X-ray Degradation of Cu(OH)2

X-ray induced degradation of copper (II) species can complicate interpretation of results. One way to mitigate this issue is to perform the analysis of the Cu 2p (and Cu LMM Auger line) first and in as few scans as possible, then perform subsequent needed analysis afterward (e.g. other high resolution spectra, survey scans).  If the mechanism of reduction is due to thermal effects, as it appears to be for Cu(OH)2, cooling of the material can reduce the amount of degradation significantly. The charts below for copper (II) hydroxide samples analyzed by XPS at normal operating temperatures (top) and cooled to -100C (bottom) using the same X-ray source (15 kV, 14 mA, monochromatic Al K(alpha)) and charge neutralizer conditions (Kratos AXIS Ultra system) show that degradation is slowed significantly for the cooled sample.  Notably, degradation is minimal for the cooled sample for the initial window of analysis. 

X-ray induced degradation of Cu(OH)2 - normal operating temperatures.

X-ray induced degradation of Cu(OH)2 - sample cooled to -100C.

Video: Advanced Analysis of Copper XPS Spectra

2020 Kratos North American User Meeting talk by Dr. Mark Biesinger, Director of Surface Science Western at Western University, London, Ontario, Canada.  Various strategies for the analysis of Cu XPS (X-ray photoelectron spectroscopy) spectra.

Rhenium

Re 4f7/2 binding energy values [1].

Re 4f7/2 - 4f5/2 splitting: 2.41 eV +/- 0.01 eV, [2] sets it at 2.43 eV
Re 5s: 99 eV
Re 4d5/2: 260 eV
Re 4d3/2: 274 eV
Re 4p3/2: 446 eV
Re 4p1/2: 518 eV
Re 4s: 625 eV

Re 4f spectrum of sputter cleaned Re metal.

Peakshape for Re 4f peaks from a Re metal standard in the figure above is LA(1.16,3,10).

Re 4d spectrum of sputter cleaned Re metal.

Peakshape for Re 4d peaks from a Re metal standard in the figure above is GL(80).

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] 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.

Rubidium

Rb 3d5/2 binding energies.

Rb 3d5/2 - 3d3/2 splitting: ~ 1.48 eV
Rb 3p3/2: 240 eV
Rb 3p1/2: 249 eV
Rb 3s: 325 eV
Rb 4p: 16 eV
Rb 4s: 31 eV

Reference:
[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.

Potassium

K 2p3/2 binding energies.

K 2p3/2-1/2 splitting is 2.77 to 2.8 eV.

Notes: Often will see K 2p peaks in high resolution C 1s spectra. Be careful not to confuse these peaks with contributions from C 1s fluorocarbon peaks or pi -> pi* shake-up structure.

For quantification in survey scans it is often easier to use the K 2s peak (~380 eV) as the K 2p peak may not be fully separated from the C 1s peak at lower resolutions.