Phonon Directionality Impacts Electron–Phonon Coupling and Polarization of the Band-Edge Emission in Two-Dimensional Metal Halide Perovskites

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 the tunability of mechanical and optoelectronic properties. The band-edge emission is defined by the structure and composition of both organic and inorganic layers, and electron–phonon coupling plays a crucial role in the recombination dynamics. However, the nature of the electron–phonon coupling and what kind of phonons are involved are still under debate. Here we investigate the emission, reflectance, 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 reveals that several bands of low-frequency phonons with nonorthogonal directionality contribute to the band-edge emission. Such complex electron–phonon coupling requires adequate models to predict the thermal broadening of the emission and provides opportunities to design polarization properties.