  In the last years, transition metal dichalcogenides (TMDs), especially at the
two-dimensional (2D) limit, gained a large interest due to their unique optical
and electronic properties. Among them, MoS2 received great attention from the
scientific community due to its versatility, workability, and applicability in
a large number of fields such as electronics, optoelectronics and
electrocatalysis. To open the possibility of 2D-MoS2 exploitation, its
synthesis over large macroscopic areas using cost-effective methods is
fundamental. In this study, we report a method for the synthesis of large-area
(~ cm2) few-layers MoS2 via liquid precursor CVD (L-CVD), where the Mo
precursor (i.e. ammonium heptamolybdate AHM) is provided via a solution that is
spin-coated over the substrate. Given the capability of organic and inorganic
molecules, such as alkaline salts, to enhance MoS2 growth, we investigated the
action of different inorganic salts as seed promoters. In particular, by using
visible Raman spectroscopy, we focused on the effect of Na(OH), KCl, KI, and
Li(OH) on the thickness, morphology, uniformity and degree of coverage of the
grown MoS2. We optimized the process tuning parameters such as the volume of
spin-coated solution, the growth temperature, and the seed promoter
concentration, to synthesise the lowest possible thickness which resulted to be
2 layers (2L) of the highest quality. We witnessed that the addition of an
inorganic seed promoter in the solution improves the extension of the grown
MoS2 promoting lateral growth front, and therefore the degree of coverage. From
this study, we conclude that, amongst the investigated seed promoters, K-based
salts proved to grant the growth of high-quality two-layer MoS2 with optimal
and uniform coverage of the SiO2/Si substrate surface.
