Real time optical observation and control of atomically thin transition metal dichalcogenide synthesis

Understanding the mechanisms involved in chemical vapour deposition (CVD) synthesis of atomically thin transition metal dichalcogenides (TMDCs) requires precise control of numerous growth parameters. All the proposed mechanisms and their relationship with the growth conditions are inferred from characterising intermediate formations obtained by stopping the growth blindly. To fully understand the reaction routes that lead to the monolayer formation, real time observation and control of the growth are needed. Here, we demonstrate how a custom-made CVD chamber that allows real time optical monitoring can be employed to study the reaction routes that are critical to the production of the desired layered thin crystals in salt assisted TMDC synthesis. Our real time observations reveal the reaction between the salt and the metallic precursor to form intermediate compounds which lead to the layered crystal formation. We identified that both the vapour-solid-solid and vapour-liquid-solid growth routes are in an interplay. Furthermore, we demonstrate the role H2 plays in the salt-assisted WSe2 synthesis. Finally, we observed the synthesis of the MoSe2/WSe2 heterostructures optically, and elucidated the conditions required for both lateral and vertical heterostructure syntheses.