Heat dissipation in atomic-scale junctions

An innovative technique based on scanning tunnelling probes with integrated thermocouples is developed and used to measure heat dissipation in the electrodes of atomic and molecular junctions. Heat in nanoscale junctions The flow of electronic charge across nanoscale junctions has been extensively studied, but it is difficult to measure the heat generated by such small currents. A new technique, based on custom-made scanning tunnelling probes with integrated thermocouples, now makes it possible to study heat dissipation at the atomic scale. The technique has been used to observe asymmetric heat dissipation — in which the amount of heat generated depends on the voltage bias polarity and majority charge carrier. The results relate the charge-carrying properties of atomic-scale junctions to heat dissipation and can be used to describe a range of mesoscopic systems in which charge transport is mainly elastic, with no exchange of energy in the contact region. Such systems include semiconductor nanowires, two-dimensional electron gases, semiconductor heterostructures, carbon nanotubes and graphene. Atomic and single-molecule junctions represent the ultimate limit to the miniaturization of electrical circuits 1 . They are also ideal platforms for testing quantum transport theories that are required to describe charge and energy transfer in novel functional nanometre-scale devices. Recent work has successfully probed electric and thermoelectric phenomena 2 , 3 , 4 , 5 , 6 , 7 , 8 in atomic-scale junctions. However, heat dissipation and transport in atomic-scale devices remain poorly characterized owing to experimental challenges. Here we use custom-fabricated scanning probes with integrated nanoscale thermocouples to investigate heat dissipation in the electrodes of single-molecule (‘molecular’) junctions. We find that if the junctions have transmission characteristics that are strongly energy dependent, this heat dissipation is asymmetric—that is, unequal between the electrodes—and also dependent on both the bias polarity and the identity of the majority charge carriers (electrons versus holes). In contrast, junctions consisting of only a few gold atoms (‘atomic junctions’) whose transmission characteristics show weak energy dependence do not exhibit appreciable asymmetry. Our results unambiguously relate the electronic transmission characteristics of atomic-scale junctions to their heat dissipation properties, establishing a framework for understanding heat diss