Thermal conductance and thermoelectric figure of merit of C60 -based single-molecule junctions: Electrons, phonons, and photons

Abstract

Motivated by recent experiments, we present here an ab initio study of the impact of the phonon transport on the thermal conductance and thermoelectric figure of merit of C60-based single-molecule junctions. To be precise, we combine density functional theory with nonequilibrium Green's function techniques to compute these two quantities in junctions with either a C60 monomer or a C60 dimer connected to gold electrodes, taking into account the contributions of both electrons and phonons. Our results show that for C60 monomer junctions phonon transport plays a minor role in the thermal conductance and, in turn, in the figure of merit, which can reach values on the order of 0.1, depending on the contact geometry. In C60 dimer junctions, phonons are transported less efficiently, but they completely dominate the thermal conductance and reduce the figure of merit as compared to monomer junctions. Thus, claims that by stacking C60 molecules one could achieve high thermoelectric performance, which have been made without considering the phonon contribution, are not justified. Moreover, we analyze the relevance of near-field thermal radiation for the figure of merit of these junctions within the framework of fluctuational electrodynamics. We conclude that photon tunneling can be another detrimental factor for the thermoelectric performance, which has been overlooked so far in the field of molecular electronics. Our study illustrates the crucial roles that phonon transport and photon tunneling can play, when critically assessing the performance of molecular junctions as potential nanoscale thermoelectric devices.

Thermal conductance of metallic atomic-size contacts: Phonon transport and Wiedemann-Franz law

“…This regime is still challenging for experimentalists, since signals are rather small and contamination of surfaces may play a crucial role [79]. Moreover, as the gap between the electrodes increases, at some point the thermal radiation via photon tunneling may give a contribution of similar order [54]. show that in the tunneling regime the electronic Lorentz ratio L el /L 0 stays close to 1.…”

“…(5), we use our previous work [23,40,53,54] and combine DFT and NEGF techniques in the same spirit as for the electron transport. Briefly our starting point is the description of the phonons or vibrational modes of the atomic contacts within the harmonic approximation.…”

Tuning the thermal conductance of molecular junctions with interference effects

“…The goal is to show that in these junctions the thermal transport is dominated by phonons and that the interference effects, predicted here, should therefore be observable experimentally. We neglect contributions to the thermal conductance from near field radiative heat transfer, whose significance can be controlled by the macroscopic shape of the electrodes 57 . As in the phononic case we assume that the electronic transport is dominated by elastic tunneling processes.…”

“…We compute the transmission function τ pn (E) by means of a combination of DFT and non-equilibrium Green's function techniques, as we have described in detail in Refs. [55][56][57].…”

Single-Molecule Analysis Methods Using Nanogap Electrodes and Their Application to DNA Sequencing Technologies

“…A major problem that currently remains for singlemolecule analysis methods that has yet to be solved is that thermal conductivity measurements of SMJs are difficult to conduct. 135,136 In addition to technical problems such as how to apply a thermal gradient to an SMJ that is only several nanometers in size or how to actually measure the temperature of an SMJ, there is also the fundamental issue that the temperature obtained from such measurements has not yet been defined. This problem also applies to the temperature obtained from the lifetime measurements in an SMJ.…”