Activating Intrinsic Self-Trapped Exciton Emission\nin Bismuth Oxyhalides by Edge Iodine Doping

Bismuth\noxyhalides (BiOX, X = F, Cl, Br, I) have gained great attention\nin optoelectronics, radiation detectors, and photocatalysis due to\ntheir unique layered structure and electronic properties. However,\nthe elusive understanding of photoexcited-state properties, particularly\nthe broad sub-bandgap emission previously associated with defect-induced\nstates in BiOX, impedes the relevant photoinduced-species regulation.\nHerein, by taking BiOBr as an example, we elucidate the sub-bandgap\nemission to originate from the radiative decay of self-trapped excitons\n(STEs). Such a radiative process could be effectively activated by\nedge iodine doping. Spectroscopic analyses and theoretical calculations\nverify enhanced exciton–phonon interaction, and symmetry breaking\naccount for the STE-emission activation in BiOBr with iodine incorporation.\nBesides, by tuning halide composition, the energies of STEs in BiOCl<sub><i>x</i></sub>Br<sub>1–<i>x</i></sub> could\nbe facilely regulated from 2.5 to 2.16 eV, promisingly enabling photoexcited\napplications such as photoluminescence and energy-transfer-mediated\nphotocatalytic small molecule activation. This work uncovers the intrinsic\nSTE states in BiOX and provides new insights into the regulation of\nthe involved photoexcited-state properties.