  By using molecular dynamics simulations, we study thermal conductivity of
silicon nanowires (SiNWs) with different cross sectional geometries. It is
found that thermal conductivity decreases monotonically with the increase of
surface-to-volume ratio (SVR). More interestingly, a simple universal linear
dependence of thermal conductivity on SVR is observed for SiNWs with modest
cross sectional area (larger than 20 nm^2), regardless of the cross sectional
geometry. As a result, among different shaped SiNWs with the same cross
sectional area, the one with triangular cross section has the lowest thermal
conductivity. Our study provides not only a universal gauge for thermal
conductivity among different cross sectional geometries, but also a designing
guidance to tune thermal conductivity by geometry.
