Mapping the Cleanliness of Wafer-Scale 2D Materials

Researchers at the University of Cambridge and the ISIS Facility have pioneered a new method to track and remove microscopic contamination on 2D materials, a breakthrough that could significantly improve the reliability of next-generation electronic devices.

The study, published in Advanced Functional Materials, utilizes Scanning Helium Microscopy (SHeM) to detect ultra-thin layers of “adventitious carbon”—a ubiquitous contaminant that persists even in ultra-high vacuum (UHV) environments.

The Challenge of “Invisible” Contamination

Two-dimensional materials like molybdenum disulfide are prized for their high carrier mobility and atomically thin structures. However, because they are essentially “all surface,” their electronic properties are extremely sensitive to even a single layer of foreign atoms.

“Trace amounts of sub-monolayer contamination impact the performance and reproducibility of 2D materials-based devices,” the researchers noted. Conventional tools often struggle to detect these lightly-bound contaminants across large “wafer-scale” areas without damaging the sample.

A Gentler Way to See

The team turned to SHeM, which uses a beam of low-energy neutral helium atoms to probe surfaces. Unlike high-energy X-rays or electron beams, the helium atoms are completely non-destructive and highly sensitive to surface order.

UHV is Not Enough: The study found that even in ultra-high vacuum, adventitious carbon grows on MoS2​surfaces over 10 to 30 hours, eventually masking the material’s crystalline structure.

Low-Temperature Cleaning: While previous methods required high-temperature annealing, this study demonstrated that these contaminants can be removed by heating to just 350 K.

Structural Susceptibility: Using SHeM’s micrometer-scale resolution, the researchers discovered that flat, highly crystalline regions are actually more susceptible to re-contamination than rougher, delaminated regions.

Paving the Way for Industrial Scale-Up

The ability to map cleanliness at the wafer scale is a major step toward industrializing 2D materials. By establishing SHeM as a powerful tool for characterization, the research offers a pathway to ensure consistent quality in the mass production of 2D-based sensors and electronics.

“This enables quantitative monitoring of surface cleanliness over extended areas,” the authors concluded, “a capability critical for ensuring reproducibility in 2D material devices”.