## 5 Transition from Bulk to Nanoscale  
Two-dimensional (2D) materials are crystalline nanomaterials that generally exhibit two macroscopic or mesoscopic dimensions with a third dimension that can be as thin as one atom thick. They typically exhibit drastically different properties compared to their bulk counterparts from which they are derived due to confinement of their electronic states to within the two dimensions. Notable examples include the electric field effect in graphene [78] and the increase in band gap energy and/or nature (e.g., direct, indirect) in a range of layered semiconductors [79; 80; 81]. Due to its orthorhombic layered structure, the weak van der Waals forces between crystalline layers of SnS also allow exfoliation of this material into the 2D limit (Figure 5). An increase in band gap from bulk to a few layers has been found to occur in a wide range of nanomaterials due to quantum confinement [80; 81; 83]. Quantum confinement effects in SnS occur at 14 nm, which is approximately twice the exciton Bohr radius for this material [29; 84]. Ab initio simulations of these effects in the main group chalcogenides and black phosphorus have been reported by [PERSON] et al. using the Heyd-Scuseria-Ernzerhof Vienna ab initio simulation package (HSE VASP). The band gap energy of SnS was calculated to change from 1.20 eV (1033 nm, NIR) for the bulk to 1.60 eV (775 nm, far red visible) for bilayer SnS and was predicted to be as high as 1.96 eV for monolayer SnS (632 nm, red/orange vis), which means that the band gap energies of nanostructured SnS can potentially span a wide range of energies across the Vis-NIR region of the electromagnetic spectrum [1]. Note, however, that DFT simulations tend to underestimate these energy gaps, which can be seen in the discrepancy for the calculated bulk values. A similar band gap value for the monolayer at 2.09 eV was reported by [PERSON] et al. using HSE VASP simulation methods [85]. SnS remains an indirect band gap from bulk to monolayer as opposed to similar monochalcogenides such as GeSe and SnSe, which appear to transform to a direct band gap at the monolayer [81]. Other calculations of band gap from DFT performed by [PERSON] et al. have obtained band gap values of 2.72 eV, 1.57 eV and 1.07 eV for monolayer, bilayer, and bulk, respectively (Table 5) [28]. A change in band gap was  
Figure 5: Bilayer SnS, views along; (**a**) the \(a\)-axis (zigzag direction); (**b**) the \(b\)-axis (zigzag direction); and (**c**) the \(c\)-axis. The orthorhombic unit cell (\(a=3.98\), \(b=4.33\), \(c=11.18\) Å; \(\alpha=\beta=\gamma=90^{\circ}\)) is marked in all three cases [82]. found to be present for both orthorhombic \(\alpha\)- and \(\beta\)-SnS simulated films with (100), (010) and (001) surfaces [86]. A combined experimental and computational study by [PERSON] and [PERSON] used a density functional theory tight binding (DFTB) model to predict a band gap energy between 1.6 and 1.7 eV for bilayer SnS, and compared these values to experimentally purified few-layer and bilayer SnS, the latter being the first example of exfoliation of SnS to the near-2D limit by a top-down process [1]. The density of states (DOS) and optical band gaps predicted by the quantum chemical simulations corresponded well with the experimental data. The study paved the way for many more studies of two dimensional IV-VI semiconductors. In Figure 6, calculations of optimised lattice vectors for monolayer SnS performed by [PERSON] et al. using HSE VASP gave results: \(a\) = 4.44 A, \(b\) = 4.15 A and \(c\) = 11.5 A [80], similar to previous work performed by [PERSON] et al. which showed \(a\) = 4.334 A, \(b\) = 3.987 A and \(c\) = 10.181 A [87]. [PERSON] et al. have showed via DFT methods that monolayer SnS has the space group pmn2\({}_{1}\) with \(a\) = 4.05 A and \(b\) = 4.34 A, similar to experimental values which showed \(a\) and \(b\) to be 3.99 A and 4.34 A, respectively [85]. Experimental analyses of layer thicknesses have found a potential SnS monolayer thickness of 7 A obtained by [PERSON] et al. [88].