## 8 Applications of Two-Dimensional Nanoscale SnS
had a response of 115 A W\({}^{-1}\), a detectivity of 4.1 \(\times\) 10\({}^{11}\) Jones, a quantum efficiency of 3.9 \(\times\) 10\({}^{4}\)% with a band gap of 1.13 eV, and a rise and fall time of around 1 s and 2 s, respectively, was also obtained. The device was prepared on a polyethylene terephthalate substrate and was found to retain photoresponsivity over 100 bending cycles, indicating that SnS may be useful in flexible photodetectors and wearable devices [153]. [PERSON] et al. have successfully produced a SnS-based four-terminal device that can function along the zigzag and armchair planes of a SnS nanosheet. An 808 nm laser was used as the illumination source. It was found that the zigzag device had a faster rise and fall time (0.2 s, 0.2 s) compared to the armchair configuration (0.4 s, 0.4 s). Both the dark and light currents were higher for zigzag compared to armchair planes: 31.8 uA vs. 19.3 uA, respectively, for dark, and 54.2 uA vs. 36.4 uA, respectively, for light. The photoconductivity ratio (\(\sigma_{\rm zigzag}\)/\(\sigma_{\rm armchair}\)) was 1.49. The differences are due to differences in the effective masses of carriers along the two directions. A similar, two-terminal device was found to exhibit a reduction in the photoconductivity ratio after oxygen plasma treatment to introduce defects. After 300 s of treatment, the anisotropic response was almost completely removed. Decoration with Au nanoparticles was found to increase photoresponsivity from 365 A W\({}^{-1}\) to 635 A W\({}^{-1}\) and quantum efficiency from 5.7 \(\times\) 10\({}^{4}\)% to 9.92 \(\times\) 10\({}^{4}\)% [152]. Photodetectors prepared by [PERSON] et al. via a physical vapour deposition method have been determined to have a responsivity of 1.6 \(\times\) 10\({}^{6}\) mA W\({}^{-1}\), an external quantum efficiency of 2.34 \(\times\) 10\({}^{5}\)%, a detectivity of 3.42 \(\times\) 10\({}^{11}\) Jones, and rise and fall times of 7.6 ms and 29.9 ms, respectively. The responsivity was significantly higher than other SnS-based devices [130]. [PERSON] et al. have prepared a SnS nanoribbon-based photodetector device via a polyol reflux method. The nanoribbons had thicknesses of 10-20 nm. A photocurrent density of 87 uA cm\({}^{-2}\) was obtained under 1 sun illumination at 100 mW cm\({}^{-2}\), alongside a rise and fall time of 0.1 s. Flexible photodetectors were successfully doctor-bladed onto polyethylene terephthalate (PET) substrates, allowing the creation of flexible SnS devices. The nanoribbons were found to have a higher photocurrent than nanoparticles prepared for a similar device [18]. As well as bulk SnS, 2D SnS has also been used in the formation of solar cells. [PERSON] et al. have grown vertically aligned SnS nanosheets on clean glass substrates as well as those coated with Ag and Au buffer layers. Sheet thicknesses of 80 nm, 37 nm and 51 nm were found for the buffer free glass, Ag, and Au layers, respectively. A flat layer of SnS was formed on the Ag and Au buffer layers before the growth of the vertically aligned nanosheets. The growth method was believed to occur via the vapour liquid solid (VLS) route, where nanosheets have a droplet of metal or alloy which controls the diameter, growth, and orientation, with the droplets acting as active sites for growth. Absorption coefficients determined by UV-Vis were 37%, 90%, and 70%, for free coated SnS, Ag, and Au buffer layers, respectively, with corresponding band gaps of 1.5-1.6 eV, \(\approx\)1.4 eV, and \(\approx\)1.7 eV, respectively. The figures of merit for the Ag-coated device were: V\({}_{\rm OC}\): 0.36 V, J\({}_{\rm SC}\): 5.6 mA cm\({}^{-2}\), fill factor: 0.48, PCE: 4.83%, R\({}_{\rm ss}\): 19 \(\Omega\) cm\({}^{-2}\), R\({}_{\rm sh}\); 302 \(\Omega\) cm\({}^{-2}\); the efficiency was 0.97%, significantly higher than free-coated SnS (0.29%) (Figure 14) [155]. Two-dimensional SnS has also been successfully used as a counter electrode to improve efficiency in I\({}_{3}\)-/I\({}^{-}\) dye sensitised solar cells. [PERSON] et al.