Voltage Dependence of Molecule–Electrode Coupling in Biased Molecular Junctions

Biased interfaces between individual molecules and metallic electrodes are central in many nanoscale devices. A key quantity that governs the nature of the metal–molecule interface is the degree of hybridization between frontier orbitals of the molecule and electrode continuum states. This hybridization leads to the broadening of molecular resonances, a measure of the electronic coupling to the surface. In this work, we present detailed ab initio calculations of the evolution of this electronic coupling for selected frontier orbitals with bias voltage in molecular junctions, nanoscale devices where metal–molecule interface effects are dominant. We focus on symmetric and asymmetric junctions based on an experimentally well-studied system, Au–bipyridine–Au, as examples of our method. The nature of the bias-dependent coupling that emerges from our ab initio calculations provides new physical insight into experimentally achievable systems and is a marked quantitative improvement over simple Lorentzian models often used in interpreting device measurements. Beyond molecular junctions, our approach can be straightforwardly generalized to other molecule–metal interfaces.