Many of the experimental techniques that we have used and developed to study buried interfaces and surfaces are related to X-ray photoelectron spectroscopy (XPS). This technique is one in which a surface is irradiated with X-rays, that are then absorbed by the surface. The absorbed X-rays can then ionise the core electrons from the atoms in the surface, causing the emission of photoelectrons. These photoelectrons can then be detected and can be used to identify not only the element that has emitted them, but also the chemical environment that the element is in (such as the difference between an oxidised or pristine surface species). The photoelectrons interact very heavily with material and so XPS is performed under ultra-high vacuum conditions, with electrons only escaping from the surface layers (top 3-8 nm) of the material that is being studied.
Hard X-ray photoelectron spectroscopy (HAXPES) is a method that can be used to get a better depth resolution from the photoelectron signal. By utilising higher energy (or “harder”) X-rays the ejected photoelectrons have a higher kinetic energy. These faster photoelectrons can escape from deeper levels of the surface, providing more information on the elemental and chemical structure further from the surface. This technique can be used to look at thickener surface layers, such as the solid-electrolyte interphase in batteries. Such experiemtns can be performed in collaberation with external facilities (Beamline I09 at the Diamond Light Source, Royce Institute at Manchester HAXPES).
Near-ambient pressure (NAP) XPS utilises differentially pumped chambers to enable the study of surfaces under conditions that are more realistic, such as a 10 mbar water vapour atmosphere. Such a technique can be utilised for the study of reaction mechanisms on catalyst surfaces, as the different chemical species can be detected at different stages in the surface-based reactions. We collaborate with Beamline B07 at the Diamond Light source for a variety of experimental projects.