The Use of X-Ray Photoelectron Spectroscopy in Materials Science
Authors: Castle James 
Published: March 2019

This review will attempt to show how XPS now makes an important contribution to Materials Science and to highlight the developments which have brought it to this position. XPS is now a mature technique for surface analysis but it has in addition a major role as a specialised tool, being essential to studies in which derivitization methods are used to tag surface groups.

The requirements of users in this field have led to the development of X-ray sources which were not envisaged in the early development of the spectroscopy. The usual sources of aluminium Kα and magnesium Kα have limitations for those elements beyond magnesium in the periodic table which would have the Is lino as the principal peak - aluminium, silicon, oulphur and phosphorus for example. Higher energy sources such as silicon Kα or zirconium and silver Lα have made it possible to utilise the Is lines up to chlorine and have the additional advantage that a strong and well resolved series of Auger lines also becomes available. The higher energy radiations are thus particularly suited to the determination of relaxation energies in materials by use of relative shifts between the photo- and Auger lines of the spectrum. Such has been the utility of such relaxation energies that use is often made of Auger lines derived from the Bremmstrahlung component of the normal x-ray sources to make a similar measurement. This measurement is used in the study of insulating ceramics in which electrostatic charging makes measurement of binding energies uncertain.

Modern materials technology is particularly concerned with the manufacture of composites; particulate, fibre and laminate composites are all well known and the key to their success often lies within the interface between the phases. Transfer of load across the interface places particular requirements on adhesion at the phase boundary and an understanding of the locus of failure during destructive testing is crucial to the development of satisfactory bonding processes. In coated and laminated products there is no problem in the use of XPS, with its excellent chemical sensitivity but there is a problem of increasing magnitude in fibre and particulate composites as the substructures become finer. This stems, of course, from the difficulty of providing a focused source of X-rays of sufficient magnitude. Imaging XPS is slowly becoming a reality with several systems having a capability of 10μm now available, and one of the markets for such instruments is that of composite materials.

There are important areas of Materials Science in which XPS has been displaced by other techniques such as SIMS. One such area is that of polymer surface analysis. The selectivity of XPS for substituent groups in the surface region is not good. Derivitization methods have made an impact, enabling acidic or basic groups to be determined, but SIMS, which has the ability to detach molecular clusters, has obvious advantages which will become increasingly exploited aa the problems of charging become solved. Until then however XPS will continue to find a role in polymer research and development.