Luminescence of defects in the structural transformation of layered tin dichalcogenides

Layered tin sulfide semiconductors are both of fundamental interest and attractive for energy conversion applications. Sn sulfides crystallize in several stable bulk phases with different Sn:S ratios (SnS₂, Sn₂S₃, and SnS), which can transform into phases with a lower sulfur concentration by introduction of sulfur vacancies (V_S). How this complex behavior affects the optoelectronic properties remains largely unknown but is of key importance for understanding light-matter interactions in this family of layered materials. Here, we use the capability to induce V_S and drive a transformation between few-layer SnS₂ and SnS by electron beam irradiation, combined with in-situ cathodoluminescence spectroscopy and ab-initio calculations to probe the role of defects in the luminescence of these materials. In addition to the characteristic band-edge emission of the endpoint structures, our results show emerging luminescence features accompanying the SnS₂ to SnS transformation. Comparison with calculations indicates that the most prominent emission in SnS₂ with sulfur vacancies is not due to luminescence from a defect level but involves recombination of excitons bound to neutral V_S in SnS₂. These findings provide insight into the intrinsic and defect-related optoelectronic properties of Sn chalcogenide semiconductors.