Photoelectron spectroscopy is a workhorse technique for investigation of filled electronic states in solids. The short path length of photoelectrons excited with far UV radiation necessitates rigorous surface preparation procedures if experiments are to yield meaningful results. Much longer photoelectron path lengths are found under soft X-ray excitation and photoemission experiments conducted using this higher energy radiation are more tolerant of imperfect surface preparation. Thus, X-ray photoelectron spectroscopy (XPS) is much better suited than ultraviolet photoelectron spectroscopy
(UPS) to the study of valence band states in polycrystalline samples of the sort found in many technological applications. However, the cross-sections for ionisation of valence states with soft X-rays are much smaller than at lower vacuum UV photon energies. It is therefore a major technical challenge to use X-ray photoemission to study the exceedingly weak features arising from states near the valence band maximum, or band gap states themselves.
X-ray Photoelectron Spectroscopy (XPS) in the Group
In the group we are fortunate to have a number of XPS systems with which can study and characterise a wide range of (typically, but not always!) oxide materials. The workhorse instrument in the Thermo Fisher K-Alpha+ instrument shown below.
This instrument is capable of measuring a wide range of samples, from oxides to biological specimens, and has excellent dual-beam charge compensation (ions and electrons), as well as having a large sample plate that can accommodate multiple samples. We also have a tilt holder for surface sensitivity measurements, an vacuum transfer module – to transfer samples from an inert environment (e.g. glovebox) without exposing it to air and a bias-stage to measure the work function of a material. We also have a monotomic Ar ion source for depth profiling experiments. More details on how to access this equipment can be found here.
Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS) in the Group
We also have an APXPS system that is capable of operating up to 30 mbar in a wide variety of gases. The system is being upgraded in summer 2019 – more details to follow.