Hydrostatic Pressure Driven Band Gap Tuning and Self-Trapped Exciton Formation in (4FPEA)$_2$SnBr$_{4}$ Halide Perovskite

Abstract: Two-dimensional tin halide perovskites provide a highly tunable platform for exciton phonon coupling and local lattice distortions, enabled by their intrinsically soft lattice. We report a combined temperature and pressure dependent photoluminescence study of the layered perovskite (4FPEA)${2}$SnBr${4}$. At room temperature, its optical response is dominated by near band edge (NBE) excitons, which redshift linearly under hydrostatic pressure up to $\sim$3 GPa, indicating a rigid band edge behavior without phase transitions. Cooling reveals a broad, strongly Stokes shifted self-trapped exciton (STE) emission, evidencing a crossover from delocalized to self localized excitonic states. Strikingly, while NBE emission redshifts under pressure, STE emission exhibits an anomalous blueshift, reflecting pressure induced modification of the exciton phonon energy landscape. In contrast, the iodide analogue (4FPEA)${2}$SnI${4}$ shows no STE emission under identical conditions, highlighting the critical role of lattice rigidity and dielectric screening in stabilizing self-trapped excitons.