▎ 摘　　要

Lattice dynamics in artificial periodic structures, or "phononic crystals", have attracted significant research interest, thanks to the potential to manipulate acoustic wave propagation with more flexibility. The same control on heat conduction, however, has proven challenging due to the short wavelength of thermal phonons. In this work, we use first-principles simulations to characterize the previously unstudied thermal properties of dodecagraphene and tetragraphene, two-dimensional (2D) carbon allotropes based upon graphene but containing a secondary, in-plane periodicity. Surprisingly, we find that despite very similar atomic structure and bonding strength, they possess significantly different thermal properties than that of graphene: at room temperature, their thermal conductivity is up to 80% lower than that of graphene. We attribute these distinct properties to the presence of naturally occurring, low frequency optical phonon modes that arise from a folding of the acoustic modes due to the superstructure and the associated frequency gap opening. Furthermore, we observe significantly enhanced Umldapp scatterings in both carbon allotropes that largely suppress the hydrodynamic phonon transport in pristine graphene. Our study presents dodecagraphene and tetragraphene as ideal model systems to explore lattice dynamics in 2D and demonstrates the potential to significantly modify thermal transport of 2D materials without making drastic changes to their fundamental compositions.