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

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Abstract
In this paper, we report on atomic force microscopy (AFM) investigation of a self-assembled monolayer (SAM) system - octadecylphosphonic acid (OPA) deposited on mica. With the deposition methods employed in this work, the SAM presents a partial coverage, i.e., the OPA covers only a fraction of the mica surface and, therefore, some bare mica regions are observed. Using standard intermittent contact AFM (IC-AFM) techniques (with medium to high oscillation damping), the topographic profile of this system clearly shows the flat SAM on top of the mica surface. However, when a small oscillation damping mode is employed, the topographic profile is inverted, i.e., the mica regions 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 regions only. A semi-quantitative model is utilized to understand the experimental results.