X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA), is a surface-sensitive technique that measures the elemental composition and chemical state of the top 1–10 nm of a material. It is common in materials science, catalysis, semiconductor characterization, and corrosion research.
XPS is based on the photoelectric effect. A sample is irradiated with monochromatic X-rays, typically Al Kα (1486.6 eV) or Mg Kα (1253.6 eV), causing core-level electrons to be ejected. The kinetic energy of the emitted photoelectrons is measured, and the binding energy is calculated as EB = hν - EK - φ, where hν is the photon energy, EK is the measured kinetic energy, and φ is the spectrometer work function.
Each element has a characteristic set of binding energies, enabling qualitative identification. The chemical environment of an atom shifts its binding energy by up to several eV, providing information about oxidation state and chemical bonding. For example, the C 1s peak appears at 284.8 eV for C-C bonds, 286.4 eV for C-O bonds, and 288.8 eV for O-C=O bonds. Quantitative analysis uses relative peak areas corrected by sensitivity factors to determine atomic concentrations.
XPS probes only the outermost 10 nm because photoelectrons from deeper layers lose energy through inelastic scattering. The analysis depth depends on the inelastic mean free path of the photoelectrons, which varies with kinetic energy and material. Angle-resolved XPS varies the detection angle to provide non-destructive depth profiling.
Survey spectra (0–1200 eV) identify all elements present except hydrogen and helium. High-resolution spectra of individual peaks provide chemical state information. XPS can detect all elements with atomic number above 3 at detection limits of approximately 0.1 atomic percent. Depth profiling using argon ion sputtering reveals composition as a function of depth.
Applications include characterization of surface contamination, verification of surface functionalization, analysis of thin film composition, and failure analysis of electronic devices.
