Tuning surface d bands with bimetallic electrodes to facilitate electron transport across molecular junctions

Understanding chemical bonding and conductivity at the electrode–molecule interface is key for the operation of single-molecule junctions. Here we apply the d-band theory that describes interfacial interactions between adsorbates and transition metal surfaces to study electron transport across these devices. We realized bimetallic Au electrodes modified with a monoatomic Ag adlayer to connect α,ω-alkanoic acids (HO2C(CH2)nCO2H). The force required to break the molecule–electrode binding and the contact conductance Gn=0 are 1.1 nN and 0.29 G0 (the conductance quantum, 1 G0 = 2e2/h ≈ 77.5 μS), which makes these junctions, respectively, 1.3–1.8 times stronger and 40–60-fold more conductive than junctions with bare Au or Ag electrodes. A similar performance was found for Au electrodes modified by Cu monolayers. By integrating the Newns–Anderson model with the Hammer–Norskov d-band model, we explain how the surface d bands strengthen the adsorption and promote interfacial electron transport, which provides an alternative avenue for the optimization of molecular electronic devices. Coating Au electrodes with Ag or Cu monolayers is shown to improve molecule–electrode binding and electrical conductivity of single-molecule junctions as a result of the tuning of the surface d bands of the metal.