Probing attractive forces at the nanoscale using higher-harmonic dynamic force microscopy

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.