  The study of heat transport in two-dimensional (2D) materials reveals novel
behaviors due to quantum confinement effects, where in-plane and out-of-plane
phonons play crucial roles. In 2D materials like graphene, it is widely
recognized that the out-of-plane vibrational mode is the primary contributor to
thermal conductivity owing to the mirror symmetry. Based on this perspective,
the introduction of interlayer coupling, which breaks this symmetry, is
expected to induce a significant reduction in thermal conductivity within 2D
materials. Nevertheless, recent studies have presented unexpected findings,
indicating that interlayer coupling can actually increase thermal conductivity
of 2D materials. This controversial result suggests a nontrivial underlying
mechanism governing the effects of interlayer coupling on thermal conductivity
in 2D materials, necessitating further exploration. In our work, we investigate
the modulation of thermal conductivity through interlayer coupling in a
sandwich structure composed of hexagonal boron nitride (h-BN) and bilayer
graphene (BG), specifically a h- BN/BG/h-BN system. Through molecular dynamics
simulations, we find that the thermal conductivity from out-of-plane phonons
can be significantly reduced, while that from in-plane phonons can be
significantly increased, as the interlayer coupling strength increases. This
results in a nontrivial, coupling-strength-dependent overall thermal
conductivity. The phonon spectrum analysis conducted using our modified package
reveals that the upshift and flattening of the out-of-plane (ZA and ZO) phonon
modes are mainly responsible for these variations, and the extent of the
upshift and flattening is proportional to the strength of interlayer coupling.
This work offers new insights into manipulating the thermal conductivity of 2D
materials.
