Ultralow lattice thermal conductivity of monolayer penta-silicene and penta-germanene

TL;DR

This study reveals that monolayer penta-silicene and penta-germanene possess ultralow lattice thermal conductivities, with penta-germanene potentially having the lowest among 2D materials, due to weak phonon interactions and strong anharmonic scattering.

Contribution

First ab initio prediction of stable penta-germanene with ultralow thermal conductivity, expanding the understanding of 2D thermoelectric materials.

Findings

Penta-silicene has a thermal conductivity of 1.29 W/mK.

Penta-germanene has a thermal conductivity of 0.30 W/mK.

Penta-germanene may have the lowest thermal conductivity in 2D materials.

Abstract

We study the lattice thermal conductivity of two-dimensional (2D) pentagonal systems, such as penta-silicene and penta-germanene. Penta-silicene has been recently reported, while the stable penta-germanene, belonging to the same group IV element, is revealed firstly by our ab initio calculations. We find that both penta-silicene and penta-germanene at room temperature have ultralow lattice thermal conductivities $\kappa$ of 1.29 W/mK and 0.30 W/mK respectively. To the best of our knowledge, penta-germanene may have the lowest $\kappa$ in 2D crystal materials. We attribute ultralow $\kappa$ to the weak phonon harmonic interaction and strong anharmonic scattering. A small phonon group velocity, a small Debye frequency, a large Gruneisen parameter, and a large number of modes available for phonon-phonon interplay together lead to the ultralow $\kappa$ of penta-silicene and penta-germanene. These discoveries provide new insight into the manipulation of ultralow $\kappa$ in 2D materials and highlight the potential applications of designing silicon and germanium based high thermoelectric materials.