First-principles calculation of the thermoelectric figure of merit for [2,2]paracyclophane-based single-molecule junctions

Abstract

Here we present a theoretical study of the thermoelectric transport through [2,2]paracyclophane-based singlemolecule junctions. Combining electronic and vibrational structures, obtained from density functional theory (DFT), with nonequilibrium Green's function techniques allows us to treat both electronic and phononic transport properties at a first-principles level. For the electronic part, we include an approximate self-energy correction, based on the DFT+ approach. This enables us to make a reliable prediction of all linear response transport coefficients entering the thermoelectric figure of merit ZT . Paracyclophane derivatives offer a great flexibility in tuning their chemical properties by attaching different functional groups. We show that, for the specific molecule, the functional groups mainly influence the thermopower, allowing us to tune its sign and absolute value. We predict that the functionalization of the bare paracyclophane leads to a largely enhanced electronic contribution Z el T to the figure of merit. Nevertheless, the high phononic contribution to the thermal conductance strongly suppresses ZT . Our work demonstrates the importance to include the phonon thermal conductance for any realistic estimate of the ZT for off-resonant molecular transport junctions. In addition, it shows the possibility of a chemical tuning of the thermoelectric properties for a series of available molecules, leading to equally performing hole-and electron-conducting junctions based on the same molecular framework.

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

“…1(c) that the phonon conduction is dominated by low-lying vibrational modes with energies E < 10 meV. Let us recall that the Debye energy of the metal electrodes sets an upper limit for the energy of the vibrational modes that can contribute to the transport, which in our case is around 20 meV [38]. However, in the range between 10 and 20 meV there are no significant contributions to the phonon thermal conductance, which we attribute to the weak metal-molecule coupling for the modes in this energy range.…”

“…For this purpose, we present here a detailed study of the role of the phonon transport in both the thermal conductance and the figure of merit of monomer and dimer C 60 molecular junctions. This study is based on a state-of-the-art combination of density functional theory (DFT) with nonequilibrium Green's function (NEGF) techniques [38,39], which allows us to compute the contribution of both electrons and phonons to the different transport properties. Our results show that the phonons play a minor role in the thermal conductance of the monomer junctions, while they largely determine this property in dimer junctions.…”

Thermoelectric performance of strongly correlated quantum impurity models

“…Nonetheless, the phonon contribution to the thermal conductance is expected to be small in these systems as compared to that in bulk ones, and there is still great interest in exploring their thermoelectric properties. (Recent studies have stressed the importance of the phonon contribution κ ph in molecules, however 14,15 .) Beyond numerical studies of the thermoelectric performance of impurity models based on numerical renormalization group 3,9,16 and the nonequilibrium Green's function approach 17,18 , simple analytic results for thermoelectric coefficients have been found in the "atomic limit", in which the broadening Γ → 0 is taken to be zero, using e.g., the sequential-tunnelling approximation for transport [19][20][21] .…”

Thermopower of molecular junctions: Tunneling to hopping crossover in DNA

“…Focusing on works reporting on both G and S, we note that most computational studies of linear-response transport coefficients in single molecules are limited to the coherent limit 23 , using the Landauer formula with parameters derived from first principle calculations [24][25][26][27][28][29][30][31][32][33][34] , or other coherent-transport approaches 35 . Effects of vibrations on the thermopower were assessed using scattering approaches 36,37 , or perturbatively using non-equilibrium Green's function (NEGF) [38][39][40][41][42][43] or quantum master equation (QME) methods [42][43][44][45][46] .…”