Abstract
Due to the harmonic motion of the cantilever in Tapping Mode Atomic Force Microscopy, it is seemingly impossible to estimate the tip-sample interactions from the motion of the cantilever. Not directly observing the interaction force, it is possible to damage the surface or the tip by applying an excessive mechanical load. The tip-sample interactions scale with the effective stiffness of the probe. Thus, the reduction of the mechanical load is usually limited by the manufacturability of low stiffness probes. However, the one-to-one relationship between spring constant and applied force only holds when higher modes of the cantilever are not excited. In this paper, it is shown that, by passively tuning higher modes of the cantilever, it is possible to reduce the peak repulsive force. These tuned probes can be dynamically more compliant than conventional probes with the same static spring constant. Both theoretical and experimental results show that a proper tuning of dynamic modes of cantilevers reduces the contact load and increases the sensitivity considerably. Moreover, due to the contribution of higher modes, the tuned cantilevers provide more information on the tip-sample interaction. This extra information from the higher harmonics can be used for mapping and possibly identification of material properties of samples.
Highlights
An Atomic Force Microscope (AFM) is a versatile instrument that enables measurement and manipulation of samples at the nanoscale
Recent advances in AFM technology already carried its application beyond topography imaging, such as subsurface elasticity measurements,39 unfolding force measurements of biomolecules,21 thermal conductivity measurements,9,16 surface chemical composition mapping,22 and mechanical properties mapping
Topography imaging with Tapping Mode AFM (TM-AFM) can still be considered the most common application
Summary
An Atomic Force Microscope (AFM) is a versatile instrument that enables measurement and manipulation of samples at the nanoscale. Recent advances in AFM technology already carried its application beyond topography imaging, such as subsurface elasticity measurements, unfolding force measurements of biomolecules, thermal conductivity measurements, surface chemical composition mapping, and mechanical properties mapping.. In TM-AFM, the cantilever is excited by a dithering signal with constant amplitude and a frequency near the fundamental resonance frequency of the cantilever. The amplitude is kept constant by adjusting the distance between the cantilever and the sample. The control signal that adjusts the distance is interpreted as the topography image. The phase delay between the motion of the cantilever and excitation signal is recorded as a measure of energy dissipation and can be related to adhesion, viscoelasticity, or hysteresis in the surface energy level.
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