Tracing Sub‐Monolayer Contamination on Wafer‐Scale 2D Materials

ABSTRACT Trace amounts of sub‐monolayer contamination impact the performance and reproducibility of 2D materials‐based devices. Conventional characterization techniques have limited capability in detecting lightly‐bound sub‐monolayer contamination on wafer‐scale. Scanning helium microscopy (SHeM), a non‐destructive ultra‐sensitive neutral‐atom imaging and diffraction approach, is used to investigate surface cleanliness of monolayer and bulk MoS 2 . This study maps the growth and removal of adventitious carbon, a ubiquitous contaminant, which is found to grow on MoS 2 even in ultra‐high vacuum (UHV). This contamination can be removed by heating to , though it reappears after cooling to room temperature for a few hours. By quantifying the rate of the contamination process across different regions with micrometer lateral resolution, it is found that flat regions are more susceptible to adventitious carbon contamination than delaminated regions. Furthermore, thermally activated desorption measurements reveal an activation energy of for adventitious carbon on MoS 2 . These findings highlight that surface cleanliness of 2D materials cannot be guaranteed even under UHV, and establish SHeM as a powerful tool for microscopic 2D material cleanliness characterization. The approach offers a new route to wafer‐scale imaging of 2D material quality, with implications for improving device reproducibility and enabling clean industrial fabrication.