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

Abstract

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 Ga2XO (X = S, Se, Te) systems by using first-principles calculations. Phonon spectra analysis and molecular dynamics simulations indicate that while Ga2SO and Ga2SeO are stable even at high temperatures Ga2TeO is dynamically unstable. Inclusion of oxygen makes Ga2SO and Ga2SeO less brittle when compared to their binary constituents.While GaX monolayers have indirect band gaps, Ga2SO and Ga2SeO 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 Ga2XO are examined and compared with monolayers. Our results not only predict stable 2D ternary Ga2XO structures but also suggest them as promising materials for optoelectronic applications.