XPS (ESCA, X-ray photoelectron spectroscopy)
quantitative, surface sensitive, specific to elements and chemical bonds
XPS is a technique for the sensitive, quantitative analysis of the surface composition. The method detects all elements except H and He and yields information on their molecular bonds. Depending on the analytical question, this information can be recorded as integral spectra, spatially resolved images or in the form of depth-resolved profiles. XPS analysis is feasible on all vacuum stable materials (e.g., glass, fibers, powders, ...). Typical fields of application of the method are:
- Investigations of the elementary structure of surfaces and layers
- Oxidation analysis of surfaces (oxidation state, oxidation depth)
- Quantitative characterization of catalysts and functional groups
Details on XPS
Physical principles - Information gained - Analytical possibilities
X-ray Photoelectron spectroscopy (XPS / ESCA) is an analytical method for the qualitative and quantitative detection of elements near the surface of a solid. For the analysis, a sample is irradiated with monochromatic X-ray radiation (Al Kα) so that photoelectrons are released from the excited atoms ("photoelectric effect"). Only photoelectrons from the top approx. 5 - 10 nm of a surface can leave the sample without energy loss. With a hemispheric analyzer, the kinetic energy of these electrons is determined and the binding energies - specific for each element – are calculated. With XPS all elements (except H and He) can be identified and quantified. Depending on the chemical environment of an element, there may be shifts in the binding energies ("chemical shift"). These give information on the state of bonding or oxidation of the respective elements. The sensitivities that can be achieved depend on the element and are typically between 0.1 and 1 atom%.
With Small-Spot-XPS the element composition can be determined on a very small (spot-Ø ≥ 10 μm) or over mm²-wide measuring ranges. Point analyses along a distance across the sample surface (linescan), 2D element distributions (maps), or angle-resolved analyses (ARXPS) for the characterization of thin layers (< 10 nm) are common applications. The elemental composition of a solid as a function of depth (depth profiling) can be determined too by combining the XPS analysis with ion bombardment-induced sample erosion (sputtering). The method is applicable to a wide variety of vacuum-compatible sample systems (metals, glasses, wafers, polymers, films, ceramics, pigments, catalysts, etc.).