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

2D halide perovskites are extensively studied for optoelectronic applications owing to their outstanding excitonic performance. Studies have revealed the coexistence of the high-temperature phase and low-temperature phase in 2D perovskite (BA)(2)PbI4 flakes and the correlation between the degree of phase transition with the thickness; nevertheless, structural phase transition at surface still remains elusive. Here, the use of interlayer excitons in (BA)(2)PbI4/WSe2 heterojunctions to characterize its structural phase transitions at surface in (BA)(2)PbI4 is proposed. Two types of interlayer exciton emission caused by the phase transition of (BA)(2)PbI4 can be observed, which can be attributed to the low-temperature and high-temperature phases of (BA)(2)PbI4, 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 the same. The results provide a novel and non-destructive approach to explore the phase transition of (BA)(2)PbI4 and offer new route to further regulate its phase transition.