Revealing Substitutional Oxygen as the Dominant Defect in Flux-Grown Transition Metal Diselenides

Advancing both the fundamental understanding and technological application of two-dimensional semiconducting transition metal dichalcogenides (TMDs) hinges on precise control and identification of atomic-scale defects. Although self-flux growth yields exceptionally pure TMD crystals, the nature of residual defects has remained an open question. Here, we use scanning tunneling microscopy (STM) to directly image and identify point defects in both monolayer and bulk self-flux grown WSe2. We find that the dominant defects reside on chalcogen sites and are unaffected by exfoliation or oxygen exposure. Combining STM observations with first-principles simulations and bulk impurity analysis, we attribute these defects to substitutional oxygen (OSe). This finding goes against the prevailing wisdom that vacancies are the most common defects in exfoliated TMDs. By establishing substitutional oxygen as the dominant defect, our work provides a crucial reference point for interpreting structure–property relationships and informs ongoing efforts to further improve material quality and device performance.