Understanding Keesom Interactions in Monolayer-Based Large-Area Tunneling Junctions.

Charge transport across self-assembled monolayers (SAMs) has been widely studied. Discrepancies of charge tunneling data that arise from various studies, however, call for efforts to develop new statistical analytical approaches to understand charge tunneling across SAMs. Structure-property studies on charge tunneling across SAM-based junctions have largely been through comparison of average tunneling rates and associated variance. These early moments (especially the average) are dominated by barrier width-a static property of the junction. In this work, we show that analysis of higher statistical moments (skewness and kurtosis) reveals the dynamic nature of the tunnel junction. Intramolecular Keesom (dipole-dipole) interactions dynamically fluctuate with bias as dictated by stereoelectronic limitations. Analyzing variance in the distribution of tunneling data instead of the first statistical moment (average), for a series of n-alkanethiols containing internal amide and aromatic terminal groups, we observe that the direction of dipole moments affects molecule-electrode coupling. An applied bias induces changes in the tunneling probability, affecting the distribution of tunneling paths in large-area molecular junctions.