Temperature dependence of anisotropic thermal conductivity tensor of bulk black phosphorus

TL;DR

This study measures the anisotropic thermal conductivity tensor of bulk black phosphorus across 80-300 K, revealing longer phonon mean-free-paths in the through-plane direction and providing key insights into phonon scattering mechanisms.

Contribution

It provides the first detailed measurement of the thermal conductivity tensor of bulk black phosphorus, highlighting anisotropic phonon relaxation times and scattering processes.

Findings

Thermal conductivity tensor is >25% higher in the through-plane axis.

Phonons are scattered mainly by Umklapp processes in all directions.

Phonon relaxation time is isotropic in basal planes but anisotropic through-plane.

Abstract

To date, the intrinsic thermal conductivity tensor of bulk black phosphorus (BP), an important 2D material, is still unknown, since recent studies focus on BP flakes not on bulk BP. Here we report the anisotropic thermal conductivity tensor of bulk BP, for temperature range 80 - 300 K. Our measurements are similar to prior measurements on submicron BP flakes along zigzag and armchair axes, but are >25% higher in the through-plane axis, suggesting that phonon mean-free-paths are substantially longer in the through-plane direction. We find that despite the anisotropy in thermal conductivity, phonons are predominantly scattered by the same Umklapp processes in all directions. We also find that the phonon relaxation time is rather isotropic in the basal planes, but is highly anisotropic in the through-plane direction. Our work advances fundamental knowledge of anisotropic scattering of phonons in BP and is an important benchmark for future studies on thermal properties of BP nanostructures.