Imaging of soft matter with tapping-mode atomic force microscopy and non-contact-mode atomic force microscopy

In this work, we take topographic images of DNA molecules and nanometre-sized polystyrene spheres in air as well as in water using an atomic force microscope (AFM). We would like to compare the applied forces and the resolution for images taken with the amplitude-modulation (AM) detection, or the tapping mode, and the frequency-modulation (FM) detection, or the non-contact (NC) mode. Experiments for these two modes are carried out on the same area of the samples, with the same tip, under the same environment, and with the same oscillation amplitude. Our experiments indicate that, in the FM mode, the tip exerts a very gentle force on soft materials and provides a height measurement close to the true value. In the AM mode, the tip exerts a stronger force on soft materials and causes their deformation, especially in the liquid environment. The resolution 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 conclude that the FM mode can obtain images with a higher resolution and provides a sufficient sensitivity to image the true and fine structure of soft matter on surfaces.