Abnormal thermal conductivity enhancement in covalently bonded bilayer borophene allotrope

Thermal conductivity of two-dimensional (2D) materials has gained prominence due to the attractive applications in thermal management and thermoelectric devices. In this work, we present a new member of bilayer 2D boron allotropes, denoted as bilayer β₁₂ borophene, and study the thermal transport properties by solving phonon Boltzmann transport equation based on density functional theory. Based on quantitative chemical bonding analysis, we identify large degrees of covalent bonding of the interlayer interaction. In comparison to its monolayer counterpart, the bilayer exhibits much higher in-plane thermal conductivity despite the lower phonon group velocity and buckling structure, inferring a new physical mechanism. The thermal conductivity (κ) of bilayer β₁₂ borophene at 300 K is 140.5 (86.3) W·m⁻¹·K⁻¹ along armchair (zigzag) direction, and κ_armchair is about 52.7% higher than that of monolayer β₁₂ borophene. The abnormal enhancement is attributed to the suppressed phonon scattering possibility and elongation of phonon lifetime. More interesting, after forming bilayer β₁₂ borophene through interlayer covalent bonding, the dominated phonon branch to thermal conductivity changes to transverse acoustic phonons from out-of-plane flexural acoustic (ZA) phonons in the monolayer borophene. Our study elucidates the rich thermal transport characteristics in bilayer covalently bonded 2D materials, and injects fresh insights into the phonon engineering of 2D borophene relevant for emergent thermal management applications.