Abstract
We systematically investigate higher harmonics in the vibration spectrum of scanning force microscope cantilevers operating in the attractive regime. We show that (a) the magnitudes of the higher-harmonic signals in the vibration spectrum should be directly correlated to the local van der Waals forces for systems without significant electrostatic interactions and (b) the higher-harmonic resonances are much sharper than the fundamental harmonic. Consequently, and unlike the case of the tapping mode operation, contrast in the amplitudes of the higher harmonics over a scanned sample with small electrostatic forces reflects changes in specific chemical composition. Dynamic force-distance curves and higher-harmonic images are presented to demonstrate contrast between biological macromolecules deposited on a mica substrate. The results suggest that the systematic measurement of higher harmonics in the attractive regime can lead to highly sensitive techniques to map the chemical composition over heterogeneous samples.
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