Atomic contacts play a central role in atomic and molecular junctions. Specifically, in the context of molecular electronics and spintronics, the vast majority of break-junction studies are done with the aid of electrodes made of the same material. Here, we show that a rich family of bimetallic contacts can be fabricated in break junction setups. Moreover, their bimetallic composition can be controlled by atomically-precise electromigration. These structures include nickel-gold contacts, where the nickel electrode has a gold atomic scale apex, gold and aluminium electrodes bridged by an unusual atomic-chain made of both gold and aluminium atoms, and iron-nickel atomic contacts that act as a spin-valve break-junction without the need for sophisticated spin-valve geometry. This rich set of structures opens the door for the fabrication and study of versatile atomic and molecular junctions.

In ballistic nanoscale conductors, electronic current densities can reach extreme values. For bias voltages on the 1V scale, the highly non-equilibrium situation can lead to significant forces between the atoms, causing substantial changes in the atomic structure. While from a technological point of view, these effects pose challenges in terms of stability and reproducibility, the control of the atomic structure by external driving forces offers an enormous potential for new applications, such as memories or switches. In this contribution, we investigate the current-induced inter-atomic forces in different models of nanojunctions under a high applied bias voltage, employing first principles electronic structure and transport calculations. Our findings show how the forces on the atoms are related to the non-equilibrium charge density, chemical bonds and the electrostatic potential. We discuss how the combination of first-principles calculations with scanning probe experiments exploring currents and forces enables closely matching molecular junctions in theory and experiment.

(a) Architecture of three-terminal graphene–based single-molecule transistors. (b) Conductance map of resonant transport, displaying vibrational side-bands in the Coulomb-blocked regions. (c) Bosonic temperature dependence of vibrational signatures.

It has been hypothesized that biological systems achieve rapid, selective chemical transformations with earth-abundant metal centers through the stabilization of key transition states by powerful electric fields within enzyme cavities. In this talk, I will describe how the reactivity of synthetic earth-abundant metal complexes can be analogously induced by the application of an external electric field in a scanning tunneling microscope. This approach is applied towards C-C bond forming reactions with an otherwise unreactive nickel complex, and the expected products are detected both in-situ through single-molecule conductance measurements as well as ex-situ through high-resolution mass spectrometry. These results highlight the relevance of electric field effects to the domains of organometallic chemistry and organic synthesis, offering a sustainable new dimension for the modulation of reaction rates complementary to ancillary ligand design.