Dominant phonon polarization conversion across dimensionally mismatched interfaces: Carbon-nanotube–graphene junction

Abstract

Dimensionally mismatched interfaces are emerging for thermal management applications, but thermal transport physics remains poorly understood. Here we consider the carbon-nanotube--graphene junction, which is a dimensionally mismatched interface between one- and two-dimensional materials and is the building block for carbon-nanotube (CNT)--graphene three-dimensional networks. We predict the transmission function of individual phonon modes using the wave packet method; surprisingly, most incident phonon modes show predominantly polarization conversion behavior. For instance, longitudinal acoustic (LA) polarizations incident from CNTs transmit mainly into flexural transverse (ZA) polarizations in graphene. The frequency stays the same as the incident mode, indicating elastic transmission. Polarization conversion is more significant as the phonon wavelength increases. We attribute such unique phonon polarization conversion behavior to the dimensional mismatch across the interface, and it opens significantly new phonon transport channels as compared to existing theories where polarization conversion is neglected.