Revealing the Origins of 3D Anisotropic Thermal Conductivities of Black Phosphorus

Black phosphorus (BP) has emerged as a direct bandgap semiconducting material with great application potentials in electronics, photonics, and energy conversion. Experimental characterization of the anisotropic thermal properties of BP at the micrometer scale is extremely challenging. This study reports measurement results of the anisotropic thermal conductivity of BP along three primary crystalline orientations, using a novel time-resolved magneto-optical Kerr effect. The thermal conductivity along the zigzag crystalline direction is 84-101 W m(-1) K-1, nearly three times as large as that along the armchair direction (26-36 W m(-1) K-1). The through-plane thermal conductivity of BP ranges from 4.3 to 5.5 W m(-1) K-1. This study performs first-principles calculation to predict the phonon transport in BP along both in-plane through-plane directions, and identifies that the strong anisotropy of thermal transport in BP can be attributed to the structural-asymmetry-induced group velocity variations along different crystalline orientations, and the relaxation time variation induced by the direction of the applied temperature gradient. This work successfully unveils the fundamental mechanisms of anisotropic thermal transport along the three crystalline directions in BP, as demonstrated by the excellent agreement between the first-principles-based theoretical predictions and experimental characterizations on the anisotropic thermal conductivities of BP.