Sn 5s2 lone pairs and the electronic structure of tin sulphides: A photoreflectance, high-energy photoemission, and theoretical investigation

The effects of Sn 5s lone pairs in the different phases of Sn sulphides are investigated with photoreflectance, hard x-ray photoemission spectroscopy (HAXPES), and density functional theory. Due to the photon energy-dependence of the photoionization cross sections, at high photon energy, the Sn 5s orbital photoemission has increased intensity relative to that from other orbitals. This enables the Sn 5s state contribution at the top of the valence band in the different Sn-sulphides, SnS, Sn2S3, and SnS2, to be clearly identified. SnS and Sn2S3 contain Sn(II) cations and the corresponding Sn 5s lone pairs are at the valence band maximum (VBM), leading to ∼1.0-1.3 eV band gaps and relatively high VBM on an absolute energy scale. In contrast, SnS2 only contains Sn(IV) cations, no filled lone pairs, and therefore has a ∼2.3 eV room-temperature band gap and much lower VBM compared with SnS and Sn2S3. The direct band gaps of these materials at 20 K are found using photoreflectance to be 1.36, 1.08, and 2.47 eV for SnS, Sn2S3, and SnS2, respectively, which further highlights the effect of having the lone-pair states at the VBM. As well as elucidating the role of the Sn 5s lone pairs in determining the band gaps and band alignments of the family of Sn-sulphide compounds, this also highlights how HAXPES is an ideal method for probing the lone-pair contribution to the density of states of the emerging class of materials with ns2 configuration.