## 7 Synthesis of Two-Dimensional SnS
The highly centrifuged sheets had a lateral size of the order of tens of nm with a thickness of 2.7 \(\pm\) 1.7 nm (Figure 9) [67]. The nanosheets obtained had direct band gaps of 1.25 eV and 1.86 eV, respectively, as determined by UV-Vis spectroscopy, which is striking because it meant that the addition of lead changes the indirect band gap of 2D SnS to a direct band gap in (Pb,Sn)S. DFT calculations showed that this was a consequence of strain on the SnS lattice introduced by the Pb ions that altered the electronic band structure of the material. The latter is a significant result, because it showed that by simple alloying of an isovalent element an indirect to direct transition could be garnered in SnS, which is an important feature for  
Figure 8: (**a**) Histograms of nanosheet thickness for nanosheets centrifuged at 180\(\times\)\(g\) (black, 7.8 nm, ca. 10 layers average) and 11,620\(\times\)\(g\) (red, 4.1 nm, ca. 2 layers average). (**b**) Calculated band gaps of SnS (from DFTB) at different layer thicknesses obtained compared with experimentally determined band gaps 180\(\times\)\(g\) (SnS sol A) and 11,620\(\times\)\(g\) (SnS sol B). Adapted with permission from _J. Am. Chem. Soc._**2015**, 39, 12689–12696. a material to be used in thin film optoelectronic devices; it typically increases absorption of light because there is no requirement for an accompanying change in crystal momentum. Traditionally, in materials such as transition metal dichalcogenides, this indirect to direct transition had to be elicited by thinning the material to a true monolayer, which is extremely low-yielding for 2D materials produced by liquid-phase exfoliation, and too time-consuming to be produced in bulk from micromechanical exfoliation. The approach has yet to be reported in other IV-VI semiconductors. Alternative solvents have been utilised for the exfoliation of SnS nanosheets. [PERSON] et al. have successfully exfoliated SnS nanosheets in isopropanol, which has the advantage of a low boiling point, hence being easier to remove. The nanosheets obtained were of an irregular size and shape, with lateral size of 50 nm and a thickness of 6 nm. The nanosheets were found to be stable in solution over a period of 30 days, as determined by UV-Vis spectroscopy [107]. [PERSON] et al. have reported the use of acetone to successfully exfoliate SnS. The SnS was bath-soniated for 20 h at 100 W and 40 kHz under a nitrogen environment followed by nanosheet separation via 15 min of centrifugation at 8000 rpm. The nanosheets obtained had an average thickness of 1.1 nm, with all nanosheets being between 0.8 and 1.5 nm thick, indicating that all sheets were only several monolayers in thickness, with the 0.8 nm sheet being close to SnS monolayer thickness (0.7 nm) [108]; selected-area electron diffraction (SAED) patterns confirmed that the nanosheets were single crystalline at the clear rhombus-like lattice fringes. The A\({}_{g}\) Raman mode was found to shift by 3.46 cm\({}^{-1}\), which was attributed to the evolution of coupling between electronic transitions and photons [89]. Further work performed by [PERSON] et al. suggests that other solvents that can disperse SnS include dimethyl formamide (DMF), ethanol, dimethyl sulfoxide (DMSO), and methanol; as determined by UV-Vis and Raman, the size of the nanosheets produced and further characterisation was not obtained [109]. The effectiveness of nanomaterial exfoliation and dispersal is dependent on the Hansen solubility parameters: \(\delta_{d}\), \(\delta_{p}\) and \(\delta_{h}\), which correspond to the dispersion forces, the polar forces, and the hydrogen bonding forces between molecules, respectively. From these parameters, it can be determined whether a material will dissolve or disperse.