Title: Phonon Directionality Determines the Polarization of the Band-Edge Exciton Emission in Two-Dimensional Metal Halide Perovskites

Abstract: Two-dimensional metal-halide perovskites are highly versatile for light-driven applications due to their exceptional variety in material composition, which can be exploited for tunability of mechanical and optoelectronic properties. The band edge emission is governed by the exciton fine structure that is defined by structure and composition of both organic and inorganic layers. Moreover, electronic and elastic properties are intricately connected in these materials. Electron-phonon coupling plays a crucial role in the recombination dynamics. However, the nature of the electron-phonon coupling, as well as which kind of phonons are involved, is still under debate. Here we investigate the emission and phonon response from single two-dimensional lead-iodide microcrystals with angle-resolved polarized spectroscopy. We find an intricate dependence of the emission polarization with the vibrational directionality in the materials, which clearly reveals that several bands of the low-frequency phonons of the inorganic lead-iodide perovskite lattice play the key role in the band edge emission. Our findings demonstrate how the emission spectrum and polarization of two-dimensional layered perovskites can be designed by their material composition, which is essential for optoelectronic applications, where fine control on the spectral and structural properties of the light is desired.