Ballistic transport through individual molecules and through ordered monolayers are fundamentally different processes. A key to understanding those differences are collective electrostatic effects that arise from (ordered) assemblies of polar groups. State-of-the art ballistic transport simulations (often in conjunction with experiments) suggest that such polar groups can be incorporated intentionally into the SAM-forming molecules to switch the charge-transport polarity, to tune the transition voltage, to induce rectification, or to control the electronic coupling between the transmissive channels in the SAM and the electrodes. In fact, such collective electrostatic effects in SAMs are inevitable as a consequence of polar docking groups and bonding-induced interfacial charge transfer.  In addition to the electrostatic coupling between neighboring molecules in a junction, also quantum-mechanical interactions are relevant. The interplay between the two effects can be best understood when considering transport through molecular clusters as intermediates between molecular and monolayer junctions. In order to separate the said coupling effects from other influences like, e.g., electrode roughness or molecular conformation, we here discuss clusters of various size in otherwise identical situations. In this way we find that overall, the impact of collective electrostatic effects dominates over quantum-mechanical interactions resulting in (i) a highly non-linear scaling of the current per molecule with cluster size and (ii) pronounced edge effects resulting in strongly varying currents through different parts of the cluster. Interestingly, the latter would also occur when only considering quantum-mechanical coupling, but the underlying energy landscape would be fundamentally different.

Electrostatic vs. quantum-mechanical coupling in self-assembled monolayers and its impact on ballistic transport through self-assembled monolayers

255th National Meeting (2018)

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