DOI: https://doi.org/10.29363/nanoge.incnc.2021.034
Publication date: 8th June 2021
Organic-inorganic halide perovskite nanocrystals (PNCs) are gaining increasing attention in contemporary research due to their promising performance in light-emitting as well as solar technology. Photoexcitation of these PNCs with an ultrashort laser pulse can produce coherent phonons along the lattice displacement coordinate, leading to lattice vibrations. Here, we employ femtosecond pump-probe spectroscopy to initiate the photophysics by the formation of coherent phonons and to observe the subsequent coherent vibrational dynamics of highly luminescent formamidinium lead halide (FAPbX3, X=Br, I) PNCs.[1] We find that the FAPbX3 PNCs generate halide-dependent coherent vibronic wave packets upon non-resonant excitation, and the dominant contributions are attributed to the Pb–X bending and stretching modes of PbX64- octahedral units of the lattice. More importantly, for the first time, we observe higher harmonic vibrational modes in FAPbI3 PNCs, which points to a more anharmonic potential energy surface in the case of FAPbI3 as compared to FAPbBr3 PNCs. This is likely due to the weaker interaction between the central FA moiety (which sits in a larger octahedral interstitial site) and the inorganic cage for FAPbI3 PNCs, and thus the PbI64- unit can vibrate more freely (Fig. 1a). This weakening reveals the intrinsic anharmonicity in the Pb-I framework, and thus facilitating the energy transfer into overtone and combination bands (Fig. 1b). Furthermore, our control experiment with MAPbBr3 reveals the energy transfer between framework phonons due to the intrinsic anharmonicity of the lead-halide framework is indeed influenced by the interaction between the framework and the organic molecules, and not only by the halide nature. The insights interestingly not only unravel the underlying reason for the halide-dependent stability of these materials but also shed light on their charge-carrier mobility and light emission properties.
We acknowledge financial support by the Bavarian State Ministry of Science, Research, and Arts through the grant “Solar Technologies go Hybrid (SolTech)”, the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germanys Excellence Strategy—EXC 2089/1-390776260, the Alexander von Humboldt Foundation (T.D. and A.D.), the German-Israeli Foundation for Scientific Research and Development (GIF, Project I-1512-401.10/2019), the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 839042 (H.H.). S.V.L. was supported by RFBR and INSF, project number 20-53-56065.
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