Atmospheric-pressure chemical vapor deposition (CVD) is used to grow monolayer MoS 2 two-dimensional crystals at elevated temperatures on silicon substrates with a 300 nm oxide layer. Our CVD reaction is hydrogen free, with the sulfur precursor placed in a furnace separate from the MoO 3 precursor to individually control their heating profiles and provide greater flexibility in the growth recipe. We intentionally establish a sharp gradient of MoO 3 precursor concentration on the growth substrate to explore its sensitivity to the resultant MoS 2 domain growth within a relatively uniform temperature range. We find that the shape of MoS 2 domains is highly dependent upon the spatial location on the silicon substrate, with variation from triangular to hexagonal geometries. The shape change of domains is attributed to local changes in the Mo:S ratio of precursors (1:>2, 1:2, and 1:<2) and its influence on the kinetic growth dynamics of edges. These results improve our understanding of the factors that influence the growth of MoS 2 domains and their shape evolution.

Synthetic 2D crystal films grown by chemical vapor 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, 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 an Ar environment do no show this effect, confirming that oxidation is the main role in the structural change. Statistical analysis of grain boundary (GB) angles shows dominant mirror formation. Electrical biasing across the GB is shown to lead to 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.

Additive manufacturing has provided a pathway for inexpensive

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Growing monolayer MoS2 films that are continuous with large domain sizes by chemical vapor deposition is one of the major challenges in 2D materials research at the moment. Here, we explore how atmospheric pressure CVD can be used to grow centimeter scale continuous films of monolayer MoS2 films directly on Si substrates with an oxide layer whilst also obtaining large domain sizes exceeding 20 μm within the films. This is achieved by orientating the growth substrate in a vertical position to improve the uniformity of precursor feed-stock compared to horizontally orientated growth substrates. This leads to continuous films of monolayer MoS2 over a significantly larger area without the need for low-pressure vacuum systems or volatile precursors. This provides important insights into novel approaches for maximizing domain sizes within MoS2 films, with concomitant large area uniform coverage.