First-principles quantum transport modeling of thermoelectricity in single-molecule nanojunctions with graphene nanoribbon electrodes

We overview the nonequilibrium Green function combined with density functional theory (NEGF-DFT) approach to modeling of independent electronic and phononic quantum transport in nanoscale thermoelectrics with examples focused on a new class of devices where a single organic molecule is attached to two metallic zigzag graphene nanoribbons (ZGNRs) via highly transparent contacts. Such contacts make possible injection of evanescent wavefunctions from the ZGNR electrodes, so that their overlap within the molecular region generates a peak in the electronic transmission around the Fermi energy of the device. Additionally, the spatial symmetry properties of the transverse propagating states in the semi-infinite ZGNR electrodes suppress hole-like contributions to the thermopower. Thus optimized thermopower, together with diminished phonon thermal conductance in a ZGNR|molecule|ZGNR inhomogeneous heterojunctions, yields the thermoelectric figure of merit ZT≃0.4 at room temperature with maximum ZT≃3 reached at very low temperatures T≃10 K (so that the latter feature could be exploited for thermoelectric cooling of, e.g., infrared sensors). The reliance on evanescent mode transport and symmetry of propagating states in the electrodes makes the electronic-transport-determined power factor in this class of devices largely insensitive to the type of sufficiently short organic molecule, which we demonstrate by showing that both 18-annulene and C10 molecule sandwiched by the two ZGNR electrodes yield similar thermopower. Thus, one can search for molecules that will further reduce the phonon thermal conductance (in the denominator of ZT) while keeping the electronic power factor (in the nominator of ZT) optimized. We also show how the often employed Brenner empirical interatomic potential for hydrocarbon systems fails to describe phonon transport in our single-molecule nanojunctions when contrasted with first-principles results obtained via NEGF-DFT methodology.