Molecular tunnel junctions based on mixed SAMs: exponential correlation of the average metal-HOMO coupling with SAM/metal work function

We report the transport characteristics of molecular tunnel junctions based on binary mixed self-assembled monolayers (SAMs) of aromatic molecules using the conducting probe atomic force microscopy platform. The molecules include terphenyl dithiol and two derivatives, 2′,3′,5′,6′-tetrafluoroterphenyl dithiol and 4,4′-(bicyclo[2.2.2]octane-1,4-diyl)dibenzenethiol, whose junction conductances differ from terphenyl dithiol by factors of 10 and 100, respectively. The junction conductance G varies exponentially with binary SAM composition, not linearly, as might be expected. By employing an analytical model for the off-resonant current-voltage (I-V) behavior, we extract the average electronic density of state parameters for junctions with Au tip and substrate contacts as a function of binary SAM composition. The average HOMO to Fermi level offset ϵ_h is weakly dependent on SAM composition, reflecting commonly observed HOMO level pinning. In stark contrast, and in correspondence with the conductance results, the average metal-HOMO coupling Γ depends exponentially on composition, not linearly as simple composition-based averaging predicts. On the other hand, Kelvin probe measurements of binary mixed SAM-metal work functions reveal that work function (or work function change ΔΦ) varies linearly with SAM composition; it is a simple sum of the single component SAM work functions weighted by their relative surface coverages. Thus, G and Γ are both exponentially correlated with ΔΦ; variation of ΔΦ by 360 meV (∼7%) tunes G by >100× and Γ by a factor of 10, with smaller work functions (and larger ΔΦ values) leading to larger G and Γ values. Qualitatively, a correlation between Γ and ΔΦ, which both reflect interfacial charge transfer, appears reasonable, but a fundamental understanding requires a first principles model that can also account for the simultaneous pinning of ϵ_h.