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 $\sim 1.0$–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 $\sim 2.3$ 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.

中文翻译：

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Sn5s2lone对和硫化锡的电子结构：光反射，高能光发射和理论研究

锡的影响 $5s$利用光反射，硬X射线光发射光谱（HAXPES）和密度泛函理论研究了锡硫化物不同相中的孤对。由于光电离截面的光子能量依赖性，在高光子能量下，Sn$5s$相对于其他轨道，轨道光发射的强度有所增加。这使锡$5s$ 不同Sn硫化物SnS在价带顶部的状态贡献， ${锡}_2{\mathrm{小号}}_3$和 ${锡}_2$，以便明确识别。锡和${锡}_2{\mathrm{小号}}_3$ 包含Sn（II）阳离子和相应的Sn $5s$ 孤对在价带最大值（VBM），导致 $〜1.0$–1.3 eV带隙和绝对能量规模上较高的VBM。相反，${锡}_2$ 仅包含Sn（IV）阳离子，没有填充的孤对，因此具有 $〜2.3$ eV室温带隙和与SnS和 ${锡}_2{\mathrm{小号}}_3$。使用光反射法发现，对于SnS，这些材料在20 K时的直接带隙为1.36、1.08和2.47 eV，${锡}_2{\mathrm{小号}}_3$和 ${锡}_2$分别进一步突出了在VBM处具有孤对状态的影响。以及阐明锡的作用$5s$ 孤对在确定Sn硫化物族的带隙和能带排列中的作用，这也凸显了HAXPES是探索孤对对新兴材料态密度的贡献的理想方法。 $\mathrm{ñ}{s}^2$ 组态。