What is X-ray Photoemission Spectroscopy?

Dave Cockett XPS

X-ray photoelectron spectroscopy (XPS) is a technique that uses X-rays to characterise the surface of materials by allowing the identification of elements and chemical species. XPS can detect the composition of the top layers of a material, having a depth of analysis up to 10 nanometers, and it is sensitive to all elements of the periodic table–with the only exception of hydrogen and helium. XPS helps characterize various materials (like metals, oxides, polymers, etc.) used in various applications (catalysis, coatings, batteries, corrosion, etc).

Understanding X-ray Photoelectron Spectroscopy

XPS is based on the photoelectric effect: a surface is irradiated with X-rays of a specific energy (hν). This energy is absorbed by the atoms in the material, producing the ejection of electrons from the atoms’ core levels. The emitted electrons, now called photoelectrons, will have a kinetic energy (KE) that equals the irradiated X-ray energy (hν) minus the electrons’ binding energy (BE).

An energy analyser separates the photoelectrons based on their KE, and electrons with the same energy are ‘counted’ by a detector. The distribution of energy is then recorded on a spectrum, and energy peaks can be directly associated with specific atom core levels, indicating the presence of different elements on the materials’ surface.

Benefits of X-ray Photoelectron Spectroscopy

A material’s surface has different properties than its bulk; the structure is ‘interrupted’, leaving ‘dangling bonds’ that can react with the environment and, in most cases, form top layers of different compositions. For this reason, to fully understand a material’s behavior, it is essential to use surface-sensitive techniques. A good understanding of surface properties is particularly useful when studying applications like corrosion, adhesion, coatings, paints, catalysis, and generally any phenomenon based on surface reactions. Understanding this behaviour is the necessary first step for R&D to optimise the composition of products and obtain the desired result.

X-ray Photoelectron Spectroscopy provides qualitative and semi-quantitative information of the surface of a material, with the identification of elements, how they are bonded to each other, and in what ratio. This technique offers unique insights into the composition of a material, and it is complementary to other characterisation techniques, such as SEM, EDS, Auger etc. Thanks to high sensitivity and small beam sizes XPS can have a spatial resolution down to the micron range. This helps in studying heterogeneous surfaces, like coating defects, alloy precipitates, etc. via XPS imaging.

XPS systems can also perform depth profiling. An ion gun is used to etch away the top layers of a surface, allowing analysis of layers just underneath. Depth profiling is very useful for studying the composition of multi-layer devices, like semiconductors, and more generally for monitoring how the composition changes with depth.

Industrial Applications of X-Ray Photoelectron Spectroscopy

A variety of industries utilize the abilities of X-ray photoemission spectroscopy:

  • Paints and adhesives: Their effectiveness and durability heavily depends on the type of bonds that form at the interface between the paint/adhesive and the substrate. Understanding this interaction helps designing the best product formulation and substrate pre-treatment.
  • Metal/Steel: XPS helps to study metal/alloy composition, passivation layers, corrosion rates, presence of contaminants, etc
  • Automotive/Aerospace: XPS helps study components properties and suitable surface treatments and coatings, depending on the type of part designed and it’s use.
  • Energy: Efficiency of batteries, fuel cells, solar cells etc depends on the chemical properties of the materials. These can be studied before and after cycling, to monitor degradation and improve design.
  • Packaging: The materials for packaging can be analyzed in relation to their surface chemistry. This means the material’s characteristics, such as its wettability and adhesion, can be established.
  • Semiconductors and Electronics: The capabilities of semiconductors and dielectrics depend on their composition, XPS depth profiling proves to be very useful for multilayer devices characterisation.

These are among the main industry applications, but the list is wide, as many other applications are affected by surface properties, like catalysis, optical devices, graphene, polymers, functional materials, pharma, etc.

Discover More About Your Materials Through X-Ray Photoemission Spectroscopy

Nowadays, materials produced for tools and technology cannot afford to be compromised by their own elemental composition. Their reliability ultimately ensures a product can continue to work. Therefore, X-ray photoemission spectroscopy tools are needed to prove materials will act as required.

If you are considering applying X-ray photoemission spectroscopy to your own work, BlueScientific can help. We provide scientific equipment for a range of areas, including materials science. So, visit our website to see our X-ray photoemission spectroscopy tools.