## 8 Applications of Two-Dimensional Nanoscale SnS  
Two-dimensional SnS has several applications; due to the band gap of thin layer SnS, many examples of photonic devices have been created via a variety of methods. Exfoliated nanoscale SnS has been used in the creation of tuneable bandgap photodetectors [106]. Physical vapour deposition of SnS of 42.6 nm thickness was found to exhibit differing responses for the armchair and zigzag directions. A photoresponsivity of 365 A W\({}^{-1}\) was determined with an external quantum efficiency up to 9.92 \(\times\) 10\({}^{-4}\)[152]. SnS films of 15 nm thickness, formed by physical vapour deposition, have been used in photodetectors alongside Bi\({}_{2}\)Te\({}_{3}\), showing a high responsivity (115 A W\({}^{-1}\)) and high quantum efficiency (3.9 \(\times\) 10\({}^{4}\)%) [153]. Devices prepared by [PERSON] et al. from 2D SnS have been found to remain stable under potentially harsh conditions, with the device remaining functional over six months in 0.1M Na\({}_{2}\)SO\({}_{4}\) along with other conditions, including an acidic environment and with neutral electrolytes [106]. The devices consisted of cascade centrifuged SnS nanosheets on ITO-coated glass as the working electrode, platinum as the counter electrode, and a saturated calomel electrode as a reference electrode submerged in an electrolyte solution. Nanosheets separated between 2000-4000 rpm, 4000-6000 rpm, and 6000-8000 rpm (with corresponding band gaps of 1.32 eV, 1.59 eV, and 1.71 eV, respectively) were used in the electrodes, with aqueous H\({}_{2}\)SO\({}_{4}\), NaOH and Na\({}_{2}\)SO\({}_{4}\) used as the electrolytes. A maximum detectivity of 1.92 \(\times\) 10\({}^{8}\) Jones for 0.5 M Na\({}_{2}\)SO\({}_{4}\) with SnS centrifuged between 4000-6000 rpm was obtained. A rise and recovery time of 0.1 s and 0.3 s, respectively, was obtained for this device with a photoresponse of 1590 nA cm\({}^{-2}\). The high photoresponse of 4000-6000 rpm centrifuged nanosheets was due to the favourable optical absorption properties alongside the number of active sites present. The photoresponsivity was higher for the lower wavelength of 365 nm. The detectivities and responsivities are higher for the Na\({}_{2}\)SO\({}_{4}\) electrolyte. The SnS was found to be more stable under acidic than basic conditions. DFT calculations suggested that this was due to the formation of Sn-O bonds, which leads to the lengthening of the Sn-S bonds, making them weaker. The nanosheet devices were found to have self-powered photoresponses, with voltage improving the current response [106]. Vertically grown 2D SnS has also been formed into a composite SnS/perovskite complex, providing a broadband response. The pure SnS device gave a 0.05 uA photocurrent at 365 nm and 4 mW cm\({}^{-2}\); with addition of the CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\) perovskite, the photocurrent increased to 7.1 uA. The rise and fall times for the composite device were 9 ms and 42 ms, respectively. It was determined that there was efficient charge transfer between the SnS and perovskite, as confirmed via time-correlated single photon counting [154]. Figure 13: (**a**) Atomic force microscopy (AFM) image of synthesised SnS monolayer; (**b**) AFM profile of SnS nanosheet; and (**c**) Tauc plot of SnS produced. Adapted with permission from _Nat. Commun._**2020**, \(1\), 3449. Other examples of 2D SnS-based photodetectors have been created by colloidal synthesis; the devices produced by this method by [PERSON] et al. were found to have a detectivity of 4 \(\times\) 10\({}^{9}\) Jones, with a spectral response of 3 \(\times\) 10\({}^{3}\) AW\({}^{-1}\) and a sensitivity of 0.95 as calculated by (I\({}_{\rm ill}\)-I\({}_{\rm dark}\))/I\({}_{\rm dark}\). Fast transitions between light and dark were also observed. The nanosheets tested were between 24 nm and 29 nm thickness with lateral sizes between 150 nm and 500 nm [122]. SnS nanosheet film-based photodetectors of 15 nm thickness with Bi\({}_{2}\)Te\({}_{3}\) electrodes prepared by [PERSON] et al.