Mechanical interactions in STM imaging of large insulating adsorbates

STM images of 10.7 nm dendrimer microspheres were obtained in order to provide a model study of STM imaging of large insulating objects. Negative contrast in these images, observed before only on biological samples, demonstrates that this phenomenon cannot be explained by the biological specificity of a specimen. Large insulating objects may appear as depressions on the constant current STM image due to the interplay of electronic and mechanical interactions between the tip, the substrate and the adsorbate. Utilization of macroscopic elasticity constants in a model of three elastic spheres gives a good correspondence with the reported experimental data. This model can be modified in future in order to obtain microscopic mechanical properties of nm-size objects in STM experiments.