Influence of chemical potential on shape evolution of 2D-MoS2 flakes produced by chemical vapor deposition

High-quality, ultrathin 2D-MoS2 layers with large area were grown on SiO2/Si substrates by using atmospheric pressure chemical vapor deposition (APCVD) at elevated temperatures. The growth precursors (MoO3 and S) were placed separately inside the double-zone furnace to control the growth parameters individually for better flexibility in the growth process. In this study, it was found that the shape and edge structure of the evolved MoS2 flakes were significantly influenced by the chemical potential of the Mo and S precursor concentration. In keeping with the concentration gradient of the Mo precursor (MoO3) on the substrate surface, the shape of MoS2 flakes changed from hexagonal to truncated triangle and then to triangular shapes, owing to the Mo-rich to S-rich conditions. The surface roughness and thickness of the differently shaped MoS2 flakes were studied by using atomic force microscope (AFM). Additionally, Raman and photoluminescence (PL) techniques were employed to characterize the crystalline quality, number of grown layers and optical performance of the as-grown MoS2 layers. Auger electron spectroscopy (AES) analysis and scanning electron microscopy (SEM) confirmed that the equilibrium crystal shape of the MoS2 was hexagonal under Mo-rich conditions. However, the shape of the MoS2 crystal changed to a triangle under S-rich conditions. Furthermore, the influence of chemical potential on the edge structure of the monolayer MoS2 and its effect on the equilibrium shape of the crystal were studied.