X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) is a comprehensive technique for analyzing the surface layers of semiconductors, metals, plastics and fibers. The method is used for determining the elemental composition of the surface, the chemical states of the elements as well as the electronic structure of the compounds.

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What is XPS?

X-ray photoelectron spectroscopy (XPS) is a technique for analyzing the chemistry and properties of 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 its 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 found out.

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 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 which the electron belongs to. Therefore, the elements in 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 XPS used for?

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

Examination of the surfaces of materials 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. Because of this, 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.

The surface of the material has different physical and chemical properties compared to the bulk material. Since the surface is on the edge of the bulk material and the environment phase, the topmost surface atoms are not surrounded by other atoms on all sides, unlike the atoms in the bulk material. Due to this, the surface atoms have bonding potential, which makes them more reactive than the atoms in the bulk. When the physical and chemical interactions between the surface atoms and external environment or another material at the interface of two layers are understood with the help of XPS, solving problems related to the manufacturing of materials becomes significantly easier.

What is the difference between XPS, XRF and XRD?

All of these 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 in 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 individual 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 the analysis. Therefore, the samples can be anything from non-stick cookware coatings to thin-film electronics and bio-active layers. Usually the sample must be cut smaller (about an inch in diameter) before the analysis and volatile components have to be removed from the sample.

Need an XPS analysis?

Measurlabs offers XPS analyses of high quality with fast results and affordable prices. If you have any questions about your sample or it’s suitability for the method, our experts are always happy to help. You can contact us through the form below or by emailing us at info@measurlabs.com.

Suitable sample matrices

  • Solid samples
  • Ultra-thin films, barrier layers and coatings
  • Semiconductors
  • Microelectronics and microcircuits
  • Plastics and plastic coatings
  • Metals
  • Glass
  • Fibers and fiber composites

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Frequently asked questions

What is Measurlabs?

Measurlabs offers a variety of laboratory analyses for product developers and quality managers. We perform some of the analyses in our own lab, but mostly we outsource them to carefully selected partner laboratories. This way we can send each sample to the lab that is best suited for the purpose, and offer high-quality analyses with more than a thousand different methods to our clients.

How does the service work?

When you contact us through our contact form or by email, one of our specialists will take ownership of your case and answer your query. You get an offer with all the necessary details about the analysis, and can send your samples to the indicated address. We will then take care of sending your samples to the correct laboratories and write a clear report on the results for you.

How do I send my samples?

Samples are usually delivered to our laboratory via courier. Contact us for further details before sending samples.