d⁰ Cation-Based Spinel-Type Sulfide Semiconductors with Color-Tunable Direct-Gap and Ambipolar Dopability

This work proposes a materials design concept based on chemical bonding and atomic orbital symmetry to realize a direct bandgap and both n- and p-type dopability in spinel-type AB₂X₄, which have been overlooked as optoelectronic semiconductors. By combining a heavy s⁰ cation, a d⁰ cation, and sulfur at the A, B, and X sites, the conduction band minimum is formed mainly by the deep s⁰ orbital, whereas the valence band maximum is formed by the shallow nonbonding sulfur p orbitals. These are appropriately aligned for n- and p-type doping. Furthermore, the anisotropic d⁰ orbitals of B suppress dispersion of competing bands, resulting in a direct bandgap. In the candidate ZnSc₂S₄, the band gap and photoluminescence are widely tuned via isovalent substitution. Additionally, the n- and p-type conductivities can be controlled through chemical doping by over 9 orders of magnitude from the insulating state, validating the effectiveness of our concept in exploring next-generation semiconductors suitable for photoabsorbers of solar cells and green light-emitting diode sources.