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Direct Observation of Ultrafast Lattice Distortions during Exciton–Polaron Formation in Lead Halide Perovskite Nanocrystals
ACS Nano ( IF 15.8 ) Pub Date : 2023-01-18 , DOI: 10.1021/acsnano.2c06727 Hélène Seiler 1, 2 , Daniela Zahn 1 , Victoria C A Taylor 1 , Maryna I Bodnarchuk 3 , Yoav William Windsor 1, 4 , Maksym V Kovalenko 3, 5 , Ralph Ernstorfer 1, 4
ACS Nano ( IF 15.8 ) Pub Date : 2023-01-18 , DOI: 10.1021/acsnano.2c06727 Hélène Seiler 1, 2 , Daniela Zahn 1 , Victoria C A Taylor 1 , Maryna I Bodnarchuk 3 , Yoav William Windsor 1, 4 , Maksym V Kovalenko 3, 5 , Ralph Ernstorfer 1, 4
Affiliation
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The microscopic origin of slow hot-carrier cooling in lead halide perovskites remains debated and has direct implications for applications. Slow hot-carrier cooling of several picoseconds has been attributed to either polaron formation or a hot-phonon bottleneck effect at high excited carrier densities (>1018 cm–3). These effects cannot be unambiguously disentangled with optical experiments alone. However, they can be distinguished by direct observations of ultrafast lattice dynamics, as these effects are expected to create qualitatively distinct fingerprints. To this end, we employ femtosecond electron diffraction and directly measure the sub-picosecond lattice dynamics of weakly confined CsPbBr3 nanocrystals following above-gap photoexcitation. While we do not observe signatures of a hot-phonon bottleneck lasting several picoseconds, the data reveal a light-induced structural distortion appearing on a time scale varying between 380 and 1200 fs depending on the excitation fluence. We attribute these dynamics to the effect of exciton–polarons on the lattice and the slower dynamics at high fluences to slower sub-picosecond hot-carrier cooling, which slows down the establishment of the exciton–polaron population. Further analysis and simulations show that the distortion is consistent with motions of the [PbBr3]− octahedral ionic cage, and closest agreement with the data is obtained for Pb–Br bond lengthening. Our work demonstrates how direct studies of lattice dynamics on the sub-picosecond time scale can discriminate between competing scenarios proposed in the literature to explain the origin of slow hot-carrier cooling in lead halide perovskites.
中文翻译：
直接观察卤化铅钙钛矿纳米晶体中激子-极化子形成过程中的超快晶格畸变
卤化铅钙钛矿中缓慢热载流子冷却的微观起源仍然存在争议，并且对应用具有直接影响。几皮秒的缓慢热载流子冷却归因于极化子形成或高激发载流子密度 (>10 18 cm –3 ) 下的热声子瓶颈效应。仅靠光学实验无法明确地解释这些效应。然而，它们可以通过直接观察超快晶格动力学来区分，因为这些效应预计会产生质量上不同的指纹。为此，我们采用飞秒电子衍射并直接测量弱约束 CsPbBr 3纳米晶体在上间隙光激发后的亚皮秒晶格动力学。虽然我们没有观察到持续几皮秒的热声子瓶颈的特征，但数据揭示了光引起的结构畸变，其时间尺度在 380 到 1200 fs 之间变化，具体取决于激发通量。我们将这些动力学归因于激子-极化子对晶格的影响，以及高通量下较慢的动力学导致较慢的亚皮秒热载流子冷却，从而减慢了激子-极化子群体的建立。进一步的分析和模拟表明，畸变与[PbBr 3 ] -八面体离子笼的运动一致，并且与Pb-Br键延长的数据最一致。我们的工作证明了对亚皮秒时间尺度上的晶格动力学的直接研究如何区分文献中提出的竞争场景，以解释卤化铅钙钛矿中缓慢热载流子冷却的起源。
更新日期：2023-01-18
中文翻译：
直接观察卤化铅钙钛矿纳米晶体中激子-极化子形成过程中的超快晶格畸变
卤化铅钙钛矿中缓慢热载流子冷却的微观起源仍然存在争议，并且对应用具有直接影响。几皮秒的缓慢热载流子冷却归因于极化子形成或高激发载流子密度 (>10 18 cm –3 ) 下的热声子瓶颈效应。仅靠光学实验无法明确地解释这些效应。然而，它们可以通过直接观察超快晶格动力学来区分，因为这些效应预计会产生质量上不同的指纹。为此，我们采用飞秒电子衍射并直接测量弱约束 CsPbBr 3纳米晶体在上间隙光激发后的亚皮秒晶格动力学。虽然我们没有观察到持续几皮秒的热声子瓶颈的特征，但数据揭示了光引起的结构畸变，其时间尺度在 380 到 1200 fs 之间变化，具体取决于激发通量。我们将这些动力学归因于激子-极化子对晶格的影响，以及高通量下较慢的动力学导致较慢的亚皮秒热载流子冷却，从而减慢了激子-极化子群体的建立。进一步的分析和模拟表明，畸变与[PbBr 3 ] -八面体离子笼的运动一致，并且与Pb-Br键延长的数据最一致。我们的工作证明了对亚皮秒时间尺度上的晶格动力学的直接研究如何区分文献中提出的竞争场景，以解释卤化铅钙钛矿中缓慢热载流子冷却的起源。
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