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The controlled synthesis of high-quality multilayer (ML) MoS flakes with gradually shrinking basal planes by chemical vapor deposition (CVD) is demonstrated. These CVD-grown ML MoS flakes exhibit much higher mobility and current density than mechanically exfoliated ML flakes due to the reduced contact resistance which mainly resulted from direct contact between the lower MoS layers and electrodes.

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Two‐dimensional (2D) PtSe2 shows the most prominent layer‐dependent electrical properties among various 2D materials and high catalytic activity for hydrogen evolution reaction (HER), and therefore, it is an ideal material for exploring the structure–activity correlations in 2D systems. Here, starting with the synthesis of single‐crystalline 2D PtSe2 with a controlled number of layers and probing the HER catalytic activity of individual flakes in micro electrochemical cells, we investigated the layer‐dependent HER catalytic activity of 2D PtSe2 from both theoretical and experimental perspectives. We clearly demonstrated how the number of layers affects the number of active sites, the electronic structures, and electrical properties of 2D PtSe2 flakes and thus alters their catalytic performance for HER. Our results also highlight the importance of efficient electron transfer in achieving optimum activity for ultrathin electrocatalysts. Our studies greatly enrich our understanding of the structure–activity correlations for 2D catalysts and provide new insight for the design and synthesis of ultrathin catalysts with high activity.

Developing controlled approaches for synthesizing high-quality two-dimensional (2D) semiconductors is essential for their practical applications in novel electronics. The application of chemical vapor transport (CVT), an old single-crystal growth technique, has been extended from growing 3D crystals to synthesizing 2D atomic layers by tuning the growth kinetics. Both single crystalline individual flakes and continuous films of 1 L MoS were successfully obtained with CVT approach at low growth temperatures of 300-600 °C. The obtained 1 L MoS exhibits high crystallinity and comparable mobility to mechanically exfoliated samples, as confirmed by both atomic resolution microscopic imaging and electrical transport measurements. Besides MoS , this method was also used in the growth of 2D WS , MoSe , Mo W S alloys, and ReS , thus opening up a new way for the controlled synthesis of various 2D semiconductors.