  We study thermoelectric devices where a single 18-annulene molecule is
connected to metallic zigzag graphene nanoribbons (ZGNR) via highly transparent
contacts that allow for injection of evanescent wave functions from ZGNRs into
the molecular ring. Their overlap generates a peak in the electronic
transmission, while ZGNRs additionally suppress hole-like contributions to the
thermopower. Thus optimized thermopower, together with suppression of phonon
transport through ZGNR-molecule-ZGNR structure, yield the thermoelectric figure
of merit ZT ~ 0.5 at room temperature and 0.5 < ZT < 2.5 below liquid nitrogen
temperature. Using the nonequilibrium Green function formalism combined with
density functional theory, recently extended to multiterminal devices, we show
how the transmission resonance can also be manipulated by the voltage applied
to a third ZGNR electrode, acting as the top gate covering molecular ring, to
tune the value of ZT.
