Phonon coherences reveal the polaronic character of excitons in two-dimensional lead-halide perovskites

TL;DR

This study reveals that in two-dimensional lead-halide perovskites, excitons exhibit diverse polaronic characters due to complex lattice dynamics, with vibrational modes linked to specific lattice reorganization signatures.

Contribution

It demonstrates the coexistence of diverse excitonic resonances with distinct polaronic characters and identifies their vibrational signatures using advanced spectroscopy and theoretical calculations.

Findings

Different excitons induce specific lattice reorganizations.

Vibrational wavepacket dynamics reveal polaronic signatures.

Low-frequency phonons are involved in exciton-lattice interactions.

Abstract

Hybrid organic-inorganic semiconductors feature complex lattice dynamics due to the ionic character of the crystal and the softness arising from non-covalent bonds between molecular moieties and the inorganic network. Here we establish that such dynamic structural complexity in a prototypical two-dimensional lead iodide perovskite gives rise to the coexistence of diverse excitonic resonances, each with a distinct degree of polaronic character. By means of high-resolution resonant impulsive stimulated Raman spectroscopy, we identify vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarriers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various low-frequency ($\lesssim 50$\,cm$^{-1}$) optical phonons involving motion in the lead-iodide layers. We thus conclude that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. This insight on the energetic/configurational landscape involving globally neutral primary photoexcitations may be relevant to a broader class of emerging hybrid semiconductor materials.