Orientation of Few-Layer MoS₂ Films: In-Situ X-ray Scattering Study During Sulfurization

Abstract

Some of the distinct optical, catalytical, and electronic properties of few-layer MoS2 films arise from a specific orientation of the MoS2 layers. The growth of horizontally or vertically aligned MoS2 during the sulfurization of predeposited Mo film can be controlled by various physical conditions such as temperature, heating rate, Mo film thickness, or sulfur vapor pressure. However, due to the inherent limitations of performing real-time and in situ experiments during sulfurization in a standard growth chamber, only a limited number of analytical techniques can be used to elucidate the in-process phase transformation. Here, we present a comprehensive real-time study of the growth of few-layer MoS2 films by sulfurization of Mo films using in situ grazing-incidence wide-angle X-ray scattering. We demonstrate that the process gas flow, and thus the sulfur partial vapor pressure, is the key control parameter for the few-layer MoS2 layer orientation while all other process parameters remain fixed. Tracking the crystallization of few-layer MoS2 layers in real-time allowed us to estimate the activation energy required for both horizontal and vertical orientations. Growth of either horizontal or vertical MoS2 was observed without a metastable transition between them throughout the sulfurization.

Lithium-Induced Reorientation of Few-Layer MoS2 Films

“…Higher sulfur vapor pressure during sulfurization leads to the formation of vertical MoS 2 layers, while slower sulfur evaporation results in horizontally aligned layers even in the case of a thicker initial Mo layer. This finding was confirmed by an in situ X-ray scattering study …”

“…This finding was confirmed by an in situ X-ray scattering study. 28 One of the unique properties of MoS 2 (and similar layered materials) is the ability to intercalate guest species into their van der Waals gaps. Intercalation may change the electronic structure and optical and electrical properties directly by electron doping or indirectly by inducing a phase transition or structural and compositional disorder.…”

Modulating the Growth of Epitaxial MoS2 on Au(111) Surfaces via an Ultra-High-Vacuum-Interconnected Apparatus

“…The competition among growth nuclei ultimately facilitates an increase in the nucleation density on the substrate surface, thereby decreasing the coverage of the MoS 2 domains. As for the growth orientation, other researchers have demonstrated that the multioriented growth of MoS 2 domains is suppressed under a deficient Mo precursor content condition because of the higher S/Mo ratio, which helps maintain an elevated sulfur concentration, suppresses the formation of alternative species, enhances the thermodynamic stability of MoS 2 , and ultimately reduces the generation of MoS 2 with different orientations. − Furthermore, appropriately increasing the S/Mo ratio improves the morphological regularity and reduces the distribution at multiorientation on the substrate.…”