Imaging elastic sample properties with an atomic force microscope operating in the tapping mode

Many delicate samples cannot be imaged without artefacts by contact-mode AFM. However, tapping-mode AFM is well suited for topographic imaging of this kind of sample. When the delicate samples laterally consist of different material phases, ambiguities in the interpretation of the topographic image can occur. Unfortunately, at present for tapping-mode imaging there is no technique available for the acquisition of material-contrast images. In this paper we present a way for tapping-mode material-contrast imaging. The material property utilised is the differing deformability of distinct material phases. Therefore, the controlled elastic deformation of the sample in the tapping mode is the first investigation carried out in this study. The novel method of tapping-mode material-contrast imaging has been validated for two different kinds of samples, both consisting of hard substrates partially covered by deformable organic adsorbates. In both cases deformabilities could be measured and material-contrast images have been acquired. The smallest visible adsorbed objects in the material-contrast images had a width of about 5 nm.