Orbital Alignment as a Key Determinant of Nanotransport in Molecular Tunnel Junctions Despite Fermi Level Pinning: Exponential Correlation with Molecule–Electrode Coupling

At first glance, interrogating the impact of molecular orbital alignment on charge transport in molecular tunnel junctions may appear out of place, given the well-documented strong Fermi level pinning effect. To demonstrate that the contrary is true, we investigated molecular junctions based on self-assembled monolayers (SAMs) of alkyl monothiols (CnT) and dithiols (CnDT) with Ag, Au, and Pt electrodes using the conducting probe atomic force microscopy (CP-AFM) platform. Analysis of these data reveals that the HOMO-metal electronic coupling Γ and the low bias conductance <i>G</i> are exponentially correlated with the HOMO energy offset relative to the Fermi level <i>ε</i>₀ = <i>E</i>_<i>MO</i> - <i>E</i>_<i>F</i> (Γ ∝ exp(-α̅|ε₀|), <i>G</i> ∝ exp(-α|ε₀|), α ≈ 2α̅). This impact is remarkably strong: for CnT junctions, a reduction by only 25% in |ε₀| translates into an increase of Γ by 1 order of magnitude and of <i>G</i> by 2 orders of magnitude. More broadly, this exponential correlation of Γ and <i>G</i> with <i>ε</i>₀ offers deeper insight into nanotransport and extends understanding of the previously reported exponential dependence of Γ and <i>G</i> on the SAM-induced work function shift ΔΦ, which reflects the linear correlation between <i>ε</i>₀ and ΔΦ. From a fundamental perspective, it is crucial to highlight that our data validate a formula for Γ (a property which depends on both electrodes of a junction), which features (i) <i>ε</i>₀ rather than ΔΦ (i.e., a property of a full junction versus a property of a "half a junction") and (ii) <i>ε</i>₀ rather than its square root in the exponent, thereby invalidating the widely employed tunneling barrier picture.