Opposite thermal conductivity trends in graphene and silicene driven by distinct interlayer coupling mechanisms

Interlayer coupling fundamentally governs the structural, electronic, and thermal properties of two-dimensional (2D) materials, yet its role in phonon transport remains incompletely understood. Here, we employ first-principles calculations combined with the Boltzmann transport theory to investigate heat conduction in monolayer and bilayer graphene and silicene, which exhibit contrasting interlayer interactions: weak van der Waals coupling in graphene and strong covalent bonding in silicene. We show that van der Waals interactions in bilayer graphene suppress thermal conductivity by ∼33%, primarily by breaking the reflection symmetry-based selection rules and reducing the lifetime of flexural acoustic (ZA) phonons. In stark contrast, bilayer silicene exhibits a 106% enhancement in thermal conductivity compared to its monolayer counterpart. This unusual trend originates from covalent interlayer bonding, which linearizes ZA phonon modes, suppresses their contribution, and activates enhanced contributions from transverse acoustic and optical branches. Our analysis further reveals that graphene supports hydrodynamic phonon transport driven by long-lived ZA modes, whereas in silicene, the hydrodynamic effect is strongly suppressed by buckling and interlayer covalency. These findings establish interlayer coupling as a decisive factor in phonon transport, providing fundamental insights for the design of 2D materials in energy conversion and thermal management applications.