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

摘要

In this paper, we report on atomic forcemicroscopy (AFM) investigation of a self-assembled monlayer(SAM) system-octadecylphosphonic acid (OPA) deposited onmica. With the deposition methods employed in this work, theSAM presents a partial coverage, i.e., the OPA covers only afraction of the mica surface and, therefore, some bare micaregions 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 SAMon top of the mica surface. However, when a small oscillationdamping mode is employed, the topographic profile is inverted,i.e., the mica regions appear higher than the surrounding OPAlayer. AFMexperiments, carried out to assess the origin of thiseffect, yield strong evidences that it is related to the presence of awater contamination layer on the bare mica regions only. Asemi-quantitative model is utilized to understand the experimental results.