X-ray Photoelectron Spectroscopy (XPS)
The operating principle of XPS is the photoelectric effect which occurs when an electron is ejected from an element after being exposed to a photon whose energy exceeds a critical value. The ejected electron escapes with a kinetic energy equal to the difference between the incident energy of the photon and its original binding energy (or the energy required to remove the electron from its orbital into space). The relation is expressed by the following equation: EK=hn-EB.
An XPS spectrophotometer operates by striking the sample with a high energy photon (an X-ray from a Mg or Al based source) and measuring the kinetic energy of all of the ejected electrons. The binding energies of these electrons are then calculated and from these a distribution of atomic species is determined. The lightest element that XPS can detect is lithium. Even though the X-ray photons can penetrate deeply into the material, scattering limits only electrons from the top 5-10 nm to be ejected without attenuation of their kinetic energy. Thus, XPS is considered a surface sensitive technique.
In addition to determining the atomic composition of the surface, XPS can also provide semi-quantitative information (expressed in units of atomic percent) by dividing the peak areas of each element by sensitivity factor, which is simply the probability of a particular element to emit an electron when struck by a photon. The sensitivity of XPS is element dependent, with larger elements (which have larger sensitivity factors) being more sensitive than smaller elements.
XPS Facilities at the CMSC
A Perkin Elmer Phi 5600 ESCA system is used for XPS analysis with a magnesium Ka X-ray source at a take off angle of 45°. Sample submitted for analysis must be solid and vacuum compatible (i.e. samples should not have a low sublimation pressure or be excessively oily, etc.). The maximum sample dimensions are roughly 2 cm in diameter and 1 cm in height. A pump down time is required to bring the sample to a proper vacuum level.