Surface Analysis Using X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS), or electron spectroscopy for chemical analysis (ESCA), is a common surface characterization tool used for a huge range of applications from the everyday (e.g., waterproofing fabrics or non-stick cookware coatings), to advanced R&D (e.g. clean energy materials, organic thin-film electronics, and bio-active surfaces). XPS measures the elemental composition, chemical, and electronic state of atoms on a material's surface to analyze its surface chemistry. 

XPS spectra are obtained by irradiating a solid surface with a beam of X-rays and measuring the kinetic energy of electrons that are emitted through the photoelectric effect (observed by Hertz in 1887 and explained by Einstein in 1905) from the material. A photoelectron spectrum is recorded by counting ejected electrons over a range of kinetic energies. The energies and intensities of photoelectron peaks enable identification and quantification of all surface elements, except for hydrogen. 

XPS offers a significant advantage in detecting subtle changes in the position of peaks that reflect the chemical state of surface elements, such as metallic or oxidized states, and diverse bonding states in polymers. The other key advantage to XPS is that the analysis depth is limited to a few nanometers due to the strong electron-matter interactions. Electrons lose energy quickly as they interact with matter, preventing their detection as part of a peak. This limits the depth from which the signal is detected to around 10 nm, depending on the material, which makes XPS extremely surface sensitive.