Electronic gap stability of two-dimensional tin monosulfide phases: Towards optimal structures for electronic device applications

Abstract

It is well known that tin monosulfide is a potential environment-friendly solar cell material that maximizes light-energy conversion efficiency, given its band gap at the peak of the solar emission spectrum. However, the presence of Sn and S vacancies at the surface of SnS hinders the performance of these solar cells. In the present work, we have prepared tin sulfide platelets on graphite using vapor phase deposition under a known temperature gradient. Remarkably, we observe two types of growth modes: (i) a dominant one with platelet-like flat crystals and (ii) a less favorable one, formed by spiral terraces. Both structures present a chemical composition compatible with SnS. Using scanning tunneling microscopy (STM) and spectroscopy (STS), we observe that, whereas flat SnS platelets exhibit fluctuations in the band gap and doping, spiral platelets exhibit stable, homogeneous band gap, and negative doping. Using selected area electron diffraction (SAED), we determine that, whereas the platelets of the unstable phase are associated with the common orthorhombic structure of SnS, the minor stable spiral platelets are polycrystalline and are in the metastable cubic phase. The production of this less favorable phase on large surface areas is of potential interest for the production of more efficient tin sulfide-based solar cells. In addition, our analysis of solar cell materials using STM/STS and SAED provides a framework for the rapid determination of the suitability and applicability of 2D materials for other potential optoelectronic applications.