  In standard molecular junctions, a molecular structure is placed between and
connected to metal leads. Understanding how mechanical tuning in such molecular
junctions can change heat conductance has interesting applications in nanoscale
energy transport. In this work, we use nonequilibrium molecular dynamics
simulations to address the effect of stretching on the phononic contribution to
the heat conduction of molecular junctions consisting of single long-chain
alkanes and various metal leads such as Ag, Au, Cu, Ni, and Pt. The thermal
conductance of such junctions is found to be much smaller than the intrinsic
thermal conductance of the polymer and significantly depends on the nature of
metal leads as expressed by the metal-molecule coupling and metal vibrational
density of states. This behavior is expected and reflects the mismatch of
phonon spectra at the metal molecule interfaces. As a function of stretching,
we find a behavior similar to what was observed earlier [J. Chem. Phys. 153,
164903 (2020)] for pure polymeric structures. At relatively short electrode
distances, where the polyethylene chains are compressed, it is found that the
thermal conductances of the molecular junctions remain almost constant as one
stretches the polymer chains. At critical electrode distances, the thermal
conductances start to increase, reaching the values of the fully-extended
molecular junctions. Similar behaviors are observed for junctions in which
several long-chain alkanes are sandwiched between various metal leads.
  These findings indicate that this behavior under stretching is an intrinsic
property of the polymer chain and not significantly associated with the
interfacial structures.
