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