X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA), is a highly surface-sensitive, quantitative, chemical analysis technique that can be used to solve a wide range of materials problems. XPS is the measurement of photoelectrons ejected from the surface of a material that has been irradiated with X-rays. The kinetic energy of the emitted photoelectrons is measured. This energy is directly related to the photoelectrons’ binding energy within the parent atom and is characteristic of the element and its chemical state.
Only electrons generated near the surface can escape without losing too much energy for detection. This means that XPS data is collected from the top few nanometers of the surface. It is this surface selectivity, coupled with quantitative chemical state identification, that makes XPS so valuable in a vast array of application areas.
What is X-ray photoelectron spectroscopy (XPS)?
XPS can measure the elemental composition, empirical formula, chemical state, and electronic state of the elements within a material. XPS spectra are obtained by irradiating a solid surface with a beam of X-rays while simultaneously measuring the kinetic energy of electrons that are emitted from the top 1–10 nm of the material being analyzed.
Small area or selected area X-ray photoelectron spectroscopy (SAXPS) is used to analyze small features on a solid surface, such as a surface blemish or particle. SAXPS maximizes the detected signal coming from a specific area and minimizes the signal from the surrounding area.
When the surface is electrically insulating the emission of electrons causes a positive charge to accumulate at the surface, severely affecting the XPS spectrum. Charge compensation neutralizes the charge on the surface by replenishing electrons from an external source. This stabilizes and controls the charging to within a few electron volts of the neutral state.
Depth profiling is a process that slowly removes material using an ion beam and then collects data after each etching cycle. Depth profiling enables a composition profile with high depth resolution to be measured. Depth profiles can be used to see how the composition changes from surface to bulk; for example, due to corrosion, oxidation of the surface, or to understand the chemistry at interfaces where different materials are joined together.
While used to identify points or small features at the surface, XPS can also be used to image the surface of a sample. This is useful in understanding the distribution of chemistries across a surface, for finding the limits of contamination, or even examining the thickness variation of an ultra-thin coating. There are two approaches for obtaining XPS images: mapping (serial acquisition) or parallel imaging (parallel acquisition).
Angle resolved XPS (ARXPS) varies the emission angle at which the electrons are collected, thereby enabling electron detection from different depths. ARXPS provides information about the thickness and composition of ultra-thin films.
In XPS instruments, X-rays are generated by bombarding a metallic anode with high-energy electrons. The energy of the emitted X-rays depends on the anode material, and the beam intensity depends on the electron current striking the anode and its energy.
The Thermo Scientific MAGCIS dual mode ion source enables depth profiling analysis and surface cleaning of both soft and hard materials on the same XPS instrument. New gas cluster ion sources enable the analysis of several classes of materials previously inaccessible to XPS depth profiling.
UV photoelectron spectroscopy (UPS) is a technique very similar to XPS but which uses UV photons rather than X-ray photons to excite photoelectrons from the surface. As UV photons have lower kinetic energy, the photoelectrons that are detected are from the lower binding energy levels involved in bonding.
Ion scattering spectroscopy (ISS) or low-energy ion scattering (LEIS) is a highly surface-sensitive technique used to probe the elemental composition of the first atomic layer of a surface. For the probe, it uses a beam of noble gas ions, which are scattered from the surface. The kinetic energy of the scattered ions is measured.
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