Tuning the thermal conductivity of silicon nanowires by surface passivation

Using large scale molecular dynamics simulations, we study the thermal conductivity of bare and surface passivated silicon nanowires (SiNWs). For the cross-sectional widths $w \le 2$ nm, SiNWs become unstable because of the surface amorphosization and also due to the evaporation of a certain fraction of Si atoms. The observed surface (in-)stability is related to a large excess energy $\Delta$ of the surface Si atoms with respect to the bulk Si, resulting from the surface atoms being less coordinated and having dangling bonds.We first propose a practically relevant method that uses $\Delta$ as a guiding tool to passivate these dangling bonds with hydrogen or oxygen, stabilizing the SiNWs. These passivated SiNWs are used to calculate the thermal conductivity coefficient $\kappa$.While the expected trend of $\kappa \propto w$ is observed for all SiNWs, surface passivation provides an added flexibility of tuning $\kappa$ with the surface coverage concentration $c$ of passivated atoms.Indeed, with respect to the bulk $\kappa$, passivation of SiNW reduces $\kappa$ by 75-80\% for $c \to 50\%$ and recovers again by 50\% for the fully passivated samples. Analyzing the phonon band structures via spectral energy density, we discuss separate contributions from the surface and the core to $\kappa$. Our results also reveal that surface passivation increases SiNW stiffness, contributing to the tunability in $\kappa$.