Spectral Phonon Analysis of Coherent and Incoherent Phonon Transport in Nanomeshes of 2D and 3D Materials

Phononic crystals, particularly nanomeshes—porous thin films of 2D or 3D materials—have garnered significant interest due to their distinctive thermal, optical, and chemical properties. This study investigates the spectral thermal transport characteristics of silicon and graphene nanomeshes, featuring nanoholes of varied sizes arranged periodically and aperiodically. Our simulations demonstrate a pronounced reduction in thermal conductivity (k) for small-hole nanomeshes, attributable to aperiodic hole distributions along heat flow directions, in contrast to large-hole counterparts. Spectral phonon analysis reveals significant suppression of low-frequency phonons in small-hole nanomeshes, akin to behavior seen in 1D superlattices. Additionally, spectral energy density analysis identifies well-defined coherent phonon modes within the 0-3 THz range in small-hole silicon nanomeshes and, moreover, spectral thermal conductivity analysis reveals that these modes ballistically transport across the nanomesh. Furthermore, a comparative analysis of the phonon transport behaviors between silicon and graphene nanomeshes highlights distinct differences in phonon-boundary scattering and phonon coherence, influenced by their respective 3D film and 2D single-layer structures.