Electronic Band Gap Tuning and Calculations of Mechanical Strength and Deformation Potential by Applying Uniaxial Strain on MX₂ (M = Cr, Mo, W and X = S, Se) Monolayers and Nanoribbons

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 (sigma) and Young's modulus (Y) and observed that sigma 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 (Delta p) in the linear regime near the equilibrium position(epsilon < 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 (mu) 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.