We have compared the applied forces and the resolution for AFM topographic images taken with the amplitude-modulation (AM) detection, or the tapping mode, and the frequency-modulation (FM) detection, or the non-contact 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 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 matters on surfaces. (Nanotechnology 18, 084009 (2007))
