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 (FAPbX(3), X=Br, I) perovskite nanocrystals (PNCs) via the combination of femtosecond pump-probe spectroscopy and density functional theory calculations. We find that the FAPbX(3) 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 FAPbI(3) PNCs as compared to FAPbBr(3) PNCs. This is likely due to the weaker interaction between the central FA moiety and the inorganic cage for FAPbI(3) PNCs, and thus the PbI64- 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. Using a combination of femtosecond pump-probe spectroscopy and first-principles calculations, Debnath et al. elucidated the halide-dependence of the excited state vibrational coherences in hybrid organic-inorganic perovskite nanocrystals. The study revealed an intrinsic anharmonicity of lead-halide framework, which correlates with perovskite stability and is influenced by the interaction between the framework and the organic molecules.

Automated summary

The study reveals that halide ions in organic-inorganic hybrid perovskites can influence the lattice vibrations of nanocrystals, leading to different coherent vibronic wave packets generation. Additionally, it shows that the PbI64- unit in FAPbI(3) nanocrystals can vibrate more freely compared to FAPbBr(3) nanocrystals, resulting in higher harmonics of fundamental modes. This intrinsic anharmonicity in the lead-halide framework plays a key role in the stability and energy transfer of perovskites.