  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 $\kappa$.
