XPS Fundamentals

X-ray photoelectron spectroscopy (XPS) or electron spectroscopy for chemical analysis (ESCA) is a surface analysis technique capable of providing elemental and chemical state information from the outer 5 to 10 nanometres of a solid surface. All elements from lithium to uranium can be detected with detection limits in the 0.1 to 0.5 atomic percent level.

When an X-ray of known energy (hν), generally Al(Kα) at 1486.7eV or Mg(Kα) at 1253.6eV, interacts with an atom, a photoelectron can be emitted via the photoelectric effect. The emitted electron’s kinetic energy (Ek) can be measured and the atomic core level binding energy (Eb) relative to the Fermi level (EF) of the sample is determined by the following equation:

Eb = hν – Ek – Φsp (1)

where Φsp is the work function of the spectrometer. Chemical information about the sample can be extracted because binding energies are sensitive to the chemical environment of the atom. Chemical environments that deshield the atom of interest (i.e. are bound to strongly electron withdrawing groups) will cause the core electrons of that atom to have increased binding energies. Conversely, decreased binding energies will be measured for core electrons of atoms that withdraw electrons from others. Essentially, binding energy will increase as chemical state number increases.

XPS, with its ability to quantify elements and determine chemical states, is used in many branches of materials science, electronics, thin film chemistry, corrosion science, polymer modification, adhesion science, coating chemistry, catalysis, mineral processing chemistry, as well as in exploring fundamental aspects of the chemistry and physics of atoms and molecules.