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

DOI: 10.1103/PhysRevMaterials.4.074602 DOI Help

Authors: Leanne A. H. Jones (University of Liverpool) , Wojciech M. Linhart (Wroclaw University of Science and Technology) , Nicole Fleck (University of Liverpool) , Jack E. N. Swallow (University of Liverpool) , Philip A. E. Murgatroyd (University of Liverpool) , Huw Shiel (University of Liverpool) , Thomas J. Featherstone (University of Liverpool) , Matthew J. Smiles (University of Liverpool) , Pardeep K. Thakur (Diamond Light Source) , Tien-lin Lee (Diamond Light Source) , Laurence J. Hardwick (University of Liverpool) , Jonathan Alaria (University of Liverpool) , Frank Jaeckel (University of Liverpool) , Robert Kudrawiec (Wroclaw University of Science and Technology) , Lee A. Burton (Shanghai University) , Aron Walsh (Imperial College London; Yonsei University) , Jonathan M. Skelton (University of Manchester) , Tim D. Veal (University of Liverpool) , Vin R. Dhanak (University of Liverpool)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Physical Review Materials , VOL 4

State: Published (Approved)
Published: July 2020
Diamond Proposal Number(s): 21431

Open Access Open Access

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.

Journal Keywords: Band gap; Density of states; Semiconductor compounds; Density functional theory; Photoemission spectroscopy

Subject Areas: Materials, Physics


Instruments: I09-Surface and Interface Structural Analysis

Documents:
PhysRevMaterials.4.074602.pdf