[111] Prediction of novel ground-state structures and analysis of phonon transport in two-dimensional Ge_xS_y compounds

We conduct this study to explore the ground-state structures of the two-dimensional (2D) variable-composition Ge_xS_y compounds, driven by the polymorphic characteristics of bulk germa- nium 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 (P-3m1), and GeS₂ (P21/c). These 2D materials exhibit signifi- cantly lower formation energies compared to their reported counterparts. We thoroughly scruti- nized the structural stability and subsequently analyzed their electronic structures. Our analysis reveals a nearly direct band gap of 0.12/0.84 eV with 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 (κl), which we determined by solving the Boltzmann transport equation for phonons. Importantly, we predict a room temperature κl value of 6.82 Wm−1K−1 for GeS, lower than its 2D orthorhombic counterpart. In the case of GeS₂, we observed an anisotropic κl value of 16.95/10.68 Wm−1K−1 along the zigzag/armchair directions at 300 K, with 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.