Computational discovery of 2D aluminium and gallium sulfides

The discovery of new two-dimensional (2D) materials presents a major challenge in modern materials science, as these materials offer significant promise for advancing technology and applications. In this study, we investigate a new family of 2D aluminum-sulfur and gallium-sulfur compounds, $2\mathrm{D}\phantom{\rule{0.16em}{0ex}}{\mathrm{A}}_{\mathrm{x}}{\mathrm{S}}_{\mathrm{y}}$ (A = Al, Ga), using an evolutionary algorithm alongside density functional theory (DFT). In both Ga- and Al-S systems, five structural types show up as stable compounds, including $2\mathrm{D}\phantom{\rule{0.16em}{0ex}}\mathrm{P}\text{\ensuremath{-}}6m2$ AS, $C2/m$ AS, $P{2}_{1}/m\phantom{\rule{0.16em}{0ex}}{\mathrm{A}}_{2}{\mathrm{S}}_{3}$, Pmm2 ${\mathrm{AS}}_{2}$, and Cm ${\mathrm{AS}}_{2}$. We assessed the viability of each 2D phase based on its thermodynamic, dynamical, and thermal properties. Additionally, we examined their structural, bonding, electronic, charge carrier mobility, piezoelectric, and mechanical characteristics in detail. Using the HSE06 functional, these 2D materials demonstrated a wide range of electronic behaviors, from metallic to semiconducting (with band gaps of 1.87-- 3.41 eV), along with extremely high charge carrier mobilities ranging from 1 to $14\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. The in-plane and out-of-plane piezoelectric properties of these $2\mathrm{D}\phantom{\rule{0.16em}{0ex}}{\mathrm{A}}_{\mathrm{x}}{\mathrm{S}}_{\mathrm{y}}$ materials ranged from \ensuremath{-}1.91 to 6.31 pm/V and from \ensuremath{-}39.09 to 0.41 pm/V, respectively. Our findings not only extend the range of 2D aluminum-sulfur and gallium-sulfur compounds but also highlight their promising potential for applications in optoelectronic and piezoelectric devices.