Modulation of the Reaction Mechanism via S/Mo: A Rational Strategy for Large-Area MoS2 Growth

Tremendous effort has been devoted to the growth of large-area two-dimensional (2D) transition-metal dichalcogenide (TMD) such as MoS2 via chemical vapor deposition (CVD). However, few studies have quantitatively and systematically investigated the effect of the reaction mechanism, that is, homogeneous reaction (in vapor phase) or heterogeneous reaction (on the substrate) in MoS2 crystal growth. Herein, we present a new strategy for regulating the reaction mechanism of CVD-grown 2D MoS2 crystals based on modulating the sulfur (S) and molybdenum (Mo) precursor concentration ratio (S/Mo). By systematically studying crystal density and substrate coverage as a function of the precisely controlled S/Mo, it was found that there is an optimal value of S/Mo (R op), above which the homogeneous reaction becomes dominant. Experimental results show that a relatively low S/Mo favors the heterogeneous (as opposed to homogeneous) reaction, resulting in sparse, large-area, and smooth MoS2 crystals, whereas a relatively high S/Mo favors the homogeneous reaction, resulting in dense, small-area, and rough-surfaced MoS2 crystals. In other words, to achieve high-quality MoS2 crystal growth via CVD, it is advantageous to promote the heterogeneous reaction by maintaining S/Mo at its optimal value. This new observation is demonstrated by incorporating growth temperature optimization, consistently achieving MoS2 crystals with lateral dimensions above 500 μm, which is among the largest ever grown. Findings from this work provide important insights into the rational process design for 2D MoS2 growth and would also be an instrumental reference toward the growth of other high-quality TMD materials.