X-ray photoelectron spectroscopy

What is XPS and what is it used for?

X-ray photoelectron spectroscopy (XPS) is a technique for analyzing the chemistry and properties of material surfaces, thin films, and coatings. XPS can determine the elemental composition of the surface (elements and their relative concentrations) along with the empirical formulas of compounds on the surface. Also, the chemical states (oxidation states or chemical groups) and electronic states (electron configurations) of the elements present in the surface material can be deduced. XPS can also be used for depth profiling when combined with ion gun etching of consecutive surface layers.

XPS is widely used in the product development and quality control of different materials, such as semiconductors, metals, and glass. XPS can be utilized when the surface structure of the material needs to be examined.

Surface analysis is important because the properties of the surface affect, for example, the corrosion rates, catalytic activity, adhesive properties, wettability, contact potential, and failure mechanisms of the material. The properties of the material can be improved with surface modification when they have first been analyzed as a function of depth or thickness with XPS. Therefore, also the efficiency of surface engineering can be examined with XPS.

How does XPS work?

During an XPS analysis, the sample is irradiated with X-rays (photons), and some of them are absorbed by the atoms in the sample. When the electron of an atom absorbs enough energy from a photon, it is ejected from the atom with a certain kinetic energy. A detector measures the kinetic energies of those ejected electrons coming from the surface (from the top 1-10 nm) of the sample and counts the number of electrons for every kinetic energy measured. These numbers represent the intensities of the different kinetic energy signals of the ejected electrons. Electrons of different energies follow different paths through the detector which allows the computer to distinguish the electrons and produce a plot (a spectrum) of their energies and relative numbers.

The atoms of a certain element emitting electrons of a particular energy produce their own characteristic peak in the spectrum. The energies and intensities of these peaks can be utilized to determine the composition of the sample’s surface. When the kinetic energy of an emitted electron has been measured and the energy of the absorbed photon (the wavelength of the original X-rays) is known, the binding energy of the emitted electron can be defined mathematically. The binding energy of the electron depends on the atomic orbital (energy level) where the electron is located and the element to which the electron belongs. Therefore, the elements on the surface can be identified based on the characteristic binding energies of the electrons emitted by their atoms, and the intensity of a certain kinetic energy signal is proportional to the amount of the element.

However, the binding energy of an electron does not only depend on its atomic orbital but also on the chemical environment of its atom. The chemical environment means the chemical bonds that the atom and its electrons partake in. Therefore, the electrons of for example a carbon atom bound to another carbon atom would have slightly different binding energies than the electrons of a carbon atom bound to an oxygen atom. These little differences in the binding energies between the electrons of the atoms of the same element make the determination of the chemical states of the atoms and the empirical formulas (e.g. (C_2H_6)x) of the compounds present in the surface layer of the material feasible. This information helps in the identification of the compound and the determination of its atom-level structure.

What is the difference between XPS, XRF, and XRD?

All three methods utilize X-rays in different ways, which is why different kinds of information can be obtained from the sample with each one of them. In XPS, the atoms of the sample’s surface absorb X-rays and emit electrons, but in XRF the atoms of the sample both absorb and emit X-rays. In XRD, the atoms of the sample do not absorb X-rays at all, they just reflect them.

When the emitted X-rays are analyzed in XRF, the elements present in the whole sample can be identified and their concentrations determined. The same information can be obtained also with XPS, but only from the surface of the sample. With XPS, the empirical formulas of the compounds on the surface can be deduced, which cannot be done with XRF or XRD.

Compared to XPS and XRF, XRD provides more information about the larger structure of the material. When the reflected X-rays are analyzed in XRD, the whole sample material or its components can be identified and the relative amounts of the components can be determined. Also, the lattice parameters of the material’s particles, as well as the crystallite size and strain can be measured.

Sample requirements and preparation

Solid samples, powders, coatings, thin films, or ultra-thin films are suitable for XPS analysis. Therefore, the samples can be anything from non-stick cookware coatings to thin-film electronics and bioactive layers. Usually, the sample must be cut smaller (about an inch in diameter), and volatile components have to be removed before analysis.

Need an XPS analysis?

Measurlabs offers XPS testing services for various sample types, including thin films, coatings, and the surfaces of bulk materials. Please contact us through the form below or by emailing us at info@measurlabs.com to request a quote. Our experts are also always happy to help if you have any questions about your sample or its suitability for the method.