Si/Ge superlattice nanowires with ultralow thermal conductivity.

The engineering of nanostructured materials with very low thermal conductivity is a necessary step toward the realization of efficient thermoelectric devices. We report here the main results of an investigation with nonequilibrium molecular dynamics simulations on thermal transport in Si/Ge superlattice nanowires aiming at taking advantage of the inherent one dimensionality and the combined presence of surface and interfacial phonon scattering to yield ultralow values for their thermal conductivity. Our calculations revealed that the thermal conductivity of a Si/Ge superlattice nanowire varies nonmonotonically with both the Si/Ge lattice periodic length and the nanowire cross-sectional width. The optimal periodic length corresponds to an order of magnitude (92%) decrease in thermal conductivity at room temperature, compared to pristine single-crystalline Si nanowires. We also identified two competing mechanisms governing the thermal transport in superlattice nanowires, responsible for this nonmonotonic behavior: interface modulation in the longitudinal direction significantly depressing the phonon group velocities and hindering heat conduction, and coherent phonons occurring at extremely short periodic lengths counteracting the interface effect and facilitating thermal transport. Our results show trends for superlattice nanowire design for efficient thermoelectrics.