Nanophysics of Soft Materials Related to Piconewton Investigations by Atomic Force Microscopy
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Two modes of operation are used to record the change of the oscillating behavior when the tip interacts with a surface:
The amplitude modulation mode (AM), often called tapping mode as a commercial name, where the tip-cantilever system is excited at a fixed-driven frequency and a fixed-amplitude excitation. The main advantage is its ability to image polymers without producing the severe damage caused by contact mode. Many works have been dedicated to studies on very soft samples but, although the AM mode provides a convenient and easy way to image soft materials, quantitative analysis of the experimental data remains uneasy.
The frequency modulation technique (FM), often called noncontact resonant mode. The FM mode was first used experimentally under ultra-high vacuum conditions to image semiconductor surfaces. The concept is to use a negative resonant frequency shift at constant oscillation amplitude as the error signal to control the distance between the tip and the surface. The interaction between the tip and the surface remains attractive so that the tip never touches the surface. The key experimental achievement was the demonstration that the FM mode was able to obtain images with contrast at the atomic scale. The experimental data that depend on the interaction between the tip and the surface are the resonance frequency variation and the damping signal extracted from the energy per cycle needed to supply in order to keep the oscillation amplitude constant. The frequency shift is related only to conservative interaction and the additional damping signal should be uniquely governed by the dissipative interaction.