Matthew Rosenberger1 Hsun-Jen Chuang1 Kathleen McCreary1 Connie Li1 Berend Jonker1

1, U.S. Naval Research Laboratory, Washington, District of Columbia, United States

Transition metal dichalcogenides (TMDs) are an exciting class of 2D materials that exhibit many promising electronic and optoelectronic properties with potential for future device applications. The properties of TMDs are expected to be strongly influenced by a variety of defects which result from growth procedures and/or fabrication. Despite the importance of understanding defect-related phenomena, there remains a need for quantitative nanometer-scale characterization of defects over large areas in order to understand the relationship between defects and observed properties, such as photoluminescence (PL) and electrical conductivity. We report direct observation of defects in monolayer WS2 with nanometer-scale precision over large length scales (up to 20 ┬Ám distances) using conductive atomic force microscopy (CAFM) at room temperature in ambient laboratory conditions. The measurements were made possible by several advances in sample preparation. The observed defects are highly conductive when probed out-of-plane with CAFM, which enables precise identification of defect locations and direct quantification of areal defect density. The defect density ranged from 2.3 x 1010 cm-2 to 4.5 x 1011 cm-2 in our samples. We correlate the measured defect density with spatial variations in PL, and observe a pronounced inverse relationship between PL intensity and defect density. Importantly, we observed the same inverse relationship on multiple different WS2 grains, which indicates that the relationship is generally true. Finally, we propose a model in which the observed electronically active defects serve as non-radiative recombination centers, and obtain good agreement with the experimental data. Performing PL measurements on both PDMS (before mechanical transfer) and on graphite (after mechanical transfer) allowed us to decouple the effect of defect-related non-radiative recombination and substrate-related non-radiative recombination. This decoupling has not been possible in previous studies because the defect density of monolayer TMDs in other studies was unknown. Our results provide important information for understanding the cause of spatial variations in TMD properties, and are a critical demonstration of a technique for mapping defect density over length scales relevant for observed TMD behaviors.