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Principle of operation of atomic force microscopes: atoms at the end of a small tip interact with atoms on the surface of the sample just below
The atomic force microscope is a force sensor whose principle is based on the interaction of a tip with the sample. It measures the force field that is established between the tip of the tip and some atoms of the surface of the sample located directly below it. As these forces depend on the distance between the tip and the sample, the measurement of these interaction forces makes it possible to know the topography of the sample scanned by the AFM tip.
The interaction forces
The interaction forces
Potential energy between the AFM tip and the sample surface as a function of the distance between them
There are many forces involved in the measurement process. They depend both on the medium in which the AFM measurements are made (air, liquid, vacuum, etc.) but also on the type of tip, sample and the possible functionalization of the tip that makes it more sensitive to a particular force. We shall see in the following paragraphs the main forces which apply to the point in the air. In all cases, the interaction forces between the AFM tip and the sample are dependent on the distance between the tip and the sample.
The Lenard-Jones model describes in a simple and approximate way these interactions the potential energy between two atoms separated by a distance. Lenard-Jones Potential
The first term of the difference corresponds to a repulsive term, dominant when the distance point to surface is small (less than nanometer). It represents the set of electrostatic repulsion forces that appear between the electronic clouds of each atom when they begin to interpenetrate. This phenomenon is known as Pauli’s principle of exclusion. The second term at power six corresponds to an attractive effect dominant at <> distance. It mainly gathers the forces of Van der Waals.
The main forces that interact on the tip when performing a topography measurement with ambient air are:
Van der Waals' strengths: are attractive and result from three effects: Orientation effects (permanent multipolar moments between two molecules), induction effects towards non-polar molecules and dispersion effects (instantaneous polarization due to fluctuations in electron positions around the nucleus). They act over very large distances, up to several hundred nanometers. They are strong enough to be able to move macroscopic objects over distances of a few micrometers, as is the case with the lever of an AFM. In air or vacuum, they are of the order of and are reduced by a factor of 10 to 100 in liquid medium.
Attractive capillary forces appear when the tip approaches the surface of a sample placed in the air. They are due to the presence of a thin layer of water on the surface of the sample which is formed by condensation of the moisture present in the ambient air. They depend on the moisture content, the radius of curvature of the tip, and the sample (hydrophilic or hydrophobic).
When the tip is close enough-a few nanometres-a meniscus is formed that strongly pulls the tip towards the surface as it approaches it, or that holds it back
The main forces of interaction between the tip and the surface of an open-air sample
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