The Optoelectronic, Vibrational, and Thermodynamic Attributes of 2D TMD-MoWX ₄ (X = S, Se) Alloys and Janus TMD-MoWS ₂ Se ₂ Alloy: A DFT Approach

This study presents a comprehensive theoretical investigation of two-dimensional transition metal dichalcogenide alloys, MoWS₄, MoWSe₄, and Janus MoWS₂Se₂employing density functional theory and density functional perturbation theory. Phonon dispersion analysis confirms the dynamic stability in all monolayers. Electronic structure calculations demonstrate direct bandgaps along the Γ - <i>Y</i> high-symmetry path, with the hybrid HSE06 functional predicting significantly larger bandgaps (2.21, 1.95, and 2.08 eV) compared to GGA-PBE (1.68, 1.47, and 1.58 eV) and PBE + SOC (1.53, 1.33, and 1.43 eV) results for MoWS₄, MoWSe₄, and MoWS₂Se₂, respectively, emphasizing the importance of exchange-correlation treatments. Projected density of states (PDOS) analysis indicates predominant contributions from Mo/W d-orbitals near the Fermi level, while chalcogen p-orbitals substantially influence optical characteristics. Optical absorption spectra exhibit similar behaviors: MoWS₄, MoWSe₄, and MoWS₂Se₂ display strong absorption peaks near 5 and 10 eV, shown to be sensitive to the plane of polarization of the incident light in the range of ultraviolet radiation to the visible spectrum. Vibrational spectroscopy reveals unique fingerprints through prominent IR-active modes at 361 cm^-1 (MoWS₄), 280 cm^-1 (MoWSe₄), and 335 cm^-1 (MoWS₂Se₂), complemented by Raman-active modes at 361 cm^-1, 289 cm^-1, and 335 cm^-1, respectively. Negative free energies across 0-1000 K evidenced thermodynamic stability, while MoWSe₄ exhibited superior stability. Molecular quantum dynamics simulations at 300 K further validate the thermal stability with minimal structural deformations. All these findings highlight the significant potential of MoWX₄-based monolayers for advanced optoelectronic, thermoelectric, and sensing applications while establishing a robust theoretical framework to guide future experimental research and property optimization efforts.