Noncontact amplitude modulated atomic force microscopy (NCAM-AFM) has successfully imaged, at the sub-nanometer scale, individual molecules of hydrophilic surfaces in aqueous solutions. It remains unclear why such resolution has not been reported for hydrophobic surfaces under water. Previously, using amplitude vs. distance AFM spectroscopy combined with molecular dynamics (MD) modeling, we showed that a ~5 Å void exists between a methyl terminated surface and the first hydration layer but is not observed over a comparable hydroxyl terminated surface. We hypothesized that the hydration layer formed over this gap creates a region of force noise between the AFM tip and surface preventing molecular imaging of hydrophobic surfaces in aqueous conditions. In this study we used nanografting to pattern separate regions of alkanethiols of similar lengths with differing terminal groups on an Au (111) surface. Although the lengths of the alkanethiols on the surface are nearly identical, our results show a difference in height between hydrophobic and hydrophilic groups. This suggests that the AFM is imaging the hydration structure over the void rather than interacting directly with the surface preventing resolution of the surface lattice. The knowledge of the contrast mechanisms at the sub-nanometer scale may inform the development of methods for reproducible sub-nanometer scale NCAM-AFM imaging of functionalized surfaces in liquid.<br/>

Identifying the barriers to sub-nanometer resolution non-contact atomic force microscopy of hydrophobic surfaces in liquid

2019 Fall National Meeting

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