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

Bismuth oxyhalides (BiOX, X = F, Cl, Br, I) have gained great attention in optoelectronics, radiation detectors, and photocatalysis due to their unique layered structure and electronic properties. However, the elusive understanding of photoexcited-state properties, particularly the broad sub-bandgap emission previously associated with defect-induced states in BiOX, impedes the relevant photoinduced-species regulation. Herein, by taking BiOBr as an example, we elucidate the sub-bandgap emission to originate from the radiative decay of self-trapped excitons (STEs). Such a radiative process could be effectively activated by edge iodine doping. Spectroscopic analyses and theoretical calculations verify enhanced exciton–phonon interaction, and symmetry breaking account for the STE-emission activation in BiOBr with iodine incorporation. Besides, by tuning halide composition, the energies of STEs in BiOClxBr1–x could be facilely regulated from 2.5 to 2.16 eV, promisingly enabling photoexcited applications such as photoluminescence and energy-transfer-mediated photocatalytic small molecule activation. This work uncovers the intrinsic STE states in BiOX and provides new insights into the regulation of the involved photoexcited-state properties.