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Force Imaging

Force Calibration Mode

Rev. D Dimension 3100 Manual 211

You can use force plots to adjust the setpoint so that minimal force is applied to the sample.

Although attractive forces appear small, the tip is extremely sharp. Because only a few nanometers

of the tip actually touch the sample, even minute forces add up quickly when distributed over an

exceedingly small area. The tip can easily dent many materials under such conditions.

The graph detailed in Figure 13.2a reveals the following types of information:

Tip-Sample Attraction

As the tip approaches the sample, various attractive forces reach out and grab the tip. This is evident

at point 2 (slight dip) in the graph above. The tip plunges toward the sample during its descent. This

is also referred to as the “jump-to-contact” point and is usually due to electrostatic attraction or

surface tension (capillary) forces.

Attraction is also evident between points 4 and 5 (sloped line) as the cantilever pulls away from the

sample. If attractive forces are strong enough, the tip clings to the sample surface as it pulls clear.

Eventually, the sample retracts and the tip rebounds sharply upward (white line between points 5

and 6). You can measure attractive forces of tip-sample interactions if you know the spring constant.

Material Elasticity

It is possible to extract 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 4. As the tip

presses further into the sample material, the cantilever ﬂexes. The amount of cantilever ﬂexion for a

given amount of downward tip movement indicates the material’s elasticity.

For example, if the material is extremely hard, pressing the tip downward results in a relatively

large amount of cantilever ﬂexion. On the other hand, if the material is soft, the cantilever will ﬂex

less during its descent. The shape and slope of the contacted portion of the force curve gives

detailed information about surface elasticity. It is possible to obtain quantitative measurements of

sample elasticity. (See Radmacher, et al. 1994. Science, Vol. 265:1577-1579).

Two imaging techniques measure and display elasticity at multiple points on a sample surface:

force modulation and force volume imaging.

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