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. 

An Overview of X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS), alternatively named electron spectroscopy for chemical analysis (ESCA), is now a well-established surface analysis technique capable of providing elemental and chemical state information from the outer 5 to 10 nm of a solid surface. It is the subject of several well-known texts[1,2,3]. There is also an excellent recent review[4] of some of the latest technical advances. 

The technique is based on the photoelectric effect, discovered by Hertz in 1887, where the emission of electrons from a material occurs under photon irradiation. While early XPS experiments showed promise it was not until much later that the true power of the technique would be revealed. In the 1950s, Kai Siegbahn (Uppsala University, Sweden), using a high-resolution spectrometer and a Cu Kα X-ray source, for the first time resolved the sharp peak at the high kinetic “edge” seen by previous studies. This enabled, for the first time, accurate determination of photoelectron kinetic energies and thus core level binding energies. In 1981 Siegbahn was awarded the Nobel Prize for Physics for this pioneering work[5].

When an X-ray of known energy (hν), generally, with laboratory-based equipment either Al Kα at 1486.7eV or Mg Kα at 1253.6eV, interacts with an atom, a photoelectron can be emitted via the photoelectric effect (Figure 1). The emitted electron’s kinetic energy (E_k) can be measured and the atomic core level binding energy (E_b) relative to the Fermi level (E_F) of the sample can be determined using the following equation:

E_b = hν – E_k – Φ_sp                 

where Φ_sp is the work function of the spectrometer (typically 4 to 5 eV). The various core level binding energies observed in a spectrum can be used to identify all the elements of the periodic table except for hydrogen and helium. Chemical state information can also be extracted because binding energies are sensitive to the chemical environment of the atom. Chemical environments that deshield the atom of interest (i.e. are bound to strongly electron withdrawing groups) will cause the core electrons of that atom to have increased binding energies. Conversely, decreased binding energies will be measured for core electrons of atoms that withdraw electrons from their neighbouring atoms. Essentially, binding energy will generally increase as chemical state number increases. As an example, niobium metal has a 3d_5/2 binding energy of 202.2 eV, while niobium 2⁺, 4⁺ and 5⁺ oxides have binding energies of 203.7 eV, 206.2 eV and 207.4 eV. Tabulations of binding energies can be found in a variety of databases[6,7].

Figure 1. Schematic of the photoemission of a Ni 2p_3/2 electron from a nickel atom.

References:
[1] S. Hufner, Photoelectron Spectroscopy, Solid State Science Series, vol. 82 Springer, Berlin, 1995.
[2] D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, John Wiley & Sons, Chichester, 1983.
[3] D. Briggs, J.T. Grant (Eds.),Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, IM Publications, Chichester, 2003.
[4] C.S. Fadley, J. Electron Spectrosc. Relat. Phenom. 178/179 (2010) 2.
[5] D. Briggs, J.T. Grant, Perspectives on XPS and AES, in: D. Briggs, J.T. Grant (Eds.), Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, IM Publications, Chichester, 2003, pp. 1-12.
[6] 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.
[7] 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.

XPS Instrument Manufacturers

The Kratos AXIS Supra (front, installed 2018) and Kratos AXIS Nova (back - installed 2007) located at Surface Science Western at the University of Western Ontario. These instruments are equipped with a variety of X-ray sources and sample preparation options.

The Kratos AXIS Ultra (front, installed 2000, decomissioned 2018) and Kratos AXIS Nova (back) located at Surface Science Western at the University of Western Ontario.

Some of the main XPS manufacturers include:
Kratos Analytical
Thermo Scientific
Physical Electronics (PHI)
VG Scienta

Example of XPS Description for Publications

The following is an example of an XPS analyses description used in the experimental section of a publication. Your description may have to be modified from this somewhat if non-standard conditions were used.

X-ray Photoelectron Spectroscopy Analyses:
The XPS analyses were carried out with a Kratos Axis Supra spectrometer using a monochromatic Al K(alpha) source (15mA, 15kV). XPS can detect all elements except hydrogen and helium, probes the surface of the sample to a depth of 5-7 nanometres, and has detection limits ranging from 0.1 to 0.5 atomic percent depending on the element. The instrument work function was calibrated to give a binding energy (BE) of 83.96 eV for the Au 4f7/2 line for metallic gold and the spectrometer dispersion was adjusted to give a BE of 932.62 eV for the Cu 2p3/2 line of metallic copper. The Kratos charge neutralizer system was used on all specimens. Survey scan analyses were carried out with an analysis area of 300 x 700 microns and a pass energy of 160 eV. High resolution analyses were carried out with an analysis area of 300 x 700 microns and a pass energy of 20 eV.  Ag 3d5/2 line FWHM at 20 eV pass energy was 0.55 eV.  All analyses were taken at a 90 degree take-off angle. Instrument base pressure was 1 x10-9 Torr. 

Spectra have been charge corrected to the main line of the carbon 1s spectrum (adventitious carbon) set to 284.8 eV. Charge neutralization was deemed to have been fully achieved by monitoring the C 1s signal for adventitious carbon. A sharp main peak with no lower binding energy structure is generally expected. Spectra were analysed using CasaXPS software (version 2.3.26). A standard Shirley background was used for all spectra. 

For other examples see:
[1] M.C. Biesinger, C. Brown, J.R. Mycroft, R.D. Davidson and N.S. McIntyre, Surf. Interface Anal. 36, 1550-83 (2004).
[2] M.C. Biesinger, R. Pollack, B.R. Hart, B.A. Kobe and R. St.C. Smart, Minerals Engineering, 20, 152-162 (2007).
[3] S.C. Ghosh, M.C. Biesinger, R.R. LaPierre and P. Kruse, Journal of Applied Physics, 101, 114322 (2007).

[4] M.C. Biesinger, Applied Surface Science, 597 (2022) 153681.

XPS Fundamentals

X-ray photoelectron spectroscopy (XPS) or electron spectroscopy for chemical analysis (ESCA) is a surface analysis technique capable of providing elemental and chemical state information from the outer 5 to 10 nanometres of a solid surface. All elements from lithium to uranium can be detected with detection limits in the 0.1 to 0.5 atomic percent level.

When an X-ray of known energy (hν), generally Al(Kα) at 1486.7eV or Mg(Kα) at 1253.6eV, interacts with an atom, a photoelectron can be emitted via the photoelectric effect. The emitted electron’s kinetic energy (Ek) can be measured and the atomic core level binding energy (Eb) relative to the Fermi level (EF) of the sample is determined by the following equation:

Eb = hν – Ek – Φsp (1)

where Φsp is the work function of the spectrometer. Chemical information about the sample can be extracted because binding energies are sensitive to the chemical environment of the atom. Chemical environments that deshield the atom of interest (i.e. are bound to strongly electron withdrawing groups) will cause the core electrons of that atom to have increased binding energies. Conversely, decreased binding energies will be measured for core electrons of atoms that withdraw electrons from others. Essentially, binding energy will increase as chemical state number increases.

XPS, with its ability to quantify elements and determine chemical states, is used in many branches of materials science, electronics, thin film chemistry, corrosion science, polymer modification, adhesion science, coating chemistry, catalysis, mineral processing chemistry, as well as in exploring fundamental aspects of the chemistry and physics of atoms and molecules.

X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS or ESCA), is an analytical technique that depends upon the measurement of the energies of photoelectrons that are emitted from atoms when they are irradiated by soft X-ray photons (1 - 2 keV). When used to study solids, XPS has a number of powerful attributes, including a high (and variable) range of sensitivities to structures on the outermost surface of the solid, an ability to identify such structures chemically, a reasonable capacity for elemental quantification, as well as the ability to determine structure thickness. As a method for characterizing surface composition, there is no single other technique that can compare with XPS, in terms of the wealth of useful information, reliability of the data, and ease of interpretation. In addition to the above, an XPS imaging mode has emerged that was hardly even anticipated 10 years ago. Since its introduction in 1970, the technique has produced an extraordinary amount of useful information, both for academic and industrial scientists. These developments have had strong influences on our views of surface chemistry, physics, and engineering.
Advancements in spectrometer technology have resulted in major improvements in spectral resolution and counting efficiency over the past 20 years. This has dramatically improved the level of confidence in spectral positions, and the ability to carry out analyses in numbers that have much better statistical significance. The exploitation of the imaging developments is likely the most exciting prospect, because, historically, little research has been done using highly-resolved XPS images. The recognition of co-localization of different species (elemental or chemical) will be one of the most powerful elements shaping XPS in the future.