and the Impact of Defect Density of Photoluminescence*

Despite the importance of understanding defect-related phenomena in 2D materials, there remains a need for quantitative characterization of defect density over large areas in order to understand the relationship between defects and observed properties, such as photoluminescence (PL). We report direct observation of defects in monolayer WS₂ with nanometer-scale precision over large length scales (up to 20 µm distances) using conductive atomic force microscopy (CAFM). CAFM enables precise identification of defect locations and direct quantification of areal defect density, which ranges from 2.3 x 10¹⁰ cm^-2 to 4.5 x 10¹¹ cm^-2 in our samples. We observe a pronounced inverse relationship between PL intensity and defect density. We develop a model in which observed electronically active defects serve as non-radiative recombination centers, and obtain good agreement with experimental data. Our results provide important information for understanding the cause of spatial variations in WS₂ properties, and are a critical demonstration of a technique for mapping defect density over length scales relevant for observed 2D material behaviors.