Instruction manual

Force Imaging
Force Calibration Mode
182 MultiMode SPM Instruction Manual Rev. B
The cantilever’s deflection is plotted on the vertical axis of the graph: when the cantilever is
deflected downward, it is plotted on the graph’s downward vertical; when the cantilever is deflected
upward, it is plotted on the graph’s upward vertical.
The graph reveals at least two very important things:
• Sample-tip attraction—As the tip approaches the sample, various attractive forces
reach out and “grab” the tip. This is evidenced at point 2 (slight dip) in the graph above
(see Figure 11.2c). Notice how the tip suddenly plunges toward the sample here during
its descent. This is sometimes called the “jump-to-contact” point and is usually due to
electrostatic attraction and/or surface tension (capillary) forces.
Attraction is also evident between points 4 and 5 (sloped line) as the cantilever is pulled
away from the sample. If attractive forces are strong enough, the tip will cling to the
sample surface as it is pulled clear. Eventually, the sample “lets go” and the tip rebounds
sharply upward (dashed line, between points 5 and 6).
By knowing the spring constant of the cantilever, it is possible to measure the attractive
forces of tip-sample interactions with good precision.
Note: Although attractive forces appear small, remember that the tip is extremely
sharp. Since only a few nanometers of the tip actually touch the sample, even
minute forces are distributed over an exceedingly small area, which add up
quickly. Many materials are easily dented by the tip under such conditions.
Force plots may be used to adjust a setpoint which applies minimal force to the
sample during contact AFM. (More on this topic below.)
• Material elasticity—It is possible to extract some information regarding the elasticity
of the material by studying force curves. In the graph above, the tip is in constant
contact with the sample between points 2 and 5. As the tip is pressed further and further
into the material, the probe’s cantilever flexes. The amount of cantilever flexion for a
given amount of downward tip movement gives an indication of the material’s elasticity.
For example, if the material is extremely hard, pressing the sample upward will result in
a relatively large amount of cantilever flexion. On the other hand, if the material is soft,
the cantilever will flex less during the ascent of the sample. The shape and slope of the
contacted portion of the force curve gives detailed information about surface elasticity.
It is sometimes possible to obtain quantitative measurements of sample elasticity. (See,
for example: Radmacher, et al. 1994. Science, Vol. 265:1577-1579.)
Two imaging techniques may be employed to measure and display elasticity at multiple
points on a sample surface: force modulation (see Section 11.6 below) and force volume
imaging (see Section 11.8 below).