Three elements in the surface analysis spotlight
X-ray Photoelectron Spectrometry (XPS) helps scientists understand the surface chemistry of materials, and it can used to explore nearly every element, with the exception of hydrogen and helium. By clicking on different elements on the XPS periodic table on our XPS Simplified webpage, researchers can obtain a wealth of information for their surface analyses including the binding energy of common chemical states for each element, experimental information, interpretation of XPS spectra, and some of the surface analysis applications in which the element appears.
It’s always fascinating to see which elements are of most interest. And from the pages that generate the most traffic, we can make some educated guesses about the types of materials and problems surface analysis professionals are investigating. Below are three elements that have been receiving a lot of website traffic as well as some tips for how to approach these elements in your surface analyses. (I realize that oxygen is high on the list, but in isolation it doesn’t tell us too much, as it always is with something else, and that covers a lot of different areas. So this time we’ll skip over oxygen, and maybe return to it at a later date).
It’s no surprise that carbon is high on the list. Carbon is important in many areas, making it the focus of many experiments. But the nature of XPS also offers a reason for why carbon is seen so frequently. XPS instruments are extremely surface sensitive as they detect the composition of the outermost atomic layers of a sample. Carbon compounds that are present in the air—for example, carbon dioxide, and even organic compounds from the people in the lab—are likely to adhere to the surface of most samples. As a result, practically every sample has a layer of carbon on its surface. The element usually appears as a mix of carbon-carbon and carbon-oxygen bonding in the C1s XPS spectrum, generally with a recognizable shape.
Once the contamination is dealt with, carbon is a compound used in multiple materials from Li-ion batteries to engineered polymers for lightweight transport. A growing area of research is carbon nanomaterials such as graphene and carbon nanotubes, which are still in their early stages of application development.
Keep in mind: Care is needed to remove the two to three nanometers of carbon contamination from a surface. Gentle ion bombardment from the system ion source (cluster ions being especially good) will usually do the trick, but it’s a good idea to monitor the material below to ensure no chemical changes are occurring.
As one of the key components of steel, iron is of interest to researchers searching for reliable ways to protect steel surfaces from corrosion. Depending on the steel composition, the outer oxide layer is typically composed of a mixture of elements different in composition from the bulk of the material. Understanding the ratio of the different oxides at the surface helps to understand why a surface is resistant to corrosion—an important property, for example, when selecting the right material for an oil, gas, or water pipeline.
Keep in mind: Because oxidation states are critical when analyzing metals, it’s important to pay attention to the additional information you get in a spectrum. Satellite features such as those seen for iron, or other effects like multiplet splitting, can help to identify oxidation states, and in some cases, even compounds.
Nitrogen is often present in a sample bonded to something else. Nitrides formed by combining nitrogen with oxygen are very useful compounds, as are many other compounds that result from combining nitrogen with a second element. For example, silicon nitride is a tough, transparent material commonly used to provide scratch resistance. One application of this material is in architectural glass, in which several ultra-thin layers of metals and oxides are applied to provide properties for temperature regulation (so-called low-e glass). These layers require protection, and a thin layer of silicon nitride is both transparent and tough enough to provide scratch resistance. How the layer interacts with the layers below it is important, and XPS depth profiling can be used to determine if the layer structure has been applied in a stable way.
Keep in mind: Some nitrogen-containing materials such as polyimide can be very sensitive to XPS surface analysis. To ensure the reliable measurement of these samples, it’s a good idea collect data from several points and average them together rather than taking a measurement from just one position.
Tim Nunney is a surface analysis marketing manager at Thermo Fisher Scientific.
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