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Using first-principles modeling, we investigate how phonon transport evolvesin layered/van der Waals materials when going from 3D to 2D, or vice versa, bygradually pulling apart the atomic layers in graphite to form graphene. Focusis placed on identifying the features impacting thermal conductivity that arelikely shared with other layered materials. The thermal conductivity $\kappa$of graphite is found to be lower than that of graphene mainly due to changes inthe phonon dispersion driven by van der Waals coupling. Specifically, as theatomic layers are brought closer together, the acoustic flexural phonons ingraphene form low-energy optical flexural phonons in graphite that possesslower in-plane velocities, density-of-states and phonon occupation, thusreducing $\kappa$. Similar dispersion changes, and impact on thermalconductivity, can be expected in other van der Waals materials whentransitioning from 2D to 3D. Our findings also indicate that the selectionrules in graphene, which reduce phonon-phonon scattering and contribute to itslarge $\kappa$, effectively hold as the atomic layers are brought together toform graphite. While the selection rules do not strictly apply to graphite, inpractice similar scattering behavior is displayed due in part to the weakinter-layer coupling. This suggests that van der Waals materials, in bulk 3Dform, may have lower phonon-phonon scattering rates than other non-layered bulkmaterials.

Impact of dimensional crossover on phonon transport in van der Waals materials: a case study of graphite and graphene