Controlled preferential oxidation of grain boundaries in monolayer tungsten disulfide for direct optical imaging.

Synthetic 2D crystal films growth by chemical vapour deposition are typically polycrystalline and determining grain size within domains and continuous films is crucial for determining their structure. Here we show that grain boundaries in the 2D transition metal dichalcogenide WS2, grown by CVD can be preferentially oxidized by controlled heating in air. Under our developed conditions, the bulk WS2 remains relatively unaffected, whilst preferential degradation at the grain boundaries causes an increase in their physical size due to oxidation. This increase in size enables their clear and rapid identification using a standard optical microscope. We demonstrate that similar treatments in Ar environment do no show this effect, confirming that oxidation is the main role in the structural change. Statistical analysis of grain boundary angles shows dominant mirror formation. Electrical biasing across the GB is shown to also lead changes at the GB and their observation under an optical microscope. Our approach enables high-throughput screening of as-synthesized WS2 domains and continuous films to determine their crystallinity and should enable improvements in future CVD growth of these materials