Prediction of novel ground-state structures and analysis of phonon transport in two-dimensional GeS compounds

We conducted this study to explore the ground-state structures of two-dimensional (2D) variable-composition GeS compounds, driven by the polymorphic characteristics of bulk germanium sulfides and the promising thermoelectric performance of 2D GeS (Pmn2₁). To accomplish this, we utilized the highly successful evolutionary-algorithm-based code USPEX in conjunction with VASP for total energy calculations, leading to the discovery of three previously unexplored structures of Ge₂S (P2/c), GeS (P3̄m1), and GeS₂ (P2₁/c). These 2D materials exhibit significantly lower formation energies compared to their reported counterparts. We thoroughly scrutinized the structural stability and subsequently analyzed their electronic structures. Our analysis reveals a nearly direct band gap of 0.12/0.84 eV with the PBE/HSE06 functional for 2D Ge₂S and an indirect band gap for 2D GeS and GeS₂. Their semiconducting nature highlights the crucial importance of lattice thermal conductivity (κ_l), which we determined by solving the Boltzmann transport equation for phonons. Importantly, we predict a room temperature κ_l value of 6.82 W m^-1 K^-1 for GeS, lower than its 2D orthorhombic counterpart. In the case of GeS₂, we observed an anisotropic κ_l value of 16.95/10.68 W m^-1 K^-1 along the zigzag/armchair directions at 300 K, with an in-plane anisotropy ratio of 1.59, surpassing that of 2D IV-VI compounds. We delve into detailed discussions regarding the role of lattice anharmonicity, group velocities, phonon lifetimes, and three-phonon weighted phase space in the overall thermal conductivity analysis.