Stepwise Sulfurization from MoO3 to MoS2 via Chemical Vapor Deposition

Chemical vapor deposition (CVD) is used widely to synthesize monolayer and few-layer transition metal dichalcogenide molybdenum disulfide (MoS2), a two-dimensional (2D) material with various applications in nanoelectronics, catalysis, and optoelectronics. However, the CVD synthesis of 2D MoS2 is highly sensitive to small changes in growth parameters and the growth mechanism has not been extensively studied. This work systematically investigates the effect of sulfur concentration on CVD synthesis of MoS2 using molybdenum trioxide (MoO3) and sulfur precursors. We find that with increasing concentration of sulfur vapor, intermediate products of molybdenum dioxide (MoO2) and molybdenum oxysulfide (MoOS2) can form during a stepwise sulfurization of MoO3 to the final product of MoS2. The intermediate MoOS2, formed due to sulfur vapor deficiency, can be fully converted to MoS2 with further sulfurization. We show that the local sulfur to molybdenum vapor ratio at the growth substrate critically determines the growth products. This study thus highlights the importance of keeping the molar ratio of sulfur to molybdenum vapor well in excess of the stoichiometrically required ratio of 3.5:1 in order to grow 2D MoS2.