Multiterminal single-molecule-graphene-nanoribbon junctions with the thermoelectric figure of merit optimized via evanescent mode transport and gate voltage

We propose thermoelectric devices where a single molecule is connected to two metallic zigzag graphene nanoribbons (ZGNRs) via highly transparent contacts that allow the injection of evanescent wave functions from ZGNRs. Their overlap generates a peak in the electronic transmission that is largely insensitive to the type of the short conjugated molecule, while ZGNRs additionally suppress hole-like contributions to the thermopower. Thus optimized thermopower, together with suppression of phonon transport through a ZGNR-molecule-ZGNR structure, yields the thermoelectric figure of merit ZT similar to 0.5 at room temperature and 0.5 <ZT <2.5 below liquid nitrogen temperature. Using density functional theory combined with the nonequilibrium Green's function formalism for multiterminal devices, we show how the transmission resonance can be manipulated by the voltage applied to a third ZGNR top-gate electrode covering the molecule to further tune the value of ZT.