Coherent and incoherent phonon thermal transport in isotopically modified graphene superlattices

Understanding the nature of phonon transport is of both fundamental and technological importance. In this paper, we demonstrate unambiguous coherent and incoherent phonon transport in C-12/C-13 graphene superlattices using large-scale molecular dynamics simulations. Coherent phonon modes preserve their phases as they propagate through multiple interfaces. For these phonons, the superlattice can be treated as a homogeneous material with its own unit cell and phonon dispersion. We observe length-dependent thermal conductivity of the C-12/C-13 graphene superlattices, which indicates the existence of coherent phonons that transport ballistically over large distances, By changing the period length of the superlattices and thus the interface density, we observe a minimum in thermal conductivity, which implies the crossover from incoherent to coherent phonon transport. The thermal conductivity of the superlattices can be further decreased as we disrupt the coherence of phonons by manipulating and randomizing the superlattice structure. Our results show that graphene - a two-dimensional material with intrinsically weak anharmonic phonon scattering - is an ideal platform for studying the nature of phonons. The ability of manipulating thermal conductivity using superlattice-based two-dimensional materials can also potentially open up opportunities for thermoelectric applications given existing reports on their high thermoelectric power factors. (C) 2014 Elsevier Ltd. All rights reserved.