## 7 Synthesis of Two-Dimensional SnS
The formation mechanism involved the initial evaporation of SnO\({}_{2}\) at 400 \({}^{\circ}\)C and the formation of SnO\({}_{2}\) clusters, which acted as SnS nucleation sites. At lower temperatures, the following reaction occurs: SnO\({}_{2}\) (s) + 1.5 S\({}_{2}\) (g) \(\rightarrow\) SnS\({}_{2}\) (s) + SO\({}_{2}\) (g). At higher temperatures, the reaction: SnO\({}_{2}\) (s) + S\({}_{2}\) (g) \(\rightarrow\) SnS (s) + SO\({}_{2}\) (g) occurs. SnS was formed at 550 \({}^{\circ}\)C with SnS\({}_{2}\) formed at 425 \({}^{\circ}\)C. The SnS produced was square faceted and horizontally aligned, with lateral sizes of over tens of \(\upmu\)m with thicknesses of around 30 layers [126]. [PERSON] et al. have developed a method of chemical vapour deposition of nanoscale SnS films at a range of temperatures from 300 \({}^{\circ}\)C to 545 \({}^{\circ}\)C. The films were formed from a mixture of H\({}_{2}\)S and SnCl\({}_{4}\) introduced into a nitrogen gas stream for 30 s and 3 min of deposition. A platelet structure was formed at 475 \({}^{\circ}\)C, with ovoid, needle-like and agglomerated structures at 300 \({}^{\circ}\)C, 525 \({}^{\circ}\)C, and 545 \({}^{\circ}\)C, respectively. The platelets formed had lateral dimensions of 800 nm by 800 nm with 25 nm thickness. At higher temperatures, SnS is produced as opposed to SnS\({}_{2}\). The films obtained at 300 \({}^{\circ}\)C at 3 min deposition had greater dimensions (1000 nm \(\times\) 500 nm) compared to 30 s deposition (500 nm \(\times\) 100 nm) [48]. Physical vapour deposition was performed by [PERSON] et al. via the use of commercial and high purity SnS powders and nitrogen carrier gas onto freshly cleaved mica. The deposition temperatures were 410 \({}^{\circ}\)C, 440 \({}^{\circ}\)C, and 470 \({}^{\circ}\)C at 10 Pa pressure. The commercial SnS powder successfully formed a SnS monolayer with a  
Figure 12: Schematic of the formulation and shape evolution of SnS nanostructures under differing reaction conditions LS, lateral size; TN, nanosheet thickness; TOP = trioctyl phosphine. Adapted with permission from _J. Mater. Chem. C_, **2018**, 35, 9410–9419. thickness of 0.8 nm and a lateral size of \(\upmu\)m scale. Submicron grains of Sn\({}_{2}\)S\({}_{3}\) were observed for SnS produced from the commercial powder. The lateral growth of the SnS monolayer from impure SnS was believed to be facilitated by the enhanced surface distribution of SnS precursor molecules. The mechanism is believed to be due to evaporation of the S in Sn\({}_{2}\)S\({}_{3}\) before SnS, facilitating the surface diffusion of the SnS precursor molecules [127]. [PERSON] et al. have also enhanced the lateral size of chemical vapour transport-deposited nanosheets via the addition of sulfur to high purity SnS, although no monolayer was formed. Without sulfur pre-exposure the thicknesses varied, with values up to 400 nm and an average lateral size of 3.5 \(\upmu\)m. With sulfur pre-exposure, the lateral size reached up to 7.9 \(\upmu\)m, with 30% of nanosheets having thicknesses under 50 nm. Thinner nanosheets (under 20 nm) have circular morphologies which transitioned to a square morphology as the thickness increased. The change in the proposed growth mechanism was that the additional sulfur leads to a transient surface passivation of SnS flakes during growth, reducing reactivity of the top facet and limiting vertical growth relative to the horizontal [128]. [PERSON] et al. have also prepared SnS nanosheets via a physical vapour deposition method. The SnS was grown on mica substrates under 20-300 Torr pressure in high purity argon with the heating zone set to 600-800 \({}^{\circ}\)C. The SnS produced had lateral sizes of several \(\upmu\)m with thicknesses ranging from 5.5 nm to 14.6 nm. The nanosheets produced were square, with the thinner nanosheets possessing comparatively rough edges and surfaces. An increase in temperature and pressure was found to increase the nanosheet dimensions up to 40 \(\upmu\)m lateral size with thicknesses up to 230 nm [129].