Variation with oscillation free amplitude

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.

When the tip-CL system vibrates far from the surface, it oscillates with its free amplitude Af. When the attractive force field b omes large enough (typically for distances between the tip and the surface smaller than 2nm), if the drive frequency is slightly below the resonant one, the oscillation amplitude increases. At this point the sample is retracted. For its retraction, the oscillation amplitude takes a new route than for the approach the variation of the oscillation amplitude shows an hysteresis between the approach and the retraction of the tip from the sample. The retraction route depends on the drive amplitude as shown in Figure 10b which repre nts only retractions for clarity purposes. The smaller the drive amplitude the smaller the hysteresis cycle, to a point where no hysteresis cycle can be detected. 

Figure 15 Variation with the oscillation free amplitude Af of the two parameters used for the fits of the amplitude variation widi distance for three molecular weights.

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