Contact AFM


In idealized experimental conditions (e.g. in ultrahigh vacuum) when the cantilever tip approaches the sample surface Van der Waals forces start acting upon it (Fig. 1). They are sufficiently far-ranging and are felt at the distance of a few tens of angstroms. Then at the distance of several angstroms repulsive force starts acting.

In real conditions (in ambient air) practically always some humidity is presented in air and a water layer is adsorbed on the sample and tip surfaces. When cantilever approaches sample surface the capillary force arises (Fig. 2) that holds the tip in contact with the surface and increases the minimum achievable interaction force.

Electrostatic interaction between the probe and the sample may appear rather often. This can be both attraction and repulsion. Van der Waals attraction forces, capillary, electrostatic and repulsion forces at the point where the tip touches the sample and forces acting upon the tip from the deformed cantilever compensate each other in equilibrium.

In Contact mode of operation the cantilever deflection under scanning reflects repulsive force and is used as such, in feedback circuitry or in their combination to imagine the sample surface profile.

Simultaneously with topography acquisition under scanning one can imagine some other characteristics of the investigated sample. If cantilever with tip are conductive one can imaging spreading resistance of the sample. If scanning is carried out in direction perpendicular to the longitudinal axis of cantilever (lateral direction) the friction force causes cantilever twisting. By measuring this twisting using position-sensitive four-sectional detector one can simultaneously with topography imagine the friction forces distribution throughout sample surface.

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