Ultralow thermal conductivity in Si/Ge_xSi_1-x core-shell nanowires

By applying non-equilibrium molecular dynamics simulation, it is demonstrated that the thermal conductivity of the reported Si/Ge core-shell nanowires (NWs) can be further reduced by shell doping. The thermal conductivity of Si/Ge_0.6Si_0.4 core-shell NWs is only about 66% of that of Si/Ge core-shell NWs. By analyzing the participation ratios of eigenmodes, it is revealed that the large reduction in the thermal conductivity of Si/Ge_0.6Si_0.4 core-shell NWs stems from the strong localization of the phonon modes from 1.0 THz to 2.0 THz and the modes from 9.0 THz to 16.0 THz due to both impurity scattering and interface scattering associated with peculiar structure of shell doped silicon NWs.