In-plane x-ray diffraction for characterization of monolayer and few-layer transition metal dichalcogenide films.

There is significant interest in the growth of single crystal monolayer and few-layer films of transition metal dichalcogenides (TMD) and other 2D materials for scientific exploration and potential applications in optics, electronics, sensing, catalysis and others. The characterization of these materials is crucial in determining the properties and hence the applications. The ultra-thin nature of 2D layers presents a challenge to the use of x-ray diffraction (XRD) analysis with conventional Bragg-Brentano geometry in analyzing the crystallinity and epitaxial orientation of 2D films. To circumvent this problem, we demonstrate the use of in-plane XRD employing lab scale equipment which uses a standard Cu x-ray tube for the analysis of the crystallinity of TMD monolayer and few-layer films. The applicability of this technique is demonstrated in several examples for WSe and WS films grown by chemical vapor deposition on single crystal substrates. In-plane XRD was used to determine the epitaxial relation of WSe grown on c-plane sapphire and on SiC with an epitaxial graphene interlayer. The evolution of the crystal structure orientation of WS films on sapphire as a function of growth temperature was also examined. Finally, the epitaxial relation of a WS/WSe vertical heterostructure deposited on sapphire substrate was determined. We observed that WSe grows epitaxially on both substrates employed in this work under all conditions studied while WS exhibits various preferred orientations on sapphire substrate which are temperature dependent. In contrast to the sapphire substrate, WS deposited on WSe exhibits only one preferred orientation which may provide a route to better control the orientation and crystal quality of WS. In the case of epitaxial graphene on SiC, no graphene-related peaks were observed in in-plane XRD while its presence was confirmed using Raman spectroscopy. This demonstrates the limitation of the in-plane XRD technique for characterizing low electron density materials.