Brightening of localized exciton with strong phonon coupling in ferromagnetic CrBr3

Novel two-dimensional (2D) magnetic semiconductors such as chromium tribromide (CrBr 3 ) provide a compelling platform for spintronic applications by combining intrinsic ferromagnetism and semiconducting behavior at the atomic scale. While this duality enables new opportunities to explore low-dimensional magnetism, the limited understanding of their optical performance has hindered progress in uncovering the interplay between optical phenomena and magnetic phase transitions. Here, we systematically investigate the optical response of chromium tribromide (CrBr 3 ) and uncover sharp, narrowband near-infrared emission features that are distinct from the broad intrinsic excitonic emission in the visible range. Using simple, non-invasive optical techniques, we demonstrate that these sharp peaks originate from defect-localized excitonic states with markedly modified recombination dynamics, which are tentatively attributed to Br-vacancy-related defects. We further reveal that these defects induce local phonon localization, selectively enhancing exciton–phonon coupling to a specific vibrational mode at 183 cm −1 . This selective coupling gives rise to pronounced phonon-induced peak splitting that persists up to room temperature. In addition, defect-induced localization dramatically shortens exciton lifetimes, reducing them from the microsecond to the nanosecond regime. Our results demonstrate the pivotal role of defects in tailoring light–matter interactions in 2D magnetic semiconductors and highlight defect engineering as a promising strategy for developing high-performance quantum emitters and integrated magneto-optical devices.