Influence of anchoring group on charge transport across self-assembled monolayer-based molecular tunnel junctions

Predicting the charge transport rate, mechanism and dielectric response of solid-state molecular electronics is challenging since these properties depend on many variables such as molecular backbone, electrode material, junction contact geometry, anchoring and terminal functional groups, and so on. Although the effects of the anchoring group (X) on the conductance of single-molecule junctions have been widely investigated, in large-area junctions examples are rare, although the latter makes it possible to also explore the role of dielectric properties on charge transport rates. Here we report a change of 2.5 orders of magnitude in the charge transport rate along with a factor of 3 change in the measured dielectric constant (ε_r) across monolayers of X(C₆H₄)_nH with n = 1 or 2 and X = NO₂, SH, NH₂, CN, and Pyr. Our combined study involving current-voltage measurements and impedance spectroscopy allowed us to isolate the contact (R_C) and monolayer resistance (R_SAM), and found that the R_C increased with the X order. This change in R_C goes hand-in-hand with the shift of HOMO and LUMO energy levels with respect to the Fermi levels of the electrodes explaining the large observed change in charge transport rate. Surprisingly, the increase in tunneling rates (or decrease in R_SAM) scales with ε_r. Our work provides new insights into the factors that influence the charge transport rate and dielectric response of molecular junctions besides widely studied changes to the molecular backbone or terminal functional groups.