CVD Synthesis of Intermediate State-Free, Large-Area and Continuous MoS2 via Single-Step Vapor-Phase Sulfurization of MoO2 Precursor

Abstract

The low evaporation temperature and carcinogen classification of commonly used molybdenum trioxide (MoO₃) precursor render it unsuitable for the safe and practical synthesis of molybdenum disulfide (MoS₂). Furthermore, as evidenced by several experimental findings, the associated reaction constitutes a multistep process prone to the formation of uncontrolled amounts of intermediate MoS_2-yO_y phase mixed with the MoS₂ crystals. Here, molybdenum dioxide (MoO₂), a chemically more stable and safer oxide than MoO₃, was utilized to successfully grow cm-scale continuous films of monolayer MoS₂. A high-resolution optical image stitching approach and Raman line mapping were used to confirm the composition and homogeneity of the material grown across the substrate. A detailed examination of the surface morphology of the continuous film revealed that, as the gas flow rate increased by an order of magnitude, the grain-boundary separation dramatically reduced, implying a transition from a kinetically to thermodynamically controlled growth. Importantly, the single-step vapor-phase sulfurization (VPS) reaction of MoO₂ was shown to suppress intermediate state formations for a wide range of experimental parameters investigated and is completely absent, provided that the global S:Mo loading ratio is set higher than the stoichiometric ratio of 3:1 required by the VPS reaction.