X-ray photoelectron spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA), is the standard technique for analyzing the surface chemistry of a material. The material’s surface is the point of interaction with the external environment and other materials. 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. Surface chemistry influences such factors as corrosion rates, catalytic activity, adhesive properties, wettability, contact potential and failure mechanisms.
As the demand for high performance materials increases, so does the importance of surface engineering. Surface modification can be used to alter or improve the performance and behavior of a material in a wide variety of applications including non-stick cookware coatings, thin-film electronics, and bio-active surfaces. XPS can be used to analyze the surface chemistry of a material after an applied treatment such as fracturing, cutting or scraping. Investigating the oil-resistant coatings on touch screens, measuring plasma deposited coatings of bio-medical devices, or understanding organic LEDs or solar cells requires the ability to etch and measure individual layers of materials using XPS depth profiling.
Accurately quantifying chemical compositions using XPS depends on having a clean sample. For example, analysis of metal oxide samples can be hindered by adventitious carbon contamination, which is typically present on the surface. Because XPS provides chemical information from the top 1–10 nm of a sample, any small surface layer of carbon contamination can have a noticeable effect on the quantification of the sample. Thus the sample must be thoroughly cleaned and all surface contaminants removed before it can be accurately analyzed.
Most XPS systems are equipped with an ion source that can be used to remove surface material for cleaning or depth profiling of samples. Ion sputtering is the long-established method for cleaning for surface analysis but it can damage the sample, which affects the analysis. The ion source usually produces monatomic ions of noble gas, typically argon. When metal oxides are cleaned with monatomic argon ion bombardment, the contamination will be cleaned off the surface; however these ions also penetrate the underlying sample, causing chemical damage even at low ion energies. This is typically seen as reduction of the oxide, due to preferential sputtering and removal of oxygen.
XPS instruments using an argon cluster ion source address this difficulty. Argon cluster sputtering provides exceptionally gentle removal of contaminants without damaging the sample because they do not penetrate the oxide surface, preserving chemical state information while still cleaning contamination from the sample.
Dual beam ion sources can also generate a monatomic beam when profiling into a harder inorganic substrate. With the ability to provide both cluster ions and monatomic ions,this instrumentation offers versatility and convenience for a range of sample types. Read the application note, Cleaning Metal Oxides Using Argon Cluster Ions to Prevent Surface Modification that describes how the cluster mode of this instrument can be used to clean an oxide sample without inducing reduction in the remaining surface.
Visit XPS Simplified for a more comprehensive explanation of the technology.