ABSTRACT
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 "VMo"; (ii) chalcogenide "VX"; and (iii) di-chalcogenide "VX2". Five different samples of sizes ranging from 4 × 4 to 8 × 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 VMo in MoSe2, which yielded a magnetic moment of 3.99 µB that was independent of the sample size. Moreover, the samples of MoSe2 with VMo and sizes of 4 × 4 and 5 × 5 PCs exhibited half-metallicity, where the spin-up state becomes conductive and is predominantly composed of dxy and dz2 orbital mixing attributed to Mo atoms located in the neighborhood of VMo. 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 VMo). The critical distance for the existence of FMC is estimated to be dcâ 16 Å, which allows small sample sizes in MoSe2 to exhibit half-metallicity while the FMC represents the ground state. The adsorption of atmospheric gases (H2O, O2, O3) 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.
ABSTRACT
Two-dimensional (2D) transition-metal dichalcogenides (TMDs) are new crystalline materials with exotic electronic, mechanical, and optical properties. Due to their inherent exceptional mechanical strength, these 2D materials provide us the best platform for strain engineering. In this study, we have performed first-principles calculations to study the effect of uniaxial strains on the electronic, magnetic, and mechanical properties of transition-metal dichalcogenides (TMDs) MX2 (where M = Cr, Mo, W and X = S, Se), monolayers (2D), and armchair and zigzag nanoribbons (1D). For the mechanical strength, we determined the tensile strength (σ) and Young's modulus (Y) and observed that σ and Y are higher in monolayers (most in WS2ML) as compared to nanoribbons where monolayers resist tension up to 25-28% strain while nanoribbons (armchair and zigzag) can be only up to 5-10%. Deformation potential (Δ p ) in the linear regime near the equilibrium position(ϵ < 2%) has also been calculated, and its effect on monolayers is observed less as compared to nanoribbons. In addition, unstrained nonmagnetic monolayers are direct band gap semiconductors (D) which changed to indirect band gap semiconductors (I) with the application of strain. Ferromagnetic states of metallic zigzag nanoribbons (including up spin channel of 7-CrS2NR and 7-CrSe2NR) are greatly affected by strain and show half-metal-like behavior in different strain range. The magnetic moment (µ) that is predominantly observed in zigzag nanoribbons is 2 times higher than that of other nanoribbons. This magnetism in nanoribbons is mostly caused by transition-metal atoms (M = Cr, Mo, W). Thus, our study suggests that strain engineering is the best approach to modify or control the structural, electronic, magnetic, and mechanical properties of the TMD monolayer and nanoribbons which, therefore, open their potential applications in spintronics, photovoltaic cells, and tunneling field-effect transistors.
