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

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.

Automated summary

The distinct optical, catalytic, and electronic properties of few-layer MoS2 films can be attributed to the specific orientation of the MoS2 layers, which is controlled by physical conditions during sulfurization of predeposited Mo film. A comprehensive study using in situ grazing-incidence wide-angle X-ray scattering revealed that the gas flow and sulfur partial vapor pressure are the key parameters in determining the orientation of few-layer MoS2 during growth. Real-time tracking of the crystallization of few-layer MoS2 layers allowed for estimation of the activation energy required for both horizontal and vertical orientations.