Sn 5 s 2 lone pairs and the electronic structure of tin sulphides: A photoreflectance, high-energy photoemission, and theoretical investigation



Jones, Leanne ORCID: 0000-0002-4654-3882, Linhart, Wojciech, Fleck, Nicole ORCID: 0000-0001-7800-056X, Swallow, Jack, Murgatroyd, Philip, Shiel, Huw, Featherstone, Thomas, Smiles, Matthew ORCID: 0000-0003-2530-5647, Thakur, Pardeep, Lee, Tien-Lin
et al (show 9 more authors) (2020) Sn 5 s 2 lone pairs and the electronic structure of tin sulphides: A photoreflectance, high-energy photoemission, and theoretical investigation. Physical Review Materials, 4.

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Abstract

The effects of Sn 5 s 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 5 s orbital photoemission has increased intensity relative to that from other orbitals. This enables the Sn 5 s state contribution at the top of the valence band in the different Sn-sulphides, SnS, Sn 2 S 3 , and SnS 2 , to be clearly identified. SnS and Sn 2 S 3 contain Sn(II) cations and the corresponding Sn 5 s 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, SnS 2 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 Sn 2 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, Sn 2 S 3 , and 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 5 s 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 n s 2 configuration.

Item Type: Article
Depositing User: Symplectic Admin
Date Deposited: 22 Jun 2020 09:37
Last Modified: 06 Oct 2021 19:11
DOI: 10.1103/PhysRevMaterials.4.074602
Open Access URL: https://journals.aps.org/prmaterials/abstract/10.1...
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URI: https://livrepository.liverpool.ac.uk/id/eprint/3090955