Heat dissipation in atomic-scale junctions

Atomic and single-molecule junctions represent the ultimate limit to the miniaturization of electrical circuits. They are also ideal platforms to test quantum transport theories that are required to describe charge and energy transfer in novel functional nanodevices. Recent work has successfully probed electric and thermoelectric phenomena in atomic-scale junctions. However, heat dissipation and transport in atomic-scale devices remain poorly characterized due to experimental challenges. Here, using custom-fabricated scanning probes with integrated nanoscale thermocouples, we show that heat dissipation in the electrodes of molecular junctions, whose transmission characteristics are strongly dependent on energy, is asymmetric, i.e. unequal and dependent on both the bias polarity and the identity of majority charge carriers (electrons vs. holes). In contrast, 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 dissipation in a range of mesoscopic systems where transport is elastic. We anticipate that the techniques established here will enable the study of Peltier effects at the atomic scale, a field that has been barely explored experimentally despite interesting theoretical predictions. Furthermore, the experimental advances described here are also expected to enable the study of heat transport in atomic and molecular junctions, which is an important and challenging scientific and technological goal that has remained elusive.Comment: supporting information available in the journal web site or upon reques

Molecular design and control of fullerene-based bi-thermoelectric materials

Molecular junctions are a versatile test bed for investigating nanoscale thermoelectricity and contribute to the design of new cost-effective environmentally friendly organic thermoelectric materials. It was suggested that transport resonances associated with discrete molecular levels could play a key role in thermoelectric performance, but no direct experimental evidence has been reported. Here we study single-molecule junctions of the endohedral fullerene Sc3N@C8 connected to gold electrodes using a scanning tunnelling microscope. We find that the magnitude and sign of the thermopower depend strongly on the orientation of the molecule and on applied pressure. Our calculations show that Sc3N inside the fullerene cage creates a sharp resonance near the Fermi level, whose energetic location, and hence the thermopower, can be tuned by applying pressure. These results reveal that Sc3N@C80 is a bi-thermoelectric material, exhibiting both positive and negative thermopower, and provide an unambiguous demonstration of the importance of transport resonances in molecular junctions

Structure and Energy Level Alignment of Tetramethyl Benzenediamine on Au(111)

We investigate the binding and energy level alignment of 2,3,5,6-tetramethyl-1,4-benzenediamine (TMBDA) on Au(111) through a combination of helium atom scattering (HAS), X-ray photoemission (XPS), and scanning tunneling microscopy (STM). We show that TMBDA binds to step edges and to flat Au (111) terraces in a nearly flat-lying configuration. Through combination of HAS and STM data, we determine that the molecules are bound on step edges with an adsorption energy of about 1.2 eV, which is about 0.2 eV stronger than the adsorption energy we measure on flat surface. Preferential bonding to the under-coordinated Au atoms on step edges suggests that the molecules bind to Au through the nitrogen lone pair. Finally, STM measurements on TMBDA in these two different adsorption configurations show that the highest-occupied molecular orbital is deeper relative to Fermi for the more strongly bound molecules on step edges, confirming that the nitrogen bonds through charge donation to the Au