Exciton-Polarons in Hybrid Ruddlesden Popper Metal Halides—Lessons from Coherent Spectroscopy

Ruddlesden Popper metal halides are quantum-well like structures composed of quasi-2D layers of metal-halide octahedra that are separated by long organic cations [1]. Owing to electronic and dielectric confinement within the inorganic layers, the Coulomb interactions between optically generated photo-carriers is enhanced substantially resulting in large exciton binding energies of 200 to 400 meV [2]. A deterministic element in the physics of these materials stems from the peculiar electron-phonon interactions arising from their ionic character and also from the convoluted dynamics of the hybrid organic-inorganic lattice, which is soft and highly noisy [3]. We discuss the consequences of such a unique structural and electronic landscape on the exciton quantum dynamics via optical measurement of exciton-phonon [4] and exciton-exciton [5] interactions. We present our perspective on how the coherent optical response of Ruddlesden Popper metal halides can be effectively rationalized within the “exciton-polaron” framework, in which lattice dressing of photo-carriers constitutes an integral component of excitonic wavefunction [1,6]. In addition, we demonstrate that the organic-inogranic lattice interactions critical in determining the structural parameters, and thus the quantum dynamics, can be effectively modulated by the choice of the metal cation, moving from heavier lead to lighter tin[7].<br/>[1] J. Phys. Chem. Lett., 11, 3173-3184 (2020).<br/>[2] Phys. Rev. Mater., 2, 064605 (2018).<br/>[3] Nature Materials, 18, 349-356 (2019).<br/>[4] Phys. Rev. Research., 1, 032032(R) (2019).<br/>[5] J. Chem. Phys., 153, 164706 (2020).<br/>[6] ArXiv:2107.03199 (2021).<br/>[7]Chemrxiv, DOI: 10.33774/chemrxiv-2021-qcm36-v2