Metallic atomic junctions pose the ultimate limit to the scaling of electrical contacts 1 . They serve as model systems to probe electrical and thermal transport down to the atomic level as well as quantum effects occurring in one-dimensional systems 2 . Charge transport in atomic junctions has been studied intensively in the last two decades 2,3,4,5 . However, heat transport remains poorly characterized because of significant experimental challenges. Specifically the combination of high sensitivity to small heat fluxes and the formation of stable atomic contacts has been a major hurdle for the development of this field. Here we report on the realization of heat transfer measurements through atomic junctions and analyze the thermal conductance of single atomic gold contacts at room temperature. Simultaneous measurements of charge and heat transport reveal the proportionality of electrical and thermal conductance, quantized with the respective conductance quanta 6 . This constitutes an atomic scale verification of the well-known Wiedemann-Franz law 7 . We anticipate that our findings will be a major advance in enabling the investigation of heat transport properties in molecular junctions, with meaningful implications towards the manipulation of heat at the nanoscale. IntroductionHeat transport and dissipation at the nanoscale has spurred research interest as it severely limits scaling of high performance electronic devices and circuits 8 . Atomic quantum point contacts represent an ideal platform to investigating heat transport in which quantum confinement effects cannot be neglected. The development of experimental techniques, such as scanning tunneling microscopy (STM) and mechanically controlled break junction (MCBJ) enabled the formation and manipulation of monoatomic metallic chains 3,4,5 . Using these techniques, charge transport in atomic junctions has been studied intensively 2,3,4,5 , and, more recently, Joule dissipation 9,10 and thermoelectric effects 11,12 have been successfully probed. Recently, heat dissipation was measured in current-carrying single gold-gold contacts by means of STM with an integrated micro thermocouple in the tip 9 . It was shown that heat dissipates symmetrically into the two contacts, confirming that the electron transmission function T(E) of the junction element around the Fermi energy of the metallic contacts governs this phenomenon. Despite these recent steps forward, the properties of heat conduction through atomic junctions still remain to be fully explored.Electrical conductance in atomic junctions is quantized. A single gold atom contact has a conductance equal to the quantum G0 = 2e 2 /h, where e is the electron charge and h Planck's constant. The Landauer approach used to describe charge transport can also be applied to predict heat transport 13 and predicts the validity of the Wiedemann-Franz (WF) law, which states that the thermal conductance GTH and the electrical conductance GEL are proportional to each other,where T is the absolute temperature and the proportio...

Molecular junctions exhibit a rich and tunable set of thermal transport phenomena. However, the predicted high thermoelectric efficiencies, phonon quantum interference effects, rectification, and nonlinear heat transport properties of organic molecules are yet to be verified because suitable experimental techniques have been missing. Here, by combining the break junction technique with suspended heat-flux sensors with picowatt per Kelvin sensitivity, we measured the thermal and electrical conductance of single organic molecules at room temperature simultaneously. We used this method to study the thermal transport properties of two model systems, namely, dithiol-oligo(phenylene ethynylene) and octane dithiol junctions with gold electrodes. In agreement with our density functional theory and phase-coherent transport calculations, we show that heat transport across these systems is governed by the phonon mismatch between the molecules and the metallic electrodes. This work represents the first measurement of thermal transport through single molecules and opens new opportunities for studying heat management at the nanoscale level.

Structures of the aromatic N-heterocyclic hexaazatriphenylene (HAT) molecular synthon obtained by surface-assisted self-assembly were analyzed with sub-Å resolution by means of noncontact atomic force microscopy (nc-AFM), both in the kinetically trapped amorphous state and in the thermodynamically stable crystalline phase. These results reveal how the crystallization governs the length scale of the network order for non-flexible molecular species without affecting the local bonding schemes. The capability of nc-AFM to accurately resolve structural relaxations will be highly relevant for the characterization of vitreous two-dimensional supramolecular materials.

To develop next-generation electronics and high efficiency energy-harvesting devices, it is crucial to understand how charge and heat are transported at the nanoscale. Metallic atomic-size contacts are ideal systems to probe the quantum limits of transport. The thermal and electrical conductance of gold atomic contacts has been recently proven to be quantized at room temperature. However, a big experimental challenge in such measurements is represented by the fast breaking dynamics of metallic junctions at room temperature, which can exceed the typical response time of the thermal measurement. Here we use a break-junction setup that combines Scanning Tunneling Microscopy with suspended micro electro-mechanical systems with a gold-covered membrane and an integrated heater acting also as thermometer. By using other metals as tip materials, namely Pt, PtIr and W, we show heat transport measurements through single gold atomic contacts. The dependence of the thermal conductance is analysed as function of contact size and material used. We find that by using Pt and Pt-Ir tips we can maximize the mechanical stability and probability of forming single Au atomic contacts. We then show the quantization of the electrical and thermal conductance with the verification of the Wiedemann-Franz law at the atomic scale. We expect these findings to increase the flexibility of experimental techniques probing heat transport in metallic quantum point contacts and to enable the investigation of thermal properties of molecular junctions.

Molecules are predicted to be chemically tunable towards high thermoelectric efficiencies and they could outperform existing materials in the field of energy conversion. However, their capabilities at the more technologically relevant temperature of 300 K are yet to be demonstrated. A possible reason could be the lack of a comprehensive technique able to measure the thermal and (thermo)electrical properties, including the role of phonon conduction. Here, by combining the break junction technique with a suspended heat-flux sensor, we measured the total thermal and electrical conductance of a single molecule, at room temperature, together with its Seebeck coefficient. We used this method to extract the figure of merit zT of a tailor-made oligo(phenyleneethynylene)-9,10-anthracenyl molecule with dihydrobenzo[b]thiophene anchoring groups (DHBT-OPE3-An), bridged between gold electrodes. The result is in excellent agreement with predictions from density functional theory and molecular dynamics. This work represents the first measurement, within the same setup, of experimental zT of a single molecule at room temperature and opens new opportunities for the screening of several possible molecules in the light of future thermoelectric applications. The protocol is verified using SAc-OPE3, for which individual measurements for its transport properties exist in the literature.

Structures of the aromatic N‐heterocyclic hexaazatriphenylene (HAT) molecular synthon obtained by surface‐assisted self‐assembly were analyzed with sub‐Å resolution by means of noncontact atomic force microscopy (nc‐AFM), both in the kinetically trapped amorphous state and in the thermodynamically stable crystalline phase. These results reveal how the crystallization governs the length scale of the network order for non‐flexible molecular species without affecting the local bonding schemes. The capability of nc‐AFM to accurately resolve structural relaxations will be highly relevant for the characterization of vitreous two‐dimensional supramolecular materials.