X-ray photoelectron spectroscopy (XPS) is a technique for analyzing the surface chemistry of a material. 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 and electrons that are emitted from the top 1-10 nm of the material being analyzed. A photoelectron spectrum is recorded by counting ejected electrons over a range of electron kinetic energies. Peaks appear in the spectrum from atoms emitting electrons of a particular characteristic energy. The energies and intensities of the photoelectron peaks enable identification and quantification of all surface elements (except hydrogen).
As the demand for high performance materials increases, so does the importance of surface engineering. 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 at the interfaces of a material’s layers. The surface chemistry will influence such factors as corrosion rates, catalytic activity, adhesive properties, wettability, contact potential and failure mechanisms.
The material’s surface is the point of interaction with the external environment and other materials. Therefore, surface modification can be used in a wide variety of applications to alter or improve the performance and behavior of a material. XPS can be used to analyze the surface chemistry of a material after an applied treatment such as fracturing, cutting or scraping. From non-stick cookware coatings to thin-film electronics and bio-active surfaces, XPS is the standard tool for surface material characterization.
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 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 represents a discontinuity between one phase and another and, therefore, the physical and chemical properties of the surface are different from those of the bulk material. These differences affect the topmost atomic layer of the material to a large extent. In the bulk of the material, an atom is surrounded on all sides in a regular manner by atoms composing that material. Because a surface atom is not surrounded by atoms on all sides, it has bonding potential, which makes the surface atom more reactive than atoms in the bulk.
Visit XPS Simplified for a more comprehensive explanation of the technology.
XPS Applications in the Metal/Steel Industry
XPS is a fundamental characterization tool for investigating a wide range of surface problems on metal and oxide surfaces. With its surface selectivity and quantifiable data, XPS is the ideal tool for measuring composition and thickness of protective oxide films on metals. In addition, sputter profiling the steel surface provides a full understanding of the surface composition and chemistry, which may help to diagnose failures in the passivation process. Finally, XPS can detect any contaminants that may have been introduced during the manufacturing process.
View the webinar XPS Simplified: Understanding Metal Surfaces and Oxides with X-ray Photoelectron Spectroscopy (XPS) to learn how XPS was used to investigate these surface issues.