Force Modulation

Force Modulation mode is used to image local sample stiffness or elasticity and is ideal for imaging composite materials or soft samples on hard substrates where you can obtain contrast between regions of different elasticity. This section assumes knowledge of operation of Contact AFM in air. It is useful, but not essential, to have experience operating in TappingMode AFM .

Force modulation mode is very similar to Contact Mode AFM. The NanoScope system scans the cantilever over the sample surface while trying to keep the cantilever deflection constant. The deflection setpointOperator-selected threshold used as the target of the feedback control loop. determines the average deflection during operation. In addition, the cantilever is oscillated up and down by a piezoelectric bimorph in the probe holder so that the tip indents slightly into the sample surface as it is scanned across the surface. The tip indents soft materials more easily than harder materials. The amount of cantilever deflection is inversely related to the amount of indentation.

The NanoScope system records the amplitude of the cantilever motion. For softer samples the tip penetrates further into the surface resulting in a smaller change in the angle of the cantilever. A small change in angle results in a small measured amplitude which displays as a bright area on the image. For harder samples the tip penetrates less into the surface resulting in a larger change in angle of the cantilever. A large change in angle causes a large measured amplitude which displays as a dark area on the image.

To measure the relative elasticity of the sample the system records the amplitude of the tip deflection versus position over the sample:

Force modulation requires the use of a special optional cantilever holder, shown below. This cantilever holder uses a piezoelectric bimorph to oscillate the cantilever against a sample surface. It is similar to the standard tapping cantilever holder; however, the piezo stack is much larger, allowing larger driving amplitudes:

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