Defect Dependent Electronic and Structural Properties of Monolayer Two-Dimensional Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) exhibit a wide range of electronic properties due to their structural diversity. Understanding their defect-dependent properties might enable the design of efficient, bright, and long-lifetime quantum emitters. Here, we use density functional theory (DFT) calculations to investigate the 2H, 1T, and 1T' phases of MoS₂, WS₂, MoSe₂, and WSe₂ and the effect of defect densities on the electronic band structures, focusing on the influence of chalcogen vacancies.

The pristine 2H, 1T, and 1T' phases display distinct structural and electronic characteristics. The 2H phase, thermodynamically stable, is a direct band gap semiconductor, while the 1T phase, despite its higher formation energy, exhibits metallic behavior conducive to energy-related applications. The metastable 1T' phase demonstrates unique electronic configurations, with potential applications in quantum spin Hall effect and high conductivity electrocatalysts. 1T’ phases with spin-orbit coupling show significant band inversion (0.61, 0.77, 0.24 and 0.78 eV for MoS₂, MoSe₂, WS₂ and WSe₂, respectively), which can be used for device applications.

Based on state-of-the art DFT calculations provide a detailed understanding of the structural, electronic, and phonon properties of these TMD phases. The cohesive and formation energies, lattice constants, bond lengths and Raman spectra are calculated for each phase. We discovered that for all four MX₂ systems, the energy difference between 2H, 1T and 1T’ phases decrease with increasing concentration of vacancy (from %3.13 to %21.88). The study further explores the effect of varying chalcogen vacancy concentrations on the electronic band structure, revealing a strong dependence on defect density and configuration.

Chalcogen vacancies, whether isolated or in clusters, significantly alter the electronic properties of TMDs. Here, we found a significant decrease in the band gap when we introduced chalcogen vacancies. For instance, the band gap of 2H WS₂ reduced from 1.81 (1.54 eV) at GGA (GGA+SOC) to 0.78 eV. These defects can introduce direct to indirect band gap transition, semi-metallic or magnetic band structures, influencing phonon-magnon and phonon-exciton interactions. The study highlights the necessity of optimizing molecular beam epitaxy (MBE) processes to control defect densities for targeted applications.

Overall, this study advances the understanding of defect-dependent electronic properties in TMDs, providing valuable insights for engineering advanced materials for quantum information processing, sensing, and energy conversion technologies. Our findings offer guidelines for experimental screening of 2D TMD defects, paving the way for the development of next-generation spintronic, electronic, and optoelectronic devices.