Oxygenation of monolayer gallium monochalcogenides: Design of two-dimensional ternary Ga2XO structures ( X=S,Se,Te )

The possibility of breaking structural symmetry with realization of Janus monolayers offers new possibilities in the field of two-dimensional (2D) materials, and various ternary systems including the class of group-III monochalcogenides have been suggested. However, interaction of oxygen was shown to modify optoelectronic properties of gallium monochalcogenides, and design of ternary systems with oxygen as a third component has not been considered yet. In this paper, we design and investigate 2D ${\mathrm{Ga}}₂X\text{O}$ ($X=\mathrm{S},\mathrm{Se},\mathrm{Te}$) systems by using first-principles calculations. Phonon spectra analysis and molecular dynamics simulations indicate that while ${\mathrm{Ga}}₂\mathrm{SO}$ and ${\mathrm{Ga}}₂\mathrm{SeO}$ are stable even at high temperatures ${\mathrm{Ga}}₂\mathrm{TeO}$ is dynamically unstable. Inclusion of oxygen makes ${\mathrm{Ga}}₂\mathrm{SO}$ and ${\mathrm{Ga}}₂\mathrm{SeO}$ less brittle when compared to their binary constituents. While $\mathrm{Ga}X$ monolayers have indirect band gaps, ${\mathrm{Ga}}₂\mathrm{SO}$ and ${\mathrm{Ga}}₂\mathrm{SeO}$ become direct band-gap semiconductors and the band gap can be further tuned by tensile/compressive strain. Additionally, depending on the type of the system, strong optical absorption within the infrared, visible, and/or ultraviolet region is also predicted. Finally, structural and electronic properties of bilayers of ${\mathrm{Ga}}₂X\text{O}$ are examined and compared with monolayers. Our results not only predict stable 2D ternary ${\mathrm{Ga}}₂X\text{O}$ structures but also suggest them as promising materials for optoelectronic applications.