Ferromagnetism in Defected TMD (MoX₂, X = S, Se) Monolayer and Its Sustainability under O₂, O₃, and H₂O Gas Exposure: DFT Study

Spin-polarized density-functional theory (DFT) has been employed to study the effects of atmospheric gases on the electronic and magnetic properties of a defective transition-metal dichalcogenide (TMD) monolayer, MoX2 with X = S or Se. This study focuses on three single vacancies: (i) molybdenum "V-Mo"; (ii) chalcogenide "V-X"; and (iii) di-chalcogenide "V-X2". Five different samples of sizes ranging from 4 x 4 to 8 x 8 primitive cells (PCs) were considered in order to assess the effect of vacancy-vacancy interaction. The results showed that all defected samples were paramagnetic semiconductors, except in the case of V-Mo in MoSe2, which yielded a magnetic moment of 3.99 mu(B) that was independent of the sample size. Moreover, the samples of MoSe2 with V-Mo and sizes of 4 x 4 and 5 x 5 PCs exhibited half-metallicity, where the spin-up state becomes conductive and is predominantly composed of d(xy) and d(z2) orbital mixing attributed to Mo atoms located in the neighborhood of V-Mo. The requirement for the establishment of half-metallicity is confirmed to be the provision of ferromagnetic-coupling (FMC) interactions between localized magnetic moments (such as V-Mo). The critical distance for the existence of FMC is estimated to be d(c) congruent to 16 angstrom, which allows small sample sizes in MoSe2 to exhibit half-metallicity while the FMC represents the ground state. The adsorption of atmospheric gases (H2O, O-2, O-3) can drastically change the electronic and magnetic properties, for instance, it can demolish the half-metallicity characteristics. Hence, the maintenance of half-metallicity requires keeping the samples isolated from the atmosphere. We benchmarked our theoretical results with the available data in the literature throughout our study. The conditions that govern the appearance/disappearance of half-metallicity are of great relevance for spintronic device applications.