Lattice thermal conductivity of β₁₂ and χ₃ borophene

Borophene allotropes have many unique physical properties due to their polymorphism and similarity between boron and carbon. In this work, based on the density functional theory and phonon Boltzmann transport equation, we investigate the lattice thermal conductivity \kappa of both β ₁₂ and χ₃ borophene. Interestingly, these two allotropes with similar lattice structures have completely different thermal transport properties. β₁₂ borophene has almost isotropic \kappa around 90 W/(mK) at 300 K, while \kappa of χ₃ borophene is much larger and highly anisotropic. The room temperature \kappa of χ ₃ borophene along the armchair direction is 512 W/(mK), which is comparable to that of hexagonal boron nitride but much higher than most of the two-dimensional materials. The physical mechanisms responsible for such distinct thermal transport behavior are discussed based on the spectral phonon analysis. More interestingly, we uncover a unique one-dimensional transport feature of transverse acoustic phonon in χ₃ borophene along the armchair direction, which results in a boost of phonon relaxation time and thus leads to the significant anisotropy and ultrahigh thermal conductivity in χ₃ borophene. Our study suggests that χ ₃ borophene may have promising application in heat dissipation, and also provides novel insights for enhancing the thermal transport in two-dimensional systems.