Height Amplifications of Scanning Tunneling Microscopy Images in Air

Scanning tunneling microscopy (STM) measurements in air often show variations in feature heights accompanied by surface deformation inconsistent with an idealized, noninteracting scanning tip and surface separated by a vacuum gap. Such deformation is usually ascribed to enhanced contact forces due to a highly curved liquid meniscus linking the STM (or atomic force microscope) tip to the sample, although little quantitative information is available about surface deformation, especially for STM images. We used standard STM techniques to image chemically homogeneous platinum replicas of Langmuir-Blodgett multilayers with known step heights in humid air and in a dry N₂ atmosphere. Identical surface features were amplified when imaged in humid air but reverted to their original height on imaging in dry N₂. This shows that capillary forces can be the dominant interaction between tip and sample, even in STM, and that these forces are capable of deforming soft materials.