Impact of Dimensional Crossover on Phonon Transport in Van der Waals Materials: A Case Study of Graphite and Graphene

Two-dimensional materials are the focus of intense research in part due to their unique thermal properties. With layered van der Waals materials, from which often 2D materials are isolated, the difference between bulk and monolayer originates from weak inter-layer coupling. Thus, 2D materials should retain some of the bulk character, and vice versa. In this talk, we present theoretical work investigating how phonon transport evolves in van der Waals materials when going from 3D to 2D, using graphite/graphene as a case study. The results are obtained using density functional theory combined with the phonon Boltzmann equation. To model the transition from 3D to 2D the inter-layer distance of graphite is gradually increased, to show how the phonon dispersion and scattering properties continuously evolve. Changes in the phonon dispersion, specifically the formation of low-energy optical phonons, play a significant role in the lower thermal conductivity of graphite, versus graphene. Interestingly, the 3-phonon scattering properties of graphite display similar behavior to graphene, in which selection rules restrict certain 3-phonon processes, due to the weak inter-layer coupling. Lastly, we attempt to identify generic features likely shared with other van der Waals materials.