Force modulation microscopy (FMM) is an extension of AFM imaging that includes characterization of a sample's mechanical properties. Like LFM and MFM, FMM allows simultaneous acquisition of both topographic and material-properties data.

In FMM mode, the AFM tip is scanned in contact with the sample, and the z feedback loop maintains a constant cantilever deflection (as for constant-force mode AFM). In addition, a periodic signal is applied to either the tip or the sample. The amplitude of cantilever modulation that results from this applied signal varies according to the elastic properties of the sample, as shown in Figure 6.

 

 

Figure 6. The amplitude of cantilever oscillation varies according to the mechanical properties of the sample surface. (bottom).

 

The system generates a force modulation image, which is a map of the sample's elastic properties, from the changes in the amplitude of cantilever modulation. The frequency of the applied signal is on the order of hundreds of kilohertz, which is faster than the z feedback loop is set up to track. Thus, topographic information can be separated from local variations in the sample's elastic properties, and the two types of images can be collected simultaneously. Figure 7 shows a topographic contact-AFM image (left) and an FMM image (right) of a carbon fiber/polymer composite.

 

 

 

Figure 7. Contact-AFM (top) and FMM (bottom) images of a carbon fiber/polymer composite collected simultaneously Field of view 5µm

 

 

 

 

 

 

 

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