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How does depth profiling show how a material’s composition changes below its surface?

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Moon rock crystals — surface analysis A scanning-electron-microscope photograph of pyroxene and plagioclase crystals (the long and the short crystals, respectively) that grew in a cavity in a fragment of Moon rock gathered during the Apollo 14 mission.

Depth profiling is a technique used in surface analysis to determine how a material's chemical composition changes from the surface into the bulk of the solid.1

This process typically relies on ion bombardment, where a high-energy beam of ions is directed at the sample. When these ions strike the material, they transfer momentum to the lattice atoms, causing a phenomenon known as "sputtering". This effect effectively erodes the surface, removing atomic or molecular layers over time.23

By operating in a "dynamic" mode—characterized by a high current density—the ion beam continuously bores into the material. As the beam removes successive layers, the composition of the newly exposed material can be analyzed, allowing researchers to map the vertical distribution of chemical species. While other techniques like X-ray photoelectron spectroscopy can analyze surface chemistry, they are often less effective for depth profiling because they require integration with ion sputtering and may have slower data acquisition rates.2134