We study the electronic contribution to the main thermoelectric properties of a molecular junction consisting of a single quantum dot coupled to graphene external leads. The system electrical conductivity (G), Seebeck coefficient (S), and the thermal conductivity (κ), are numerically calculated based on a Green's function formalism that includes contributions up to the Hartree-Fock level. We consider the system leads to be made either of pure or gapped-graphene. To describe the free electrons in the gapped-graphene electrodes we used two possible scenarios, the massive gap scenario, and the massless gap scenario, respectively. In all cases, the Fano effect is responsible for a strong violation of the Wiedemann-Franz law and we found a substantial increase of the system figure of merit ZT due to a drastic reduction of the system thermal coefficient. In the case of gapped-graphene electrodes, the system figure of merit presents a maximum at an optimal value of the energy gap of the order of ∆/D ∼ 0.002 (massive gap scenario) and ∆/D ∼ 0.0026 (massless gap scenario). Additionally, for all cases, the system figure of merit is temperature dependent.

The study of thermoelectricity in molecular junctions is of fundamental interest for the development of various technologies including cooling (refrigeration) and heat-to-electricity conversion . Recent experimental progress in probing the thermopower (Seebeck effect) of molecular junctions has enabled studies of the relationship between thermoelectricity and molecular structure . However, observations of Peltier cooling in molecular junctions-a critical step for establishing molecular-based refrigeration-have remained inaccessible. Here, we report direct experimental observations of Peltier cooling in molecular junctions. By integrating conducting-probe atomic force microscopy with custom-fabricated picowatt-resolution calorimetric microdevices, we created an experimental platform that enables the unified characterization of electrical, thermoelectric and energy dissipation characteristics of molecular junctions. Using this platform, we studied gold junctions with prototypical molecules (Au-biphenyl-4,4'-dithiol-Au, Au-terphenyl-4,4''-dithiol-Au and Au-4,4'-bipyridine-Au) and revealed the relationship between heating or cooling and charge transmission characteristics. Our experimental conclusions are supported by self-energy-corrected density functional theory calculations. We expect these advances to stimulate studies of both thermal and thermoelectric transport in molecular junctions where the possibility of extraordinarily efficient energy conversion has been theoretically predicted .

Molecular junctions offer unique opportunities for controlling charge transport on the atomic scale and for studying energy conversion. For example, quantum interference effects in molecular junctions have been proposed as an avenue for highly efficient thermoelectric power conversion at room temperature. Toward this goal, we investigated the effect of quantum interference on the thermoelectric properties of molecular junctions. Specifically, we employed oligo(phenylene ethynylene) (OPE) derivatives with a para-connected central phenyl ring ( para-OPE3) and meta-connected central ring ( meta-OPE3), which both covalently bind to gold via sulfur anchoring atoms located at their ends. In agreement with predictions from ab initio modeling, our experiments on both single molecules and monolayers show that meta-OPE3 junctions, which are expected to exhibit destructive interference effects, yield a higher thermopower (with ∼20 μV/K) compared with para-OPE3 (with ∼10 μV/K). Our results show that quantum interference effects can indeed be employed to enhance the thermoelectric properties of molecular junctions.

Thermal conductivity is a crucial material property for a diverse range of energy technologies, ranging from thermal management of high power electronics to thermal insulation for building envelopes. This review discusses recent advances in achieving high and low thermal conductivity (k) as relevant for energy applications, from high-k heat spreaders to low-k insulation. We begin with a brief introduction to the physics of heat conduction from both theoretical and computational perspectives. The heart of the review is a survey of recent advances in high- and low-k materials. The discussion of good heat conductors for thermal management includes inorganics and polymers in both bulk and low dimensional forms. For insulators, the discussion covers the effects of chemical composition, crystal structure, and defects and porosity. Promising areas for future research in both fundamental materials science and engineering technologies are noted.