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

Abstract

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 BiOCl_xBr_1–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.