Extraordinary thermal conductivity of gold sulfide monolayers*

Gold sulfide monolayers (α-, β-Au₂S  and α-, β-, γ-AuS) have emerged as a new class of two-dimensional (2D) materials with appealing properties such as high thermal and dynamical stability, oxidation resistibility, and excellent electron mobility. However, their thermal properties are still unexplored. In this study, based on first-principles calculations and the Peierls-Boltzmann transport equation, we report the thermal conductivity (κ) and related phonon thermal properties of all members of this family. Our results show that gold sulfide monolayers have ultralow thermal conductivity within the range of 0.04 W m^-1K^-1 to 10.62 W m^-1K^-1, with different levels of anisotropy. Particularly, our results demonstrate that β-Au₂S, having κ_aa= 0.06 W m^-1K^-1 and κ_bb= 0.04 m^-1K^-1 along the principal in-plane directions, has one of the lowest κ values that have been reported for a 2D material. This extremely low thermal conductivity can be attributed to its flattened phonon branches and low phonon group velocity, high anharmonicity, and short phonon lifetimes. Our results may provide insight into the application of gold sulfide monolayers as thermoelectric materials, and motivate future κ measurements of gold sulfide monolayers.