Intermittent Contact Tapping Mode AFM

The high lateral forces and concomitant drawbacks of contact mode are circumvented in intermittent contact (also called tapping) mode AFM (Fig. 6.5). This mode utilizes an oscillating tip-cantilever assembly and relies on a feedback from the amplitude (constant amplitude imaging). A typical cantilever for operation in air is much stiffer than a contact mode cantilever (10-100 N/m) and is excited to resonance or near resonance. The forced oscillator is damped upon interaction of the tip with the sample surface (Fig. 6.5b and c). If the cantilever spring constant or the amplitude is too low, the energy in the forced oscillator is not sufficient to overcome the adhesive interactions and the tip remains trapped in contact and is consequently dragged across the surface. 

1 Constant Amplitude Imaging The sample topography z = z(x, y) of the sample surface can be reconstructed from the recorded (i.e., known) compensation 8z for each pixel (x, y) by scanning the sample with respect to the oscillating 

The acquisition of an accurate surface profile is based, as mentioned, on the assumption of identical energy dissipation in all areas of the surface imaged. If the forced oscillation of the cantilever-tip assembly is damped in different areas to a different extent, the correction 5z does not match exactly the true surface profile. If, for instance, phase A in a heterogeneous phase-separated structure damps the oscillation more efficiently compared to phase B, the more dissipative phase A will appear too high compared to phase B. The example shown 

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The prime differences among the different AFM modes, such as CM (discussed above) and intermittent CM, as elucidated in the following section, are the feedback parameters and the choice of the cantilever. For intermittent contact (tapping) mode AFM, a stiff cantilever (k typically 10—50 N/m) with a resonance frequency of 100—400 kHz is chosen. The cantilever, which is inserted in an identical manner as for CM into the cantilever holder, is excited to vibrate by an integrated piezo actuator. Instead of deflection (contact force), the amplitude of the forced oscillating lever is detected, analyzed, and utilized in the feedback loop (Fig. 2.20). 

In addition to its capability of imaging the topography of polymer surfaces with virtually eliminated shear forces, intermittent contact (tapping) mode AFM, can also be useful to probe various surface properties, such as adhesive or surface mechanical properties. Thereby AFM can help to identify and quantify the abundance and distribution of the phases present in multicomponent systems. As shown already... 

The immobilized G4 PAMAM dendrimers can be directly visualized using intermittent contact (tapping) mode AFM. As shown in Fig. 15, the dendrimers attached to NHS - CIO SAMs appear as globular features with heights... 

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