Nature of localized phonon modes of tilt grain boundaries in graphene

Abstract

In this work, it is reported an investigation of the phonon spectrum of polycrystalline models of graphene, using ab-initio calculations of the phonon dispersion and phonon modes of these systems. Four different models of polycrystalline graphene are considered: one where the lattices of adjacent grains have a relative translation between them, with a translational grain boundary connecting the grains, and three models where the lattices of adjacent grains are tilted relative to each other, with tilt grain boundaries connecting adjacent grains. It is found that tilt grain boundaries introduce high-frequency non-dispersive phonon modes in the phonon spectrum of polycrystalline graphene, and that these modes are strongly localized in the core of the grain boundaries, while no such high-frequency localized modes are found in the case of the translational boundary system. By computing phonon group velocities and specific heats of each system in our study, and by analyzing in detail the patterns of atomic displacements of these localized modes, a consistent interpretation is provided for the experimentally observed trends of the lattice thermal conductivity of polycrystalline graphene samples, as a function of the grain-boundary tilt angle.