Tailoring Photophysical Dynamics in a Hybrid Gallium-Bismuth Heterometallic Halide by Transferring from an Indirect to a Direct Band Structure.

Low-dimensional lead-free metal halides have emerged as novel luminous materials for solid-state lighting, remote thermal imaging, X-ray scintillation, and anticounterfeiting labeling applications. However, the influence of band structure on the intriguing optical property has rarely been explored, especially for low-dimensional hybrid heterometallic halides. In this study, we have developed a lead-free zero-dimensional gallium-bismuth hybrid heterometallic halide, A(GaCl)(BiCl) (A = CHN), that is photoluminescence (PL)-inert because of its indirect-band-gap character. Upon rational composition engineering, parity-forbidden transitions associated with the indirect band gap have been broken by replacing partial Ga with Sb, which contains an active outer-shell 5s lone pair, resulting in a transition from an indirect to a direct band gap. As a result, broadband yellow PL centered at 580 nm with a large Stokes shift over 200 nm is recorded. Such an emission is attributed to the radiative recombination of an allowed direct transition from triplet P states of Sb based on experimental characterizations and theoretical calculations. This study provides not only important insights into the effect of the band structure on the photophysical properties but a guidance for the design of new hybrid heterometallic halides for optoelectronic applications.