Variable-deflection mode

In C-AFM, the tip makes physical contact with the sample. As the tip is moved across the sample, the contact force causes the cantilever to bend according to changes in topography. In constant force mode, the tip is constantly adjusted to maintain a constant deflection, and therefore constant height above the surface. It is this adjustment that is displayed as data. However, the ability to track the surface in this manner is limited by the feedback circuit. Sometimes the tip is allowed to scan without this adjustment, and one measures only the deflection. This is useful for small, high-speed atomic resolution scans, and is known as variable-deflection mode. 

AFM force imaging modes are closely related to the STM modes described previously. The modes are either constant force imaging (CFI) or variable deflection imaging (VDI) that... 

AUTO side). In this position, the controller is still in automatic mode. However, the deviation meter now indicates the approximate value of the controlled variable. The deviation meter deflects full down for zero variable value, and full up for 100% variable value. 

Images can be made in variable or constant force mode. In the latter case the difference signal from the photo detectors is used to adjust the distance between tip and surface, such that the force between the two, and thus the deflection of the cantilever, remains constant. An important advantage of working in constant force mode is that the overall orientation of the surface with respect to the z direction is not so critical, because the z piezo compensates for any inclination of the sample. 

I Pulsed Force Mode AFM In pulsed force mode AFM, a conventional contact mode AFM scan with feedback on the cantilever deflection is carried out. Simultaneously, the sample is modulated in the z direction sinusoidally (with variable frequency, e.g., 1 kHz) such that the tip breaks free from the surface periodically. The resulting plot of cantilever deflection versus time is shown in Figure 6.7. In the figure, it can be observed that the tip snaps to contact and then indents the sample. Upon retraction, the tip breaks free from the surface (pull-off) and the free cantilever rings down in air. Hence, instead of a slow acquisition of force-displacement data in... 

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