Sulfur Vacancy Related Optical Transitions in Graded Alloys of MoxW1‐xS2 Monolayers

Abstract Engineering electronic bandgaps is crucial for applications in information technology, sensing, and renewable energy. Transition metal dichalcogenides (TMDCs) offer a versatile platform for bandgap modulation through alloying, doping, and heterostructure formation. Here, the synthesis of a 2D Mo x W 1‐x S 2 graded alloy is reported, featuring a Mo‐rich center that transitions to W‐rich edges, achieving a tunable bandgap of 1.85 to 1.95 eV when moving from the center to the edge of the flake. Aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy showed the presence of sulfur monovacancy, V S , whose concentration varied across the graded Mo x W 1‐x S 2 layer as a function of Mo content with the highest value in the Mo‐rich center region. Optical spectroscopy measurements supported by ab initio calculations reveal a doublet electronic state of V S , which is split due to the spin‐orbit interaction, with energy levels close to the conduction band or deep in the bandgap depending on whether the vacancy is surrounded by W atoms or Mo atoms. This unique electronic configuration of V S in the alloy gave rise to four spin‐allowed optical transitions between the V S levels and the valence bands. The study demonstrates the potential of defect and optical engineering in 2D monolayers for advanced device applications.