Coherent vibrational dynamics reveals lattice anharmonicity in organic-inorganic halide perovskite nanocrystals

The halide ions of organic-inorganic hybrid perovskites can strongly influence the interaction between the central organic moiety and the inorganic metal halide octahedral units and thus their lattice vibrations. Here, we report the halide-ion-dependent vibrational coherences in formamidinium lead halide (FAPbX₃, X = Br, I) perovskite nanocrystals (PNCs) via the combination of femtosecond pump-probe spectroscopy and density functional theory calculations. We find that the FAPbX₃ PNCs generate halide-dependent coherent vibronic wave packets upon above-bandgap non-resonant excitation. More importantly, we observe several higher harmonics of the fundamental modes for FAPbI₃ PNCs as compared to FAPbBr₃ PNCs. This is likely due to the weaker interaction between the central FA moiety and the inorganic cage for FAPbI₃ PNCs, and thus the PbI₆^4- unit can vibrate more freely. This weakening reveals the intrinsic anharmonicity in the Pb-I framework, and thus facilitating the energy transfer into overtone and combination bands. These findings not only unveil the superior stability of Br-based PNCs over I-based PNCs but are also important for a better understanding of their electronic and polaronic properties.