Enhancing thermal radiation by graphene-assisted hBN/SiO₂ hybrid structures at the nanoscale

A graphene-assisted hBN/SiO₂ hybrid structure is proposed and demonstrated to enhance near-field thermal radiation (NFTR). Due to the complementarity between the hyperbolic phonon polaritons of hBN and the surface phonon polaritons of SiO₂ at mid-infrared frequencies, coupling modes can remarkably improve the photon tunneling probability over a broad frequency band, especially when assisted by the surface plasmon polaritons of graphene sheets. Thus, the heat flux can exceed the blackbody limit by 4 orders of magnitude at a separation distance of 10 nm and reach 97% of the infinite limit of graphene-hBN multilayers using only two layers with a thickness of 20 nm each. The first graphene layer controls most of the heat flux, while the other layers can be used to regulate and optimize. The dynamic relationship between the chemical potential μ and the gap distance d are thoroughly discussed. Optimal heat flux of our graphene-assisted hBN/SiO₂ hybrid structure with proper choices of (μ₁, μ₂, μ₃) for different d (from 10 nm to 1000 nm) is further increased by 28.2% on average in comparison with the existing graphene-hBN triple-layer structure.