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

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 Sx fragments released at high temperatures from the reactor walls or atomic S supplied by sublimation from SnS2. 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.