Force vs. distance (F vs. d) curves are used to measure the vertical
force that the tip applies to the surface while a contact-AFM image is
being taken.
It is a plot of the deflection of the cantilever versus the extension of
the piezoelectric scanner. The van der Waals force curve represents just
one contribution to the cantilever deflection. Local variations in the
form of the F vs. d curve indicate variations in the local elastic
properties. Contaminants and lubricants affect the measurement, as does
the thin layer of water that is often present when operating an AFM in
air.
A typical curve of an AFM operating in air is depicted in the
Figure. At the left side of the curve, the scanner is fully retracted
and the cantilever is undeflected since the tip is not touching the
sample. As the scanner extends, the cantilever remains undeflected until
it comes close enough to the sample surface for the tip to experience
the attractive van der Waals force. The tip snaps into the surface. As
the scanner continues to extend, the cantilever deflects away from the
surface, approximately linearly. After full extension, at the extreme
right of the plot, the scanner begins to retract. The retracting curve
is often different, because a monolayer or a few monolayers of water are
present on many surfaces. This water layer exerts a capillary force that
is very strong and attractive. As the scanner pulls away from the
surface, the water holds the tip in contact with the surface, bending
the cantilever strongly towards the surface (region c). At some point,
depending upon the thickness of the water layer, the scanner retracts
enough that the tip springs free (point d, snap-back point). As the
scanner continues to retract beyond the snap-back point, the cantilever
remains undeflected as the scanner moves it away from the surface in
free space.
If additional layers are present along with the water layer, multiple
snap-back points can occur: the positions and amplitudes of the
snap-back points depend upon the viscosity and thickness of the layers
present on the surface.
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