Evidence for the contamination layer can be easily observed with a force distance (F/D) curve that measures the interactive forces between an AFM probe and a sample surface. In an AFM, the F/D curve is measured by monitoring the deflection of a cantilever with a probe at its end, as the probe is pushed into the surface. As the probe moves closer to the sample surface, it is pulled in between the probe and the contamination layer by capillary forces. This is called the attractive force because the probe is pulled in toward the surface.
Understanding the contamination layer is important when imaging with AFMs for the following reasons:
- When doing probe tip approach, a contamination layer can create a false feedback if the probe is not able to interact with the hard forces of the sample surface.
- The contamination layer can reduce the ultimate resolution of the AFM in air. This is because the interaction volume between the probe and the surface is made larger by the contamination layer.
- Scan rates can be limited as the contamination layer can cause turbulence which then destabilizes the AFM's Z feedback while scanning a sample.
- Occasionally the AFM probe tip can skip up on the contamination layer, thereby creating image artifacts.
Once you have planned your scanning around potential sample surface contamination issues, you can obtain amazing image results from your AFM. For a discussion on techniques to remove sample surface contamination, you may want to review "Methods for Cleaning AFM Reference and Calibration Samples" or a recording of our popular webinar on AFM Sample Preparation.