Ion Scattering Spectroscopy (ISS) works by directing a beam of low-energy ions, typically noble gases like helium, at the surface of a material. When these ions collide with the surface atoms, they scatter. The kinetic energy of the scattered ions is then measured. Each element at the surface produces a distinct peak in the energy spectrum due to differences in mass and momentum transfer.

The dependence of peak energy position on atomic mass in ISS. E₀ = primary ion E_k, E₁ = primary ion E_k following a scattering process at θ° to initial trajectory, m₁ = mass of primary ion, m₂ = mass of target atom.

Ions can be detected either using an XPS analyser with a reversed polarity (ISS), or with a dedicated cylindrical mirror analyzer (LEIS).¹ ISS may be incorporated into a standard XPS system, and is commonly found on instruments from most manufacturers – however LEIS requires a dedicated standalone instrument.

When running an ISS experiment on an XPS instrument, it is typically performed in the 1-3 keV region (the energy span of the analyzer). Helium ions are lighter, and therefore do not provide as large an energy separation for heavier elements – so it is often advised to use Argon gas lines instead if one wishes to distinguish two heavy elements. The penalty of this is increased intrinsic sputtering of the surface.

General Analysis #

ISS data is output in the same format as XPS data, with kinetic energy on the X-axis instead of binding energy. As such, standard XPS fitting softwares such as CasaXPS, or Avantage can be used for analysis.

Care should be taken with background selection – since ISS data is subject to the same inelastic background considerations as XPS data.²

Qualitative Analysis #

If one simple wishes to observe the elemental identity of the terminating layer – then the peak assignment is a relatively trivial affair, requiring only the angle and the energy of the ion beam.

Energy Ratio = Kinetic Energy / Ion Beam Energy

These ratios will be consistent for a given ion gun angle, and can be computed using the binary collision model above.

Quantitative Analysis #

Quantitative analysis of scattered ions is possible by the following relationship:

Y_i = n_i . ( δσ_i / δω ) . P_i⁺ . cR

Where:

Y_i = Detected ion yield

n_i = Atomic surface concentration

δσ_i / δω = Differential scattering cross section

P_i⁺ = Ion fraction of backscattered projectiles

c = instrument factor (solid angle of acceptance, transmission function)

R = Roughness factor

For more information – see the practical guide to interpreting low energy ion scattering (LEIS) spectra article from Stanislav Průša et al.

Owing to its enhanced surface sensitivity in comparison to XPS, which has an information depth of as much as 9–10 nm (using a typical Al kα X-ray source), ISS has found much use in extracting qualitative information on elemental distributions in the very uppermost atomic layers of a sample. The ability to probe only a single monolayer allows for a more sophisticated analysis of the atomic arrangement at the surface and near surface.

This information is of critical concern in fields such as heterogeneous catalysis, wherein performance and activity are directed by surface interactions of reactants onto active sites. Ceria based mixed oxides for example, possess unique surface properties which render them highly useful in a variety of applications. Surface segregation of either dopant or ceria, determined by ISS, has enabled a deeper understanding of how different thermal treatments or synthesis methods affect the resultant materials properties as catalysts, ion conductors and oxygen storage.

Furthermore, since this is in effect an ion bombardment technique, the correct experimental parameters may induce sputtering of the surface of a material (even at low accelerating voltages), successive ISS spectra may therefore reveal an elemental gradient, affording further information which with knowledge of sputtering yields (or in comparison with alternative methods of probing the depth distribution of the outer surface) may even be used to prepare a incredibly surface sensitive elemental depth profile.

ISS may also be used to determine changes to the outermost surface following chemical or physical processing and has been used to study systems undergoing many common processes such as thermal treatments, hydrogen reduction or exposure to gases or vapours such as O₂ and H₂O, and has also been used to study particle sintering.

When considering nanoparticle or thin-film growth over a substrate (of non-matching identities), knowledge of nucleation and growth modes permits sophisticated and precise control over nanomaterial synthesis. ISS analysis combined with XPS affords a simple methodology for modelling the growth based on the calculated surface coverage or peak intensity relative ratios.

Many detailed models have been developed to understand the varying growth mechanisms of particles on films.⁴

ISS may be used during temperature profiles much in the same way as XPS, to observe structural changes as a function of increased temperature.

With careful consideration of intrinsic sputtering rates, and adsorbate removal rates, this technique has been shown to resolve CO adsorption sites from Pd nanocluster samples.⁵

ISS is ideal for studying SMSI formations, given the extreme surface sensitivity of the technique. The formation of an ultra-thin overlayer (e.g. TiO2 over Ag nanoparticles)⁶ results in a markedly different ISS signal.