Reduction of phonon lifetimes and thermal conductivity of a carbon nanotube on amorphous silica

We use molecular dynamics (MD) simulations to examine phonon lifetimes in (10,10) carbon nanotubes (CNTs), both when isolated and when supported on amorphous SiO₂ substrates. We determine the intratube phonon-phonon, interfacial, and CNT-substrate phonon scattering rates from the computed inverse lifetimes. Suspended CNTs have in-plane optical phonon lifetimes between 0.7-2 ps, consistent with recent experiments, but contact with the substrate leads to a lifetime reduction to the 0.6-1.3 ps range, which is of consequence for high-field electrical transport. A significant lifetime reduction was also observed for low-frequency acoustic phonons with strong polarization dependence. As a result, the thermal conductivity of the supported CNT is computed to be ∼33% lower than that of the suspended CNT. The calculation of the scattering rates also enables us to estimate the thermal boundary conductance (TBC) between supported CNT and substrate, in good agreement with the Green-Kubo relation and with experimental results. The results highlight that solid substrates strongly affect and could be even used to tune the thermal properties of CNTs.