Inorganic Cage Motion Dominates Excited-State Dynamics in 2D-Layered Perovskites (CxH2x+1NH3)2PbI4 (x = 4–9)

Hybrid two-dimensional (2D)-layered perovskites with formula A2PbX4, where X = I, Br, or Cl and A is an ammonium-terminated organic cation, have attracted interest due to their tunable optoelectronic properties. In particular, the nature of interactions between tightly bound excitons and the polar inorganic–organic lattice in these dielectrically and quantum-confined materials has been the subject of intense investigation. Using a combination of resonant impulsive-stimulated Raman scattering, nonresonant continuous wave Raman scattering, density functional theory, and temperature-dependent photoluminescence, we investigate the effect of alkylammonium organic cation length on exciton–phonon coupling in the series of 2D lead iodide perovskites (CxH2x+1NH3)2PbI4 with chain lengths ranging from four to nine carbons (C4–C9). We find that the motion of the inorganic Pb–I cages comprises the majority of relevant phonon modes, whose frequencies are unchanged despite more than a doubling of the organic cation length. The accompanying motion of the alkylammonium organic cations is mechanically driven by displacement of the heavy iodide ions through Coulombic attraction and hydrogen bonding, but the coherence of the organic cation motion with the inorganic cage does not extend past the fourth carbon in the chain. The consistency in exciton–phonon coupling across this series of 2D lead iodide perovskites suggests that the exciton is highly confined along the stacking axis of these layered structures, and that the nuclear displacements involved in polaron formation are dominated by inorganic cage motion, with minimal contributions from the organic cation.