  Motivated by recent experiments, we present here a systematic ab-initio study
of the length dependence of the thermal conductance of single-molecule
junctions. We make use of a combination of density functional theory with
non-equilibrium Green's function techniques to investigate the length
dependence of the phonon transport in single alkane chains, contacted with gold
electrodes via both thiol and amine anchoring groups. Additionally, we study
the effect of the substitution of the hydrogen atoms in the alkane chains by
heavier fluorine atoms to form polytetrafluoroethylenes. Our results
demonstrate that (i) the room-temperature thermal conductance is fairly
length-independent for chains with more than 5 methylene units and (ii) the
efficiency of the thermal transport is strongly influenced by the strength of
the phononic metal-molecule coupling. Our study sheds new light onto the phonon
transport in molecular junctions, and it provides clear guidelines for the
design of molecular junctions for thermal management.
