Probing Local Structural Phase Transition at the Surface of (BA)₂PbI₄ via Interlayer Exciton Emission

Two-dimensional (2D) halide perovskites have been extensively studied for optoelectronic applications owing to their outstanding excitonic performance. Studies have revealed high-temperature phase and low-temperature phase in 2D perovskite (BA)₂PbI₄flakes and the correlation with the thickness; nevertheless, structural phase transition at surface still remains elusive. Here, we propose to use interlayer excitons in (BA)₂PbI₄/WSe₂ heterojunctions to characterize their structural phase transitions at surface in (BA)₂PbI₄. Two types of interlayer exciton emission caused by the phase transition of (BA)₂PbI₄ can be observed, which can be attributed to the low-temperature and high-temperature phases of (BA)₂PbI₄, respectively. Importantly, the spatially resolved PL mapping suggests that two phases in the coexisted region distribute rather uniformly and the degree of phase transition at the surface and interior of crystal remains largely same. Our results provide a novel and non-destructive approach to explore the phase transition of (BA)₂PbI₄ and offer new route to further regulate its phase transition.