It is important to emphasize that we are not measuring a direct force in TappingMode. The curve shown in figure 7 is constructed by adding the short range repulsive and long range attractive forces.
When the probe approaches the sample, it experiences an attractive force and is pulled toward the surface until contact is made. From that point on, the repulsive interaction forces dominate the response. The probe can then be retracted and additional information can be extracted from that trace. The TappingMode AFM, while experiencing these interactions, does not actually measure this force curve, nor the direct forces between the tip and the sample for that matter. The TappingMode AFM oscillates back and forth on this curve, interacting without being in direct control of the force, and reporting only an average response of many interactions though the lock-in amplifier. One can certainly measure the reduction of cantilever amplitude as tip and sample approach each other, as is shown in figure 8, but it must be understood that each point on that curve represents an average value and not a single interaction.
While this is in no way detrimental to basic imaging, it restricts the information beyond sample topography that canbe gained and unambiguously assigned to a certain sample property. This is unlike the previously shown force-distance curve, during which one has direct control and is able to extract useful sample information.
Additionally, the inherently unstable feedback situation in TappingMode operation makes it quite difficult to automate some of the scan adjustments. Forces can vary when going away from a steady-state situation. This will occur while scanning rough surfaces, as the amplitude error at the sharp edges can correspond to interaction forces one order of magnitude higher than that of steady-state. Amplitude error incurred force is the leading cause of tip damage, and such damage occurs because the feedback is not directly controlling interaction force. On samples exhibiting high adhesion forces, a tip amplitude has to be selected that is high enough to ensure that the tip is actually leaving the sample surface. The higher the tip amplitude, the higher the energy stored in the lever and subsequently the imaging forces (see Appendix for a simple example). Operation in fluids suffers from drift due to temperature changes and/or changing fluid levels.
At this point, we have established that the adjustment of the feedback system is a task that is essential to achieving reliable information from the AFM. It is easier to control a contact mode scan when compared to a TappingMode scan due to the added complexity of the oscillating system. While past attempts have been made to adjust imaging parameters automatically in TappingMode, no method has proven competitive with an experienced user for the broad range of samples commonly studied with AFMs. This is because TappingMode operates at cantilever resonant frequency, where the cantilever dynamics are relatively complicated. For example, the cantilever dynamics can be dramatically changed by changing the amplitude set-point. This causes the highest usable gain to change, which in turn requires the optimal set-point to change. Additionally, the tapping dynamics depend strongly on the sample properties. A well-tuned feedback loop for the soft part of the sample can cause feedback oscillation for the hard part of the sample, rendering optimization of the parameters for every part of the sample very difficult. Furthermore, the long time constant (milliseconds) of the cantilever resonance also prevents instantaneous optimization at each imaging point. Finally, the direct force control of contact mode imaging and thus added information available are lost in TappingMode. TappingMode does however offer the undeniable benefit of lateral force free imaging, which has made it the dominant imaging mode in AFM to date.