XPS, also known as ESCA (electron spectroscopy for chemical analysis) is a surface analysis technique which provides both elemental and chemical state information. Modern X-ray photoelectron spectrometers can operate in both spectroscopy and imaging modes, the latter providing information on the lateral distribution of chemistry across a surface. X-ray photoelectron spectroscopy (XPS), also known as ESCA (electron spectroscopy for chemical analysis) is a surface analysis technique which provides both elemental and chemical state information virtually without restriction on the type of material which can be analysed. It is a relatively simple technique where the sample is illuminated with X-rays which have enough energy to eject an electron from the atom. These ejected electrons are known as photoelectrons. The kinetic energy of these emitted electrons is characteristic of the element from which the photoelectron originated. The position and intensity of the peaks in an energy spectrum provide the desired chemical state and quantitative information. The surface sensitivity of XPS is determined by the distance that a photoelectron can travel through the material without losing any kinetic energy. These elastically scattered photoelectrons contribute to the photoelectron peak, whilst photoelectrons that have been inelastically scattered, losing some kinetic energy before leaving the material, will contribute to the spectral background. The chemical environment of an atom alters the binding energy (BE) of a photoelectron which results in a change in the measured kinetic energy (KE). The BE is related to the measured photoelectron KE by the simple equation; BE = hν – KE where hv is the photon (x-ray) energy. The chemical or bonding information of the element is derived from these chemical shifts. Most XPS analysis starts with a survey spectrum, collecting photoelectrons across the entire binding energy range accessible using the chosen excitation source. The photoelectrons are collected from an area defined by the input lens. For AXIS spectrometers, large area spectra are collected from a 300 x 700 um analysis area. To analyse smaller areas, an aperture is inserted into the transfer lens, limiting the area from which photoelectrons are collected. The circular aperture gives analysis areas with 110 um, 55 um, 27 um or 15 um diameter. XPS imaging is used to provide elemental, or chemical state, distribution across the surface. Ultimate spatial resolution is 1 um. The fields of view available in imaging-XPS mode are 900 x 900 um, 400 x 400 um and 200 x 200 um. AXIS spectrometers are unique in having combined hemispherical and spherical mirror analysers for spectroscopy and imaging modes respectively. XPS is a mature analytical technique, widely used in materials characterisation labs in industry and academia. The technique is used for surface analysis of samples with technological applications as diverse as organic LEDs to drug eluting thin-films from the pharmaceutical industry. XPS provides useful electronic and chemical information from the uppermost 10 nm of a material. Further insight into material properties can be gained by coincident, complementary techniques.
