Probing physical origin of anisotropic thermal transport in black phosphorus nanoribbons

TL;DR

This study experimentally confirms that the anisotropic thermal transport in black phosphorus nanoribbons is primarily due to the anisotropic phonon group velocity, supported by direct measurements and first-principles calculations.

Contribution

It provides the first experimental evidence linking phonon group velocity anisotropy to thermal transport anisotropy in black phosphorus nanoribbons.

Findings

Thermal conductivity ratio matches Young modulus ratio between directions.

Anisotropic phonon group velocity explains thermal transport anisotropy.

First-principles calculations support experimental results.

Abstract

Black phosphorus (BP) has emerged as a promising candidate for next generation electronics and optoelectronics among the 2D family materials due to its extraordinary electrical/optical/optoelectronic properties. Interestingly, BP shows strong anisotropic transport behaviour because of its puckered honeycomb structure. Previous studies have demonstrated the thermal transport anisotropy of BP and theoretically attribute this to the anisotropy in both phonon dispersion relation and phonon relaxation time. However, the exact origin of such strong anisotropy lacks clarity and has yet to be proven experimentally. In this work, we probe the thermal transport anisotropy of BP nanoribbons (NRs) by an electron beam technique. We provide direct evidence that the origin of this anisotropy is dominated by the anisotropic phonon group velocity for the first time, verified by Young modulus measurements along different directions. It turns out that the ratio of thermal conductivity between zigzag (ZZ) and armchair (AC) ribbons is almost same as that of the corresponding Young modulus values. The results from first-principles calculation are consistent with this experimental observation, where anisotropic phonon group velocity between ZZ and AC is shown. Our results provide fundamental insight into the anisotropic thermal transport in low symmetric crystals.