Drastic effects of vacancies on phonon lifetime and thermal conductivity in graphene

Understanding thermal transport in 2D materials and especially in graphene is a key challenge for the design of heat management and energy conversion devices. The high sensitivity of measured transport properties to structural defects, ripples and vacancies is of crucial importance in these materials. Using a first principle based approach combined with an exact treatment of the disorder, we address the impact of vacancies on phonon lifetimes and thermal transport in graphene. We find that perturbation theory fails completely and overestimates phonon lifetimes by almost two orders of magnitude. Whilst, in defected graphene, longitudinal acoustic and transverse acoustic modes remain well defined, the out of plane acoustic (ZA) modes become marginal. In the long wavelength limit, the ZA dispersion changes from quadratic to linear and the scattering rate is found proportional to the phonon energy, in contrast to the quadratic scaling often assumed. The impact on thermal transport, calculated beyond the relaxation time approximation and including first principle phonon–phonon scattering rates as reported recently for pristine graphene, reveals spectacular effects even for extremely low vacancy concentrations. (paper)