  Recently, monolayer SnS, a two-dimensional group IV monochalcogenide, was
grown on a mica substrate at the micrometer-size scale by the simple physical
vapor deposition (PVD), resulting in the successful demonstration of its
in-plane room temperature ferroelectricity. However, the reason behind the
monolayer growth remains unclear because it had been considered that the SnS
growth inevitably results in a multilayer thickness due to the strong
interlayer interaction arising from lone pair electrons. Here, we investigate
the PVD growth of monolayer SnS from two different feed powders, highly
purified SnS and commercial phase-impure SnS. Contrary to expectations, it is
suggested that the mica substrate surface is modified by sulfur evaporated from
the Sn2S3 contaminant in the as-purchased powder and the lateral growth of
monolayer SnS is facilitated due to the enhanced surface diffusion of SnS
precursor molecules, unlike the growth from the highly purified powder. This
insight provides a guide to identify further controllable growth conditions.
