Observation of topography inversion in atomic force microscopy of self-assembled monolayers

In this paper, we report on atomic force microscopy (AFM) investigation of a self-assembled monolayer (SAM) system-octadecylphosphonic acid (OPA) depo sited on mica. With the deposition methods employed in this work, the SAM p resents a partial coverage, i.e., the OPA covers only a fraction of the mic a surface and, therefore, some bare mica regions are observed. Using standa rd intermittent contact AFM (IC-AFM) techniques (with medium to high oscill ation damping), the topographic profile of this system clearly shows the fl at SAM on top of the mica surface. However, when a small oscillation dampin g mode is employed, the topographic profile is inverted, i.e., the mica reg ions appear higher than the surrounding OPA layer. AFM experiments, carried out to assess the origin of this effect, yield strong evidences that it is related to the presence of a water contamination layer on the bare mica re gions only. A semi-quantitative model is utilized to understand the experim ental results.