Surface Analysis Techniques
Surface chemistry of materials investigated with spectroscopy techniques.
As the demand for high-performance materials increases, so does the importance of surface engineering. The material’s surface is the point of interaction with the external environment and other materials; therefore, many of the problems associated with modern materials can be solved only by understanding the physical and chemical interactions that occur at the surface or the interfaces of a material’s layers.
A surface layer is defined as being up to three atomic layers thick (~1 nm), depending upon the material. Layers up to approximately 10 nm are considered ultra-thin films, and layers up to approximately 1 μm are defined as thin films. The remainder of the solid is referred to as bulk material. This terminology is not definitive, however, and the distinction between the layer types can vary depending upon the material and its application.
The surface will influence such factors as corrosion rates, catalytic activity, adhesive properties, wettability, contact potential and failure mechanisms. Surface modification can be used to alter or improve these characteristics. Surface analysis techniques are therefore very important to understand surface chemistry of a material and investigate the efficacy of surface engineering, material failures, or the development of new devices.
X-ray photoelectron spectroscopy (XPS) is a technique commonly used to explore surface chemistry. The data obtained from XPS provides the quantified composition of the outer few nanometers of a material. In other words, it details both the elements present and the chemical states of those elements.
Knowing this information is crucial for understanding and improving the performance of a material. The surface sensitivity of XPS means that it reveals the surface chemistry at a level that other routinely used analytical techniques cannot. XPS analysis can be extended into a material through a process known as depth profiling, which slowly removes material using an ion beam, collecting 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.
XPS is a vital tool for understanding surface chemistry and is used to overcome challenges in a wide range of industries and applications; however, using only a single technique to understand a material is not always enough. To gain a complete understanding, other analyses may be required. Ideally, the different experiments should be performed on the same instrument, removing uncertainty about co-locating the analysis point and ensuring the same sample conditions.
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. As the energy of the incident beam, the mass of the ion, the scattering angle, and the energy of the scattered ion are known, conservation of momentum can be used to calculate the mass of the surface atom. Because this interaction can occur only with the outermost surface layer, ISS is very effective. It is used to investigate surface segregation and layer growth, complementing the composition information from XPS.
Reflected electron energy loss spectroscopy (REELS) is a technique used to probe the electronic structure of the material at the surface. It works in a similar fashion to ISS, but in this case, the incident particle is an electron, and it is the scattered electron beam that is measured. The incident electrons can lose energy by causing electronic transitions in the sample, and these energy losses are what are measured in the REELS experiment. Properties such as electronic band gaps or the relative energy levels of unoccupied molecular orbitals can be measured. In some cases, it is also able to detect hydrogen, which is not possible with XPS.
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. This is quite a complex region, with a lot of overlapping peaks, but it can act as a fingerprint for compounds. Usually, comparing the data collected in this region with XPS and UPS is helpful. UPS data complements REELS data for understanding electronic properties, providing information on the highest energy occupied bonding states. The width of a photoelectron spectrum can be used to measure the work function on suitable samples, and this can be done easily using a relatively short full range scale (0–22 eV or 40 eV, rather than 0–1487 eV for Al Ka X-rays).
Raman spectroscopy is a technique, which is very sensitive to structural changes, used to understand molecular bonding in materials. It is another scattering technique, but this time, photons from a laser source are used, typically in the infrared to UV wavelengths. Of the incident photons, a few undergo Raman scattering, losing energy through exciting vibrational modes of the sample. These scattered photons are detected to make a spectrum. Raman spectroscopy typically has a much larger depth of analysis compared to the other techniques listed here. However, the complementary information obtained is especially useful for understanding both polymers, where the bulk information complements the surface information, and nanomaterials, such as graphene and carbon nanotubes, where the depth scales correlate nicely.
Auger electron spectroscopy (AES) is a technique that uses a focused electron beam to measure the surface composition. The Auger emission process is caused by the relaxation of an atom after an electron has been emitted. The vacancy in a shell is filled by an electron from another orbital, and the extra energy released in this process causes the emission of another electron. Auger features are seen in XPS spectra, as this process can also occur after a photoelectron is emitted. However, for AES, an electron gun is usually used to excite the sample. Auger electron spectroscopy delivers elemental and some chemical state information, complementing XPS, and it has the advantage of higher spatial resolution.