Operando Raman probing of mode selective electron phonon coupling in two dimensional halide perovskites

Abstract

Electron phonon coupling governs charge transport, carrier relaxation, and polaron formation in halide perovskites, yet its microscopic origin in low dimensional systems remains poorly understood. Here, we combine operando, bias dependent Raman spectroscopy with density functional theory (DFT) calculations to directly probe carrier lattice interactions in two-dimensional Ruddlesden Popper perovskites,(PEA)$_2$PbI$_4$ and its fluorinated analogue, (PEA-F)$_2$PbI$_4$. Under applied electric fields, both systems exhibit mode-selective Raman linewidth broadening predominantly near 100 cm$^{-1}$, whereas other phonon modes remain largely unaffected, revealing highly selective coupling between injected carriers and specific lattice vibrations. DFT calculations identify these modes as hybrid organic inorganic vibrations involving coupled motion of the organic spacer and symmetric Pb I equatorial stretching, rather than purely inorganic phonons. Fluorination fundamentally reconstructs the vibrational landscape by modifying molecular packing, crystal symmetry, and organic inorganic coupling, resulting in changes to the phonon density of states, longer phonon lifetimes, and an enhanced carrier mediated lattice response. Notably, electrical bias produces opposite phonon lifetime evolution in thin films and single crystals: the phonon lifetime decreases by approximately 17 to 22% in thin films but increases 20 to 26% in single crystals. These contrasting responses demonstrate that structural order plays a fundamental role in determining carrier phonon interactions and phonon relaxation pathways in two-dimensional halide perovskites.