Tuning the thermal conductance of molecular junctions with interference effects

Abstract

We present an ab initio study of the role of interference effects in the thermal conductance of single-molecule junctions. To be precise, using a first-principles transport method based on density functional theory, we analyze the coherent phonon transport in single-molecule junctions made of several benzene and oligo-phenylene-ethynylene derivatives. We show that the thermal conductance of these junctions can be tuned via the inclusion of substituents, which induces destructive interference effects and results in a decrease of the thermal conductance with respect to the unmodified molecules. In particular, we demonstrate that these interference effects manifest as antiresonances in the phonon transmission, whose energy positions can be tuned by varying the mass of the substituents. Our work provides clear strategies for the heat management in molecular junctions and more generally in nanostructured metal-organic hybrid systems, which are important to determine, how these systems can function as efficient energy-conversion devices such as thermoelectric generators and refrigerators.

Thermal and Thermoelectric Properties of Molecular Junctions

“…More importantly, the destructive interference features of those crossconjugated molecules significantly reduce the thermal conductance with respect to linear conjugated analogues. Recently, Klockner et al used a DFT‐based first‐principles method to analyze the coherent phonon transport in single‐molecule junctions made of several benzene derivatives (Figure d) . They showed that the thermal conductance of these MJs can be tuned via the inclusion of substituents, which also induces destructive interference effects and results in a decrease of the thermal conductance with respect to the unmodified molecules (Figure f).…”

“…F) The corresponding phononic thermal conductance as a function of temperature for the different benzenediamine derivatives. The inset shows the room‐temperature cumulative thermal conductance as a function of energy for the junctions with X = H, Br, I. D–F) Reproduced with permission . Copyright 2017, American Physical Society.…”

“…This has led to several theoretical proposals. Harnessing the phonon quantum interference effect has been proposed as one promising approach to suppress phononic thermal transport in MJs . Recently, Famili et al proposed “Christmas tree”‐like molecules composed of “a long trunk,” along which phonons can propagate, attached to pendant molecular branches (Figure c).…”

Thermal bridging of graphene nanosheets via covalent molecular junctions: A non-equilibrium Green’s functions–density functional tight-binding study

“…On the other hand, by looking in details at the phonon transmissions for dibenzylbenzene and diphenoxybenzene shown in Figure 7b, we observe that in the former case T() is characterized by several Fano-like features throughout the spectrum. Famili et al [26] and Klockner et al [27] discussed how the emergence of phononic Fano interferences could lead to a low thermal conductance in molecular bridges. Fano resonances could arise because the CH2 linkers have masses and therefore vibrational frequencies in resonance with those of the benzene rings, whereas the mass mismatch of the oxygens breaks the mode degeneracies, removing destructive interferences in the transmission.…”

“…Fano effect, that is the effects of interference between a continuum and a resonant state, is important in the interpretation of electronic transport [25]. Indeed, Famili et al and Klockner et al have found that this effect modifies the phonon transport of molecular junctions [26,27]. For phonon transport, the Fano effects occur under Debye energies, for metallic electrodes that is typically in the range of several tens of meV (10 meV≃80.64 cm -1 ) 1 , while for graphene, the Debye temperature of ΘD ≈ 2100K makes that Fano effect limit to 1459.6 cm −1 which gives a wide window of phonon energies that contributes to thermal transport at room temperatures [28].…”