Improved structural calculations of bulk and monolayer TaX2 (X = S, Se) using DFT-D, and comparison of their electronic and elastic properties

Abstract

In recent decades, transition metal dichalcogenides have emerged as promising platforms for integrating electronic and optical properties in both bulk and monolayer forms. Among these, TaX₂ (X = S, Se) materials exhibit significant characteristics such as charge density waves, strong optical responses, and superconducting behavior, making them suitable for flexible electronics, photonics, and energy storage. This study provides a detailed calculation of van der Waals forces' effects on the structural, elastic, and electronic properties of bulk and monolayer TaX₂, utilizing the DFT-D method and comparing results with LDA and GGA approximations. DFT-D calculations reveal that the lattice constants of bulk TaS₂ and TaSe₂ differ from experimental values by only 0.54 % and 0.13 %, respectively, indicating a more accurate estimation than LDA and GGA. The bulk TaX₂ band structure shows overlapping conduction and valence bands near the Fermi level, suggesting metallic properties. However, transitioning to monolayer structures eliminates this overlap and modifies the band positions, affecting the band gap and orbital characters. The DFT-D calculations yield band gaps of 0.72 eV for monolayer TaS₂ and 0.64 eV for TaSe₂. Furthermore, mechanical analysis confirms the structural stability of TaX₂ in both bulk and monolayer forms, as verified by the Born stability criterion. Elastic constant calculations, alongside the extraction of Bulk and Shear moduli using Pugh's law (G/B < 0.571), indicate that both bulk and monolayer TaX₂ have flexible structures.