Thermoelectrics offer an attractive pathway for addressing an important niche in the globally growing landscape of energy demand. Nanoengineering existing low-dimensional thermoelectric materials pertaining to realizing fundamentally low thermal conductivity has emerged as an efficient route to achieve high energy conversion performance for advanced thermoelectrics. In this paper, by performing nonequilibrium and Green-Kubo equilibrium molecular dynamics simulations we report that the thermal conductivity of Si nanowires (NWs) in polycrystalline form can reach a record low value substantially below the Casimir limit, a theory of diffusive boundary limit that regards the direction-averaged mean free path is limited by the characteristic size of the nanostructures. The astonishingly low thermal conductivity of polycrystalline Si NW is 269 and 77 times lower with respect to that of bulk Si and pristine Si NW, respectively, and is even only about one-third of the value of the purely amorphous Si NW at room temperature. By examining the mode level phonon behaviors including phonon group velocities, lifetime, and so forth, we identify the mechanism of breaking the Casimir limit as the strong localization of the middle and high frequency phonon modes, which leads to a prominent decrease of effective mean free path of the heat carriers including both propagons and diffusons. The contribution of the propagons to the overall thermal transport is further quantitatively characterized and is found to be dramatically suppressed in polycrystalline Si NW form as compared with bulk Si, perfect Si NW, and pure amorphous Si NW. Consequently, the diffusons, which transport the heat through overlap with other vibrations, carry the majority of the heat in polycrystalline Si NWs. We also proposed approach of introducing "disorder" in the polycrystalline Si NWs that could eradicate the contribution of propagons to achieve an even lower thermal conductivity than that ever thought possible. Our investigation provides a deep insight into the thermal transport in polycrystalline NWs and offers a promising strategy to construct a new kind of semiconducting thermoelectric NW with high figure of merit.
Keywords: Polycrystalline silicon nanowire; diffusons; molecular dynamics; propagons; thermal transport; thermoelectrics.