Idealized approach-retract curve

Figure 2b Idealized approach-retract curve plot of the oscillation amplitude variation with the tip-sample distance during the approach and retraction of a sample toward an oscillating tip-cantilever system. First, when the tip is far from the sample, it oscillates with its free amplitude Af as depicted in part a. In part b, the tip-CL system interacts with the surface through an attractive field. If the drive frequency is slightly below the resonance one, the oscillation amplitude increases. Part c corresponds to the so-called AFM tapping mode where the tip comes in intermittent contact with the sample. In this part, the oscillatory amplitude A decreases linearly with the CL-surface distance d with a slope equal to 1 if the sample is hard, that is if dcAf, A(d) = d. In part d, the tip is stuck on the sample with an oscillation amplitude down to zero. The tip might be damaged this part is usually avoided.

Figure 13 Idealized force-distance curve describing a single approach-retraction cycle of the AFM tip. Modified from Shahin, V. etal. J. Cell. Scl. 2006, f f9,23-30. The AFM tip is approaching the sample surface (a). The initial contact between the tip and the surface is mediated by the attractive van der Waals forces (contact) that lead to an attraction of the tip toward the sample (b). Hence, the tip applies a constant and default force upon the sample surface that leads to sample indentation and cantilever deflection (c). Subsequently, the tip tries to retract and to break loose from the surface (d). Various adhesive forces between the sample and the AFM tip, however, hamper tip retraction. These adhesive forces can be taken directly from the force-distance curve (e). The tip withdraws and loses contact with the sample upon overcoming the adhesive forces (f). Inset experimental approach curve recorded fora silicon surface electrografted by poly(/V-succinimidyl acrylate) (PNSA) (from a 0.1 M NSA solution in DMF) with a silicon nitride tip.

The reversibility of the curve was confirmed by repeating the approach and retraction cycle for more than 60 times without breaking the crosslinker. Although decidedly non-linear, this is almost an ideal coil spring behavior at the molecular level. 

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