Doped scandium-based layered oxychalcogenides

Abstract

Layered oxychalcogenides are a branch of Mixed Anion Layered Materials, known for their electrical properties and are studied for transparent conducting applications. Their large optical band gaps and intrinsic layering permits the transport of charge carriers which may be improved by doping.

The wide band gap layered quinternary ‘325’ oxychalcogenide Sr3Sc2O5Cu2S2 has been long acknowledged as a promising candidate material for p-type transparent conductivity due to an exceptional undoped hole mobility, but has received limited research attention since its initial recognition. In Chapter 3, the electrical enhancement of Sr3Sc2O5Cu2S2 is achieved by the aliovalent doping of sodium and potassium and the introduction of strontium vacancies without compromising transparency. This is the first reported doping study of this material.

Greater potential was arguably observed by the selenide analogue, Sr3Sc2O5Cu2Se2, whose undoped conductivity is greater than Sr3Sc2O5Cu2S2 but at the expense of transparency. A comprehensive doping study of Sr3Sc2O5Cu2Se2 was carried out containing eight unique dopants or site vacancies, of which five yielded improved performance. The most impressive enhancements of the electrical conductivity of Sr3Sc2O5Cu2Se2 resulted from the doping of K+ or Na+ on the Sr2+ site, or Mg2+ on the Sc3+ site. Heavy sodium doping produced an almost five order of magnitude increase in conductivity up to 23.4 S cm-1 with no reduction in band gap. To the best of the author’s knowledge this is the most extensive doping investigation of any single material in literature.

The ‘426’ relatives of these materials, Sr4Sc2O6Cu2S2 and Sr4Sc2O6Cu2Se2, are explored within Chapter 4 to the same aim. These are not as impressive but still show an order of magnitude improved performance by doping. A delicate thermodynamic balance encountered during the synthesis of these materials is reported whereby purity is compromised by formation of the more stable Sr3Sc2O5Cu2S2 and Sr3Sc2O5Cu2Se2 phases. This also harms their electrical properties through an inability to be sufficiently densified. These oxychalcogenides were produced alongside their gallium and indium counterparts, the former yielding greater conductivities but both with band gaps too small for use in commercial transparent conducting applications.

Finally, solid-state metathesis is explored in Chapter 5 as a method for oxychalcogenide synthesis, applied to materials of this type only once before. The scandium material Sr3Sc2O5Cu2Se2 described above was synthesised successfully by metathesis for the first time, including a detailed temperature dependency experiment compared to the conventional method. This process was applied to a novel combinatorial approach whereby multiple oxychalcogenides may be yielded from a ‘one-pot’ parallel reaction, but undesirable product site mixing limits the scalability and applicability of this method in practice.

This work furthers our understanding of scandium-based layered oxychalcogenides for p-type conductivity.

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Identifiers

ORCID for Liam Paul Kemp: orcid.org/0009-0007-7110-5888

ORCID for Geoffrey Hyett: orcid.org/0000-0001-9302-9723

ORCID for Andrew Hector: orcid.org/0000-0002-9964-2163

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