Direct synthesis of vertically graded Mo1-xWxS2 multilayer alloys by confined-space sulphurisation of magnetron-sputtered MoSx/W precursors.

Scalable, wafer-compatible routes to compositionally graded transition-metal dichalcogenide multilayers remain limited, particularly in the sub-10 nm regime relevant to device stacks. Here, we demonstrate the synthesis of vertically graded Mo1-xWxS2 films by sequential magnetron sputtering of W and MoSx precursor layers, followed by confined-space sulphurisation. By varying the sulphurisation temperature (550, 800, and 850 °C) and time (45 and 60 min), we identify 800 °C for 45 min (10 °C min-1 ramp) as the optimal conditions balancing conversion and morphological stability. X-ray reflectivity and X-ray diffraction reveal continuous layered films with a strong (002) texture for sulphurisation temperatures ≥800 °C. Raman spectroscopy, supported by multipeak deconvolution of the A1g(Γ) and E12g(Γ) modes, indicates Mo-W mixing and not an abrupt MoS2/WS2 bilayer interface. Cross-sectional high-resolution transmission electron microscopy confirms a layered Mo1-xWxS2 multilayer structure under all sulphurisation conditions, with crystallinity highest under the optimised 800 °C and 45 min conditions. Scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy confirms a vertical Mo/W composition gradient. X-ray photoelectron spectroscopy reveals a temperature-dependent alloy composition, with the surface W content increasing progressively with sulphurisation temperature, consistent with thermally driven Mo/W interdiffusion across the vertical composition gradient. These results establish sulphurised MoSx/W precursor stacks as a practical thin-film route to vertically graded Mo1-xWxS2, compatible with large-area physical-vapour processing.