Study of Structural, Thermoelectric, and Photoelectric Properties of Layered Tin Monochalcogenides SnX (X = S, Se) for Energy Application

Abstract

SnS and SnSe are renowned energy materials that are applied for photoelectric and thermoelectric conversions owing to their suitable band gap, close to 1 eV, and superior figure of merit (ZT), larger than 0.1. In this paper, high-quality layered SnX (X = S, Se) crystals have been successfully grown by the chemical vapor transport (CVT) method. The crystal structure and band structure of SnX are studied, and their photoelectric and thermoelectric properties are characterized. In Raman measurement, four vibration modes with distinct angle-polarization dependence are simultaneously detected by both SnS and SnSe, verifying their similar orthorhombic layered structure with in-plane anisotropy. In-plane anisotropy of band-edge and interband transitions along a and b axes has also been measured experimentally using polarized thermoreflectance (PTR) from 0.7 to 5 eV. The anisotropic band edges of layered SnX (X = S, Se) are well matched and reproduced by first-principles calculation. Hall-effect and thermoelectric measurements revealed that SnX are p-type semiconductors with a high carrier density, larger than 1017 cm–3. According to the measurement results of the surface photovoltaic (SPV) response and ZT value, layered SnS can have a superior SPV (8.5 μV/μW) response ∼12× higher than that of SnSe, while SnSe has a ZT of 0.16, ∼4× larger than that of SnS in SnX (X = S, Se). Layered SnSe and SnS could possess great feasibility for application in thermoelectric power generation and solar energy conversion.