X-rays knock electrons out of the atoms at a surface, and the energies those electrons carry say which elements are present and what they are bonded to. It is the standard check that a film really is the compound you wanted and has not oxidised.
Going deeper
XPS reads the energies of electrons knocked out of core levels. Because those electrons escape only from the top few nanometres, it measures the surface; and because the binding energy shifts with chemical environment, a second, displaced copy of a peak shows that part of the material has reacted.
What the energies say
A soft X-ray of known energy ejects an electron from a core level. Measuring its kinetic energy gives the binding energy through E_B = hν − E_kin − φ, where φ is the spectrometer . Core-level energies are characteristic of each element, so the survey spectrum is an elemental inventory of the surface, and peak areas divided by tabulated sensitivity factors give composition – reliable to a few percent at best, not to the third decimal.
The finer information is in small shifts. An atom that has given up charge holds its remaining electrons more tightly, so oxidised molybdenum appears at a higher binding energy than molybdenum in MoS2. A spectrum therefore separates a compound from its oxide, distinguishes substitutional dopants from adsorbed ones, and follows from a .
Why it suits 2D materials – and where it misleads
The escape depth of photoelectrons is only a few nanometres, so XPS is intrinsically a surface technique – which for a means the whole sample. It is the standard check that a grown or film really has the intended , that it has not oxidised, and that transfer residues are gone.
That same sensitivity is the trap. A monolayer contributes a weak signal sitting on a strong substrate background, and adventitious carbon and adsorbed oxygen from the air are always present. Ultrahigh vacuum, or transfer without air exposure, is needed for anything about surface chemistry to be meaningful, and the sampling depth means a buried interface is only visible through a very thin cover.
Referencing and charging
The most common source of error is where the energy axis is fixed. The habit of calibrating on the adventitious carbon C 1s peak at 284.8 eV is unreliable: that carbon is a mixture whose apparent position depends on the sample and on charging, and on it can be off by an electronvolt or more – enough to invent or hide a chemical shift.
substrates make this worse, since the sample charges as electrons leave and every peak drifts. Flood guns neutralise the charge, but the residual shift still has to be corrected, and the correction reported. The safer practice is an internal reference within the sample, a stated calibration procedure, and reporting peak separations, which are immune to a rigid shift, alongside absolute positions.
For specialists
Core-level giving elemental composition and chemical state from the top few nanometres. On 2D films it fixes stoichiometry, detects oxidation and substitutional impurities, and follows through rigid core-level shifts. Binding-energy referencing is the usual source of error – adventitious carbon at 284.8 eV is not a reliable standard on a semiconductor – and charging has to be controlled on insulating substrates.