Abstract
In this work, we take topographic images of DNA molecules and nanometre-sizedpolystyrene spheres in air as well as in water using an atomic force microscope (AFM). Wewould like to compare the applied forces and the resolution for images taken with theamplitude-modulation (AM) detection, or the tapping mode, and the frequency-modulation(FM) detection, or the non-contact (NC) mode. Experiments for these two modes arecarried out on the same area of the samples, with the same tip, under the sameenvironment, and with the same oscillation amplitude. Our experiments indicate that, inthe FM mode, the tip exerts a very gentle force on soft materials and provides a heightmeasurement close to the true value. In the AM mode, the tip exerts a stronger force onsoft materials and causes their deformation, especially in the liquid environment. Theresolution of the FM mode is about the same as that of the AM mode for operation in air,but the former is significantly superior to the latter in water. We thus concludethat the FM mode can obtain images with a higher resolution and provides asufficient sensitivity to image the true and fine structure of soft matter on surfaces.
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