Phononic Fine-Tuning in a Prototype Two-Dimensional Hybrid Organic-Inorganic Perovskite System

The emerging two-dimensional (2D) lead-halide perovskite materials hold great promise for next-generation photovoltaic and optoelectronic applications, in which phonon engineering plays a crucial role. However, detailed mechanistic exploration related to phonon effects, especially from a dynamics perspective, remains rather limited. Herein, we present a systematic demonstration of phononic fine-tuning in a prototype 2D hybrid organic-inorganic perovskite (HOIP) system, i.e., phenethylammonium lead iodide [(PEA)₂PbI₄] with each hydrogen atom at positions 2 (ortho), 3 (meta), and 4 (para) on the PEA's phenyl group being replaced by a fluorine atom. Through a set of joint observations via ultrafast spectroscopy and temperature-dependent photoluminescence spectroscopy, we reveal that such a fluorination can subtly exert profound impacts on its structural distortion-induced phononic properties, including coherent phonon modes, phonon-phonon/electron-phonon interactions, and the hot-phonon bottleneck effect. This work highlights the significant importance of the atomic-level tailoring of organic cations in low-dimensional HOIP systems, which is usually ignored in conventional notion and practice.