Ultralow Thermal Conductivity and Thermal Diffusivity of Graphene/Metal Heterostructures through Scarcity of Low-Energy Modes in Graphene

In many ultralow thermal conductivity materials, interfaces of dissimilar materials are employed to impede heat flow perpendicular to the interfaces. However, when packed within a distance comparable to the phonon wavelengths, these interfaces are coupled and thus ineffective to scatter low-energy phonons, due to either coherent phonon transmission across the closely packed interfaces or weak coupling of the low-energy phonons and the interfaces. Here, we propose to block the propagation of these low-energy phonons by periodically distributed scarcity of available low-energy phonon modes using graphene/metal heterostructures of transferred graphene and ultrathin metal films. We demonstrate the effectiveness of graphene in blocking propagation of low-energy phonons by comparing the effective transmission probabilities of phonons in a wide range of multilayered structures; we find that interfaces in our graphene/metal heterostructures remain decoupled even when the spacing between interfaces is <2 nm. With the proposed strategy, we successfully achieve an ultralow thermal conductivity of Λ = 0.06 W m^-1 K^-1 and a world-record lowest thermal diffusivity of α = 2.6 × 10^-4 cm² s^-1 suitable for thermal insulation. Moreover, we demonstrate the capability to tune the electronic heat transport across the new materials by creating atomic-scale pinholes on graphene through magnetron sputtering, with electrons carrying ≈50% of heat when Λ is ≈0.15 W m^-1 K^-1. With the ultralow Λ and substantial electronic transport, the new graphene/metal heterostructures could be explored for thermoelectric applications.