Surface Passivation by Excess Sulfur for Controlled Synthesis of Large, Thin SnS Flakes

Abstract

Tin sulfide (SnS) is a part of the group IV chalcogenides (SnX, GeX; X:S, Se), a family of anisotropic layered materials considered for thin-film photovoltaics, optoelectronics, and valleytronics and predicted narrow band gap multiferroic materials. Large ultrathin SnS flakes, suitable for a variety of applications, are challenging to synthesize because of the enhanced surface reactivity due to the open layer structure of SnS, which gives rise to a competition between lateral and vertical growth. Here, we investigate the effects of added sulfur on passivating the surface and modifying the balance between lateral expansion and thickening of SnS flakes in chemical vapor transport from a SnS precursor. We investigate the growth of SnS flakes and compare the results of synthesis from a pure SnS precursor with growth, in which a slight excess of sulfur is supplied in different ways. Our results demonstrate that small amounts of excess sulfur can profoundly affect the size and thickness distributions of SnS flakes. The largest and thinnest flakes are obtained if (i) traces of sulfur are added and (ii) the sulfur source consists of either small S x fragments released at high temperatures from the reactor walls or atomic S supplied by sublimation from SnS 2 . The likely mechanism for the observed growth modifications is a transient surface passivation of SnS flakes during growth, which reduces the reactivity of the top facet of the flakes, limits vertical growth, and thus gives rise to ensembles with increased lateral size and reduced thickness.

Multilayer Lateral Heterostructures of Van Der Waals Crystals with Sharp, Carrier–Transparent Interfaces

“…[9] Large SnS seeds were grown on mica under conditions favoring thick (>25 nm) flakes bounded by long straight {110} side facets (Figure 1b). [11,12] Occasional thinner flakes show rounded shapes, consistent with previous results. [11] Subsequent GeS growth leads to a significant increase in the lateral flake size while maintaining faceted shapes (Figure 1c; Figure S1, Supporting Information), consistent with GeS attachment to the lateral edges of the SnS seeds.…”

Micrometer-scale monolayer SnS growth by physical vapor deposition

“…Indeed, qute recently, the enhancement of lateral growth size was reported by excess sulfur even though the thickness is not monolayer but mutilayer. 47…”

Realization of Large Scale, 2D van der Waals Heterojunction of SnS2/SnS by Reversible Sulfurization

“…Other rotation degree was also observed by electron microscopy. [ 48,49 ] However, our STM observation of the moiré pattern gives a direct evidence for the twisting between SnS 2 and SnS.…”

“…[ 31,50,51 ] The conversion from SnS to SnS 2 in a sulfur‐rich atmosphere has also been demonstrated before. [ 42,47,49 ] The spontaneous heterostructure formation of few‐layer SnS 2 over SnS was also reported. [ 48 ] However, none of these works has demonstrated a reversible transition, although one may get hints from previous works that this is possible.…”