LAUSR

The thermal conductivity ( k) of semiconducting nanomaterials is influenced by the geometry-dependent phonon boundary scattering mean free path ( Λ Bdy). Although prior work has calculated Λ Bdy of periodically corrugated rectangular nanowires and used these results to study phonon backscattering in nanomaterials, Λ Bdy remains unknown for recently fabricated periodic coaxial cylindrical nanowires. Here, we use phonon ray tracing simulations to comprehensively study the effect of geometric parameters on Λ Bdy in coaxial cylindrical nanowires. We find that for a fixed smaller cylinder diameter ( D 1) and cylinder length ratio, Λ Bdy of periodic nanowires can be maximized or minimized via geometric control of the pitch ( p) and larger cylinder diameter ( D 2). Our simulations show that saturated phonon backscattering for small pitch ratio ( p r) nanowires gives rise to a minimum in Λ Bdy / D 1 at p r near unity, while the maximum in Λ Bdy / D 1 for large p r nanowires can be understood using a simple thermal resistor model for two individual nanowires in series. Combining our Λ Bdy calculations with analytical phonon dispersion and bulk scattering models, we predict that k of periodic silicon nanowires with fixed D 1 can be tuned by up to 34% in the boundary scattering dominated regime by modifying D 2 and p and that variations as large as 135% can be observed in the normalized thermal conductance. Our results provide insight into geometry-dependent phonon backscattering and can be used to predict k of periodic cylindrical nanowires over a range of temperatures and geometric lengthscales.