Air stability of monolayer WSi₂N₄ in dark and bright conditions

Two-dimensional materials with chemical formula MA₂Z₄ are a promising class of materials for optoelectronic applications. To exploit their potential, their stability with respect to air pollution has to be analyzed under different conditions. In a first-principle study based on density functional theory, we investigate the adsorption of three common environmental gas molecules (O₂, H₂O, and CO₂) on monolayer WSi₂N₄, an established representative of the MA₂Z₄ family. The computed adsorption energies, charge transfer, and projected density of states of the polluted monolayer indicate a relatively weak interaction between substrate and molecules resulting in an ultrashort recovery time of the order of nanoseconds. O₂ and water introduce localized states in the upper valence region but do not alter the semiconducting nature of WSi₂N₄ nor its band-gap size apart from a minor variation of a few tens of meV. Exploring the same scenario in the presence of photogenerated electrons and holes, we do not notice any substantial difference except for O₂ chemisorption when negative charge carriers are in the system. In this case, monolayer WSi₂N₄ exhibits signs of irreversible oxidation, testified by an adsorption energy of -5.5 eV leading to an infinitely long recovery time, a rearrangement of the outermost atomic layer bonding with the pollutant, and n-doping of the system. Our results indicate stability of WSi₂N₄ against H₂O and CO₂ in both dark and bright conditions, suggesting the potential of this material in nanodevice applications.