  Contact Resonance Force Microscopy (CR-FM) is a leading AFM technique for
measuring viscoelastic nano-mechanical properties. Conventional piezo-excited
CR-FM measurements have been limited to imaging in air, since the "forest of
peaks" frequency response associated with acoustic excitation methods
effectively masks the true cantilever resonance. Using photothermal actuation
results in clean contact resonance spectra, that closely match the ideal
frequency response of the cantilever, allowing unambiguous and simple resonance
frequency and quality factor measurements in air and liquids alike. This
extends the capabilities of CR-FM to biologically relevant and other soft
samples in liquid environments. We demonstrate CR-FM in air and water on both
stiff silicon/titanium samples and softer
polystyrene-polyethylene-polypropylene polymer samples with the quantitative
moduli having very good agreement between expected and measured in both
environments.
