We present an efficient strategy to modulate tunnelling in molecular junctions by changing the tunnelling decay coefficient, β, by terminal-atom substitution which avoids altering the molecular backbone. By varying X = H, F, Cl, Br, I in junctions with S(CH2)(10-18)X, current densities (J) increase >4 orders of magnitude, creating molecular conductors via reduction of β from 0.75 to 0.25 Å−1. Impedance measurements show tripled dielectric constants (εr) with X = I, reduced HOMO-LUMO gaps and tunnelling-barrier heights, and 5-times reduced contact resistance. These effects alone cannot explain the large change in β. Density-functional theory shows highly localized, X-dependent potential drops at the S(CH2)nX//electrode interface that modifies the tunnelling barrier shape. Commonly-used tunnelling models neglect localized potential drops and changes in εr. Here, we demonstrate experimentally that $$\beta \propto 1/\sqrt{{\varepsilon }_{r}}$$ β ∝ 1 / ε r , suggesting highly-polarizable terminal-atoms act as charge traps and highlighting the need for new charge transport models that account for dielectric effects in molecular tunnelling junctions.

Recently it is discovered that molecular junctions can be pushed into theMarcus Inverted region of charge transport, but it is unclear which factors are important. This paper shows that the mechanism of charge transport across molecular wires can be switched between the normal and Marcus Inverted regions by fine-tuning the molecule-electrode coupling strength and the tunneling distance across oligophenylene ethynylene (OPE) wire terminated with ferrocene (Fc) abbreviated as S-OPE n Fc (n = 1-3). Coherent tunneling dominates the mechanism of charge transport in junctions with short molecules (n = 1), but for n = 2 or 3 redox reactions become important. By weakening the molecule-electrode interaction by interrupted conjugation, S-CH 2 -OPE n Fc, intramolecular orbital gating can occur pushing the junctions completely into the Marcus Inverted region. These results indicated that weak molecule-electrode coupling is important to push junctions into the Marcus Inverted Region.

We present an efficient strategy to modulate tunnelling in molecular junctions by changing the tunnelling decay coefficient, β, by terminal-atom substitution which avoids altering the molecular backbone. By varying X = H, F, Cl, Br, I in junctions with S(CH2)(10−18)X, current densities (J) increases > 4 orders of magnitude, creating molecular conductors via reduction of β from 0.75 to 0.25 Å−1. Impedance measurements shows tripled dielectric constants (εr) with X = I, reduced HOMO-LUMO gaps and tunnelling-barrier heights, and 5-times reduced contact resistance. These effects alone cannot explain the large change in β. Density-functional theory shows highly localized, X-dependent potential drop at the S(CH2)nX//electrode interface that modifies the tunnelling barrier shape. Commonly-used tunnelling models neglect localized potential drops and changes in εr. We demonstrate experimentally that β ∝1/√εr, suggesting highly-polarizable terminal-atom sites act as charge traps as proposed by Berlin and Ratner. Our work shows the need for new charge transport models that account for dielectric effects in molecular tunnelling junctions.

Marcus Inverted Region In article number 2200637, Christian A. Nijhuis, Damien Thompson, Enrique Del Barco, and co‐workers demonstrate that the mechanism of charge transport can be deliberately switched between the normal and Marcus Inverted regions by controlling the molecule–electrode coupling strength in a series of molecules. The cover shows two almost identical junctions. The only difference is one CH2 which lowers molecule‐electrode coupling strength pushing the junctions from direct coherent tunnelling to hopping in the Marcus Inverted region. This ability can improve the energy efficiency of molecular devices. Image credit: Niels van der Velde.