Sn 5 s 2 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, ${\mathrm{Sn}}_{2}{\mathrm{S}}_{3}$, and ${\mathrm{SnS}}_{2}$, to be clearly identified. SnS and ${\mathrm{Sn}}_{2}{\mathrm{S}}_{3}$ contain Sn(II) cations and the corresponding Sn $5s$ lone pairs are at the valence band maximum (VBM), leading to $\ensuremath{\sim}1.0$--1.3 eV band gaps and relatively high VBM on an absolute energy scale. In contrast, ${\mathrm{SnS}}_{2}$ only contains Sn(IV) cations, no filled lone pairs, and therefore has a $\ensuremath{\sim}2.3$ eV room-temperature band gap and much lower VBM compared with SnS and ${\mathrm{Sn}}_{2}{\mathrm{S}}_{3}$. 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, ${\mathrm{Sn}}_{2}{\mathrm{S}}_{3}$, and ${\mathrm{SnS}}_{2}$, 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 $\mathrm{n}{s}^{2}$ configuration.