DFT based study of charge transport across MoS2-yOx monolayers utilizing design of experiments planning and analyzing methods

Atomic scale defects strongly impact the electronic structure and charge transport properties of two dimensional catalysts, and in that way affect their performance under electrochemical operating conditions. The aim of this study is to understand how oxygen substitution defect concentration and their spatial distribution change the electronic states and charge transport through molybdenum disulfide (MoS 2 ) monolayers with and without sulfur vacancies. To achieve these objectives, we employ density functional theory (DFT), tight‑binding (TB) theory, non-equilibrium Green’s function (NEGF), Landauer–Büttiker approach, and design of experiments (DOE) methods. Our findings support that oxygen substitutions reduce the average metal–chalcogen bond length compared to a pristine structure and structures with sulfur vacancies. Further analysis reveals that the average Mo-O bond length increases as the oxygen concentration rises when the oxygen atoms are all located on the same sulfur atomic layer. This research also presents a complex mutual effect of sulfur vacancies and oxygen substitutions on the electronic structure of MoS 2 monolayers. The analysis shows that mid‑gap states, the electronic states identified at energies located within the pristine band gap (between the valence band maximum and the conduction band minimum), induced by the presence of sulfur vacancies, are eliminated when they are occupied by oxygen substitutional atoms, while oxygen substitutions near the vacancies split and shift these mid-gap electronic states. The main influence of oxygen substitutions on the electronic structure of MoS 2 monolayers is at the edge of the valence band maximum. Increasing oxygen concentration leads to a reduction in the band gap energy and the ionization potential, with the lowest values obtained for line oxygen atoms arrangements localized on a single sulfur layer. Formation energy calculations reveal that oxygen substitutions are thermodynamically favorable compared to vacancies formation, when the most preferable oxidation site is the vacancy site. The presented charge transport computations demonstrate that in structures with oxygen substitution defects only, the electric current increases with oxygen concentration, mainly when oxygen atoms are arranged in lines within a single sulfur layer. Structures combining adjacent vacancies and oxygen substitutions present enhanced conductivity, particularly when an oxygen atom is near the vacancies on the same atomic layer. Our calculated results provide valuable insight into relative trends in charge transport across different defect concentrations and spatial arrangements, demonstrating that these parameters strongly influence MoS 2 monolayer electronic properties, and thereby offering a framework for atomic scale defect engineering in two dimensional catalysts.