The retracting portion of the force curve sometimes
follows the approach curve; however, there is often hysteresis. The
most common type of hysteresis is due to some sort of adhesion, which
appears in the force curve as a deflection below the zero-deflection
line. The source of the adhesion can vary depending on the sample. In
the ideal case of a sphere interacting with a flat surface, the
adhesion force can be related to the radius of the sphere and the
surface energies of the two surfaces.
Under ambient conditions, the
main source of adhesion is the formation of a capillary bridge between
the tip and the sample. In air, most samples have several nanometers of
water adsorbed to the surface; this water layer wicks up the tip and
forms a ‘bridge’ between the tip and the sample. Pulling the tip out of
that bridge requires a large force to overcome the surface tension. In
fluid, the adhesive force depends on the interfacial energies between
the tip and sample surfaces, and the solution; varying the solution can
thus change the force of adhesion.
A different form of ‘adhesion’ occurs when a polymer
is captured between the AFM tip and the substrate. In this case, there
is a very distinctive ‘adhesive’ force as the tip is pulled away.
Typically, these curves initially retrace the approach curve near the
surface but, away from the surface, exhibit a smooth negative
deflection as the polymer is stretched until it breaks or detaches from
the tip or the substrate, and the cantilever returns to the
zero-deflection line. If multiple polymer molecules attach to the tip
and substrate, a saw-tooth pattern can be observed as individual
polymers detach. For references concerning different kinds of adhesive
forces see [1,2].
To
be useful, the force curves must be transformed into descriptions of
force as a function of distance, F(D). However, current SFMs do not
have an independent measure of D. Instead, the transformation to D is
achieved by subtracting the cantilever deflection from the z-piezo
movement.
In some cases detachment of the tip under retractive
motion of scanner occurs abrupt, and suitable force (force of adhesion)
can measured relatively correctly.
Corresponding adhesion maps are
typically produced by taking the most negative force detected during
the retraction curve as the value for adhesion and plotting that value
against the x–y position of each curve. Several types of spatially
resolved adhesion map can be produced, for example, the spatial
distribution of adhesion in grafted-polymer systems. Using a special
AFM tips, modified by antibodies or ligands , one can map the
distribution of specific proteins on the surfaces of living cells etc.
References
TIBTECH 17, 143 (1999).
Israelashvili, J.N. (1992) Intermolecular and Surface Forces, Academic Press.
