X-ray photoelectron spectroscopy (XPS/ESCA)

X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA), is the most frequently used technique for surface analysis, due to its relatively simple use and data interpretation. The sample is irradiated by X-rays with a single energy, causing electrons to be emitted from the surface of the sample. The electron energy analyser determines the binding energy of photoelectrons. The binding energy and photoelectron peak intensity are used for the determination of the element, its chemical state and its quantity. The information about surface structures or thin layer structure provided by the XPS are valuable for many industrial applications such as polymer surface modification, catalysis, corrosion, adhesion, semiconductor and dielectrics industry, magnetic media and thin coatings used in a number of industrial disciplines.

All solid materials interact with their environment through their surface. The nature of their interaction is given by the physical and chemical composition of their surfaces. The chemical composition of the surface influences several factors such as corrosion, catalytic activity, adhesiveness, wettability, contact potential and material fatigue. Therefore, surfaces have a hugely important influence on the properties of solids. In spite of the undeniable importance of surfaces, the proportion of atoms on the surface of solids relative to the amount of atoms making up the volume of the material is very small. The precise share of surface atoms depends on the shape, surface roughness and the composition of the material.

Electron spectroscopy studies electron emissions, which are analysed with an XPS detector, where their kinetic energy is transformed into information which identifies what elements are contained in the sample. The main reasons for using photoemission spectroscopy include its chemical sensitivity and suitability for exploring surfaces. The photoelectron’s binding energy also carries information about the chemical composition of the sample. Further, based on the chemical environment, which exposes nuclear electrons to, primarily, photoemission, it is possible to determine the bondability type, oxidation state, adsorption possibilities. They subsequently result in binding energy shifts.

The construction of the devices was significantly developing from very early on in experiments by Siegbahn and others. All modern XPS devises rely on the same key components – X-ray sources, input electron optics, electron energy analyser and detector. The aforementioned components are placed in an apparatus where there is an ultra high-vacuum (UHV). This apparatus is typically made of stainless steel with a high degree of electromagnetic shielding, of the so called mu-metal.