Further Dynamic AFM Modes

IMAGING POLYMER MORPHOLOGY USING ATOMIC FORCE MICROSCOPY 

2 PFT Mode AFM Related to this dynamic pulsed force mode is the so-called PFT mode. Using a cantilever with intermediate spring constant ( lN/m), a somewhat altered intermittent contact mode experiment is carried out during which the entire force-displacement curve is being captured. Unlike the intermittent contact mode, however, in which the amphtude is used as the feedback parameter, in this mode the force is controlled directly by using the maximum exerted force, the peak force, as the feedback parameter. This mode allows exquisite control of the force exerted with the tip/cantilever and provides information on sample deformation, stiffness, and adhesive forces from an analysis of the force-displacement curve (Fig. 6.8). 

In the PFT mode, as mentioned, the AFM tip is brought intermittently into contact with the sample surface with an adjustable amplitude 0.1 nm. Similar to the intermittent contact mode, this minimizes the lateral forces as the lateral displacement is slow compared to the residence time of the tip in contact, even though the tip (or the sample) is oscillated out of resonance at a frequency of 1-10 kHz, which is lower than in standard TM in air and also in liquid, and even though the imaging speeds in both modes are comparable. 

In each oscillation cycle, the force-displacement curve is recorded and analyzed. Initially, the force is zero (baseline force), which corresponds to the deflection of the undisturbed cantilever with the tip being far away from the sample surface. 

The value of the peak force threshold is held constant by the feedback loop, so that at each point on the sample scanned the force exerted on the sample is nominally the same. Provided that the lateral differences in adhesive forces are negligible, this affords constant normal force imaging conditions. 

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