Interplay of phonon directionality and emission polarization in two-dimensional layered metal halide perovskites

TL;DR

This study investigates how phonon directionality and emission polarization in 2D layered metal halide perovskites are influenced by organic cations, revealing multiple phonon bands involved in emission and their impact on optical properties.

Contribution

It provides new insights into the directional phonon modes and their role in emission processes, highlighting the influence of organic cations on vibrational and optical behaviors.

Findings

Multiple phonon bands are involved in emission processes.

Organic cations significantly affect phonon spectra and emission polarization.

Directional low-frequency phonons are highly sensitive to organic cation type.

Abstract

With polarized and angle-resolved Raman spectroscopy studies on single two-dimensional layered perovskites (2DLP) flakes with different ammonium molecules as organic cations, in 2020 we revealed the very rich phonon spectra in the low-frequency regime. Although the phonon bands at low frequency can generally be attributed to the vibrations of the inorganic lattice, we found very different phonon spectra for the same lead-bromide octahedra composition by only changing the type of the organic cations. In addition, the intensity of the different phonon modes depended strongly on the angle of the linearly polarized excitation beam with respect to the in-plane axes of the octahedra lattice. In 2022, we mapped this angular dependence of the phonon modes, which enabled to identify the directionality of the different lattice vibrations. By correlating the phonon spectra with the temperature-dependent emission for a set of 2DLPs that featured very different STE emission, we demonstrated that the exciton relaxation cannot be related to coupling with a single (longitudinal optical) phonon band, and that several phonon bands should be involved in the emission process. We performed angle-resolved polarized emission and Raman spectroscopy on the same two-dimensional lead iodide perovskite microcrystals, which revealed the impact of the organic cations on the linear polarization of the emission, and corroborated our interpretation that multiple phonon bands should be involved in the radiative recombination process. Our studies revealed a wealth of highly directional low-frequency phonons in 2DLPs from which several bands are involved in the emission process, which leads to diverse optical and vibrational properties depending on the type of organic cation in the material.