Design and characterization of novel polymorphs of single-layered tin-sulfide for direction-dependent thermoelectric applications using first-principles approaches

Advanced computational approaches have made the design and characterization of novel two-dimensional (2D) materials possible for applications in cutting-edge technologies. In this work, we designed five polymorphs of 2D tin sulfide (namely, α-SnS, β-SnS, γ-SnS, δ-SnS, and ε-SnS) and explored their potential for thermoelectric applications using density functional theory-based computational approaches. Investigations of the energetic stability showed that the generated monolayers were as stable as parent α-SnS and exhibited cohesive and formation energies comparable to those of other stable 2D materials. These monolayers demonstrated high structural anisotropy (except β-SnS), which resulted in interesting features in the effective mass of the charge carriers and the subsequent thermoelectric properties. The in-plane anisotropy yielded different effective masses of charge carriers along the 100- and 010-directions. The x- and y-components of the electrical conductivity tensors were accordingly enhanced by the p-type doping and n-type doping, respectively. We estimated the maximum thermoelectric power factors along the x- and y-axes and the corresponding optimal doping levels were recognized; this suggested that the thermoelectric performance of these monolayers along the x-direction can be improved by p-type doping and that along the y-direction can be improved by n-type doping. Moreover, the thermoelectric figures of merit of the SnS monolayers approached a benchmark value of unity at room temperature. Our results suggested that these novel polymorphs of 2D SnS are promising materials for applications in direction-dependent thermoelectric devices. The present study can provide valuable guidance for generating low-cost and non-toxic polymorphs of other layered-structure materials.