  We report a record low thermal conductivity in polycrystalline MoS2 obtained
by varying grain sizes and orientations in ultrathin films. By optimizing the
sulphurisation parameters of nanometre-thick Mo layer, we could grow MoS2 films
with tuneable morphologies. The thermal conductivity is extracted from a Raman
laser power-dependent study on suspended samples. The lowest value of thermal
conductivity of 0.27 Wm-1K-1, which reaches a similar value as teflon, is
obtained in a polycrystalline sample formed by a combination of horizontally
and vertically oriented grains, with respect to the bulk (001) monocrystal.
Analysis by means of molecular dynamics and finite element method simulations
confirm that such grain arrangement leads to lower grain boundary conductance.
We discuss the possible use of these thermal insulating films in the context of
electronics and thermoelectricity.
