Advanced Auger Analysis

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Advanced Auger Analysis

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Advanced Auger Analysis #

Mark Isaacs

Updated on August 22, 2025

Quick Overview #

Technique Overview #

The Auger parameter is a powerful tool for chemical analysis due to its sensitivity to the local chemical environment, its independence from charging, and its ability to provide insights into both initial- and final-state effects. The use of Auger parameters and Wagner plots can significantly enhance the information obtained from XPS measurements.

Applications and Fields #

  • Analysing metal-ligand bonding
  • Determining the surface stoichiometry of thin films
  • Characterizing catalysts and interfaces
  • Studying the effect of crystallinity and adsorption
  • Analysing free atoms, elemental solids, and atoms in molecules
  • Investigating the electronic properties of clusters on different supports
  • Identifying different coordination environments in materials
Wagner plot for unknown nickel samples

What is the Auger Parameter? #

If you don’t want through some of the maths behind these transitions – visit our page on the Auger parameter!
The Auger parameter is a quantity derived from both photoelectron and Auger electron kinetic energies that is used to characterize the chemical and physical state of a material. It is defined as the sum of the kinetic energy of an Auger electron and the binding energy of a core electron.(1) Because it uses the difference between two line energies from the same element in the same sample, the Auger parameter is independent of static charge corrections and work function measurements.(2) The Auger parameter is especially useful for studying the chemical environment of core-ionized atoms because its shifts are equal to twice the change in extra-atomic relaxation energy, which reflects the electronic interaction with surrounding atoms. The parameter is also a direct measure of the polarizability of the chemical environment of the core-ionized atom.(3)

What Can This Actually Tell Us? #

The Auger parameter is sensitive to the chemical environment of an atom. Changes in the Auger parameter indicate changes in the chemical state of an element. By comparing the Auger parameter of a material to reference values, the oxidation state and the type of chemical bonds can be identified.(3)
 
For example, the Auger parameter has been used to identify different chemical states of copper,(4) nickel,(5) gallium,(6) and silver(7) compounds.
 
By separating our auger parameter into initial and final state contributions we can gain insights into the ground state charge distribution of an atom, before core ionization occurs, which reflects the chemical bonds formed by the atom and the local electrostatic potential (initial state), and the relaxation energy gained from the electronic interaction with the environment after a core hole is created, and the screening efficiency of the surrounding atom.
 
This can provide more specific information depending on the system.

Auger Parameter for Characterising Small Metal Nanoparticles #

Binding Energy Shifts: #

 
Small clusters of metals supported on poorly conducting substrates generally exhibit positive binding-energy shifts. These shifts were initially thought to arise from a unit positive charge on the cluster, however, it has been shown that binding energy shifts and linewidth broadening primarily depend on the cluster size and cluster/substrate interaction, rather than solely electrostatic effects Theoretical studies show that core-level shifts arise from initial-state effects and scale inversely with cluster size.(8)
Lattice contraction in small metal particles increases d-electron involvement in bonding, contributing to initial state shifts in binding energy.(9) This means that the chemical environment of a metal atom is different in smaller particles, as compared to larger particles or bulk materials, due to the compression of the lattice. 
 

Auger Parameter Shifts #

As the size of metal nanoparticles decreases, the Auger parameter generally shifts to lower values compared to the bulk material. This shift indicates changes in the electronic environment, particularly in the screening of core holes (final state contributions).

Reduced Screening: Smaller nanoparticles exhibit reduced screening ability. This means that the core hole created during the photoemission process is less effectively shielded by the surrounding electrons compared to bulk metals.
 
Non-local Screening: The reduced screening in smaller nanoparticles is often attributed to the non-local screening mechanism, where the polarization of neighbouring atoms plays a key role. In this mechanism, the screening is not just from electrons in the same atom but from the surrounding environment, which is less effective for surface atoms of smaller particles that have fewer neighbors.

The Auger parameter is very sensitive to the chemical nature of the nearest-neighbour ligands of the core-ionized atom

Local vs Non-Local Screening: The local screening mechanism involves electron transfer from the nearest-neighbour ligands into a localized atomic orbital of the core-ionized atom, while the non-local mechanism involves population of spatially extended s and p orbitals from ligand valence orbitals. For transition metal ions with localized d orbitals, the Auger parameter is relatively independent of the chemical nature of the ligands, while the Auger parameter is more sensitive to chemical nature of the ligands in the case of non-local screening.(10)

 

Quantifying Size and Shape (11) #

Growth Shape: The shape of the nanoparticles can be inferred from the Auger parameter’s behaviour as a function of coverage or particle size, as different shapes such as columns or hemispheres result in different rates of change in the Auger parameter. For example, a columnar growth shows a sharper change in the Auger parameter compared to a hemispherical growth.
 
XPS Peak Analysis: Combining Auger parameter analysis with XPS peak shape analysis can give information about the particle size, shape and the degree of substrate coverage.

Auger Parameter for Electronic Analysis #

The Auger parameter has been shown to correlate with the band gap of materials, particularly in semiconductors and oxides. This relationship arises because the Auger parameter reflects the electronic environment and polarization properties of a material, which are also related to the band gap.(12)

Correlation between Auger Parameter and Band Gap:
A change in the Auger parameter for various semiconducting materials with respect to the metal state has been correlated to the energy gap of the material. For a series of compounds, a linear correlation has been demonstrated between their bandgap and the magnitude of the Auger parameter of their constituent cations (e.g. deposited titania films).(13)
 
Electronic Structure and Polarization:
The Auger parameter is sensitive to the electronic structure and polarization of the material. It is a measure of the extra-atomic relaxation or screening effect of the surrounding medium on the final ion state. The magnitude of the Auger parameter is a reflection of the degree of screening of a core hole in a given compound.
The band gap, which is the energy difference between the valence band and the conduction band, is also a fundamental property of the electronic structure of a material. The ability of the material to screen core holes and its band gap are both influenced by the electronic environment and the polarizability of the atoms.

Auger Parameter for Metal-Support Effects #

In small metal nanoparticles, charge transfer between the metal atoms and the support material or surface adsorbates can occur.(2,3) By analyzing the initial state contributions, it’s possible to determine how the electron density around the metal atoms is altered, which impacts their chemical and catalytic properties.(8)

Support Material and Polarizability:
The polarizability of the support plays a crucial role in determining the Auger parameter of the supported nanoparticles.
A more polarizable support can lead to a larger shift in the Auger parameter of the nanoparticle.(14)
When the support has high polarizability, the Auger parameter shift can be explained by a change in metal-ligand distance or local structure.
 
Charge Transfer:
The interaction between the metal aggregate and the oxide support can result in considerable changes in the physical properties and chemical activity of the catalyst.
Charge transfer at the interface can tune the electronic and chemical properties of the metal particle.
For example, copper and silver atoms adsorbed on a TiO2(110) surface can reduce the oxide by direct electron transfer, forming supported cations, while gold, on the other hand, may form a covalent bond and remain neutral in the same environment.(15)
 
Screening Effects:
The Auger parameter is a measure of the extra-atomic relaxation or screening effect of the surrounding medium on the final ion state, which is influenced by the support. The support can either enhance or reduce the screening of the core hole created during photoemission.
A higher Auger parameter indicates higher relaxation energy or improved screening energy.
When conduction electrons are available, more screening energy is available. For example, xenon implanted in conductors shows a larger Auger parameter, compared to xenon adsorbed on oxidized supports.(1) The Auger parameter shifts can be interpreted by taking into account the polarizability of the surrounding atoms, or the extra-atomic relaxation or polarization energy.
Small clusters deposited on poorly conducting substrates generally exhibit positive binding energy shifts, initially interpreted as arising from a unit positive charge on the cluster in the photoemission final state.
 
Local Structure and Geometry:
The Auger parameter shift is a function of the number, distance, electronic polarizability, and local geometry of the nearest-neighbour ligands around the core-ionized atom.(8)
The local geometry adopted by the ligand atoms has a strong influence on the dipoles of nearest neighbours.

Auger Parameter for Surface Basicity #

Auger parameter is a useful tool for determining the surface basicity of metal oxides – in the same way as when comparing auger parameter shifts to base metals in the above methods, we can compare to that of gaseous H₂O (which has an α value of 1038.5 eV), where an increase in α indicates higher surface polarisability. This increase, along with a decrease in the separation of specific Auger transitions KLL – KLV (ΔEk), suggests that the surface has greater Lewis basicity.

For example, in the case of MgO nanoparticles, as the particle size increases, both α and ΔEk indicate an increase in surface base strength. This is due to the growth of crystallites and the transformation from regular (100) facets to more reactive stepped (110) facets. However, once the crystallites grow beyond 10 nm, further changes in morphology or surface termination are minimal, which is reflected in a plateau in their polarisability. This plateau suggests that the surface basicity does not increase further with additional crystallite growth.(16, 17)

Auger Parameter for Refractive Index #

Robin West and Jim Castle identified a correlation between the Al and Si auger parameters with refractive index of silicates.

The auger parameter was found to scale with the polarization energy of the surrounding oxygen ions, which were correlated with measured refractive indexes to identify a new method for identifying changes to RI using XPS.(18)

References #

  1. Wagner, C. D., and A. Joshi. “The auger parameter, its utility and advantages: a review.” Journal of electron spectroscopy and related phenomena 47 (1988): 283-313. Read it online here.
  2. Wagner, C. D. “Chemical shifts of Auger lines, and the Auger parameter.” Faraday Discussions of the Chemical Society 60 (1975): 291-300. Read it online here.
  3. Moretti, Giuliano. “Auger parameter and Wagner plot in the characterization of chemical states by X-ray photoelectron spectroscopy: a review.” Journal of electron spectroscopy and related phenomena 95.2-3 (1998): 95-144. Read it online here.
  4. Moretti, Giuliano, and Horst P. Beck. “Relationship between the Auger parameter and the ground state valence charge of the core‐ionized atom: The case of Cu (I) and Cu (II) compounds.” Surface and Interface Analysis 51.13 (2019): 1359-1370. Read it online here.
  5. Biesinger, Mark C., et al. “The role of the Auger parameter in XPS studies of nickel metal, halides and oxides.” Physical Chemistry Chemical Physics 14.7 (2012): 2434-2442. Read it online here.
  6. Bourque, Jeremy L., Mark C. Biesinger, and Kim M. Baines. “Chemical state determination of molecular gallium compounds using XPS.” Dalton Transactions 45.18 (2016): 7678-7696. Read it online here.
  7. Kaushik, Vijay Kumar. “XPS core level spectra and Auger parameters for some silver compounds.” Journal of Electron Spectroscopy and Related Phenomena 56.3 (1991): 273-277. Read it online here.
  8. Moretti, Giuliano, et al. “Auger parameter and Wagner plot studies of small copper clusters.” Surface Science 646 (2016): 298-305. Read it online here.
  9. Bagus, Paul S., Andrzej Wieckowski, and Hajo Freund. “Initial and final state contributions to binding-energy shifts due to lattice strain: Validation of Auger parameter analyses.” Chemical physics letters 420.1-3 (2006): 42-46. Read it online here.
  10. Islam, Mohammed J., et al. “PdCu single atom alloys supported on alumina for the selective hydrogenation of furfural.” Applied Catalysis B: Environmental 299 (2021): 120652. Read it online here.
  11. Moretti, Giuliano. “The Auger parameter and the polarization energy: A simple electrostatic model.” Surface and Interface Analysis 16.1‐12 (1990): 159-162. Read it online here.
  12. Yubero, Francisco, et al. “Size and shape of supported zirconia nanoparticles determined by x-ray photoelectron spectroscopy.” Journal of applied physics 101.12 (2007). Read it online here.
  13. González-Elipe, Agustín R., and Francisco Yubero. “Spectroscopic characterization of oxide/oxide interfaces.” Handbook of surfaces and interfaces of materials 2.4 (2001): 147. Read it online here.
  14. Thøgersen, Annett, et al. “An experimental study of charge distribution in crystalline and amorphous Si nanoclusters in thin silica films.” Journal of Applied Physics 103.2 (2008). Read it online here.
  15. Pacchioni, Gianfranco, and Hans-Joachim Freund. “Controlling the charge state of supported nanoparticles in catalysis: lessons from model systems.” Chemical Society Reviews 47.22 (2018): 8474-8502. Read it online here.
  16. Montero, Janine M., et al. “Structure-sensitive biodiesel synthesis over MgO nanocrystals.” Green chemistry 11.2 (2009): 265-268. Read it online here.
  17. Montero, J. M., et al. “The surface chemistry of nanocrystalline MgO catalysts for FAME production: An in situ XPS study of H2O, CH3OH and CH3OAc adsorption.” Surface science 646 (2016): 170-178. Read it online here.
  18. West, R. H., and J. E. Castle. “The correlation of the auger parameter with refractive index: an XPS study of silicates using Zr Lα radiation.” Surface and Interface Analysis 4.2 (1982): 68-75. Read it online here.
Applications of XPS, Auger peaks, Catalysis, Final-State Effects, Nanomaterials
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