Single-molecule quantum-transport phenomena in break junctions

Single-molecule junctions - devices in which a single molecule is electrically connected by two electrodes - enable the study of a broad range of quantum-transport phenomena even at room temperature. These quantum features are related to molecular orbital and spin degrees of freedom and are characterized by various energy scales that can be chemically and physically tuned: level spacings, charging energies, tunnel couplings, exchange energies, vibrational energies and Kondo correlation energies. The competition between these different energy scales leads to a rich variety of processes, which researchers are now starting to be able to control and tune experimentally. In this Technical Review, we present the status of the molecular electronics field from this quantum-transport perspective with a focus on recent experimental results obtained using break-junction devices, including scanning probe and mechanically controlled break junctions, as well as electromigrated gold and graphene break junctions. Quantum aspects of transport through single molecules are observable at room temperature. In this Technical Review, we discuss the different processes and energy scales involved in charge transport through single-molecule junctions, the resulting electronic functionalities and the new possibilities for controlling these functionalities for the realization of nanoscale devices.