Influence of the Vibrational Modes from the Organic Moieties in 2D Lead Halides on Excitonic Recombination and Phase Transition

Abstract

2D metal halide semiconductors have been intensively studied in the past few years due to their unique optical properties and potential for new‐generation photonic devices. Despite the large number of recent works, this class of materials is still in need of further understanding due to their complex structural and optical characteristics. In this work, a molecular‐level explanation for the dual band emission in the 2D (C4H9NH3)2PbI4 in its bulk form is presented, demonstrating that this feature is caused by a strong exciton–phonon coupling. Temperature‐dependent photoluminescence with Raman and IR spectroscopies reveals that vibrations involving the C‐NH3+ butylammonium polar head are responsible for this exciton–phonon coupling. Additionally, experimental shifts in the mean phonon frequencies coupled with the electronic excitation, combined with a theoretical model, show that these vibrational modes present a soft‐mode behavior in the phase transition of this material.

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“…Hot electrons/holes travel through the crystal lattice and, by doing so, they dissipate energy transferring momentum to the crystal lattice via phonon scattering, the so‐called electron–phonon interaction. Phonons not only influence the hot carriers' relaxation but also influence carriers' recombination mechanisms 121,122 . For instance, excitonic states may be split into an exciton‐phonon replica before radiative recombination 122–124 .…”

“…Phonons not only influence the hot carriers' relaxation but also influence carriers' recombination mechanisms 121,122 . For instance, excitonic states may be split into an exciton‐phonon replica before radiative recombination 122–124 . Besides, the transfer of momentum from the carriers to the crystal lattice reduces the maximum power output as part of the incident photon energy is dissipated as heat (i.e., carriers‐phonon coupling are non‐adiabatic processes).…”

“…For instance, the statistical disorder of the alkyl chains in microcrystals increases the lattice strain, augmenting the exciton‐phonon coupling in these systems 125,126 . Although this phenomenon is absent in thin films, it exemplifies the influence of the organic cations in the emission dynamics of these materials 122,125,126 . The thickness‐dependent electron–phonon coupling is believed to be due to an ineffective screening of the van der Waals gap to phonon confinement in RP 2D lead halides 127 .…”

“…Indeed, increasing the distance between inorganic slabs, by either intercalating molecules within the van der Waals gap or increasing the chain length of organic molecules, causes a reversible transition between a dual‐band (phonon replica) and single‐band excitonic emission 128 . Recently, our group highlighted that coupled vibrational modes between the ammonium polar heads (R‐NH 3 + ) and the inorganic sublattice are the origin of this strong exciton‐phonon coupling in microcrystalline 2D lead iodide, originating this dual‐band emission profile 122 . Also, some vibrational modes in the organic layers of 2D lead iodide influence the phase transition of these materials, revealing a soft‐mode characteristic of these vibrations upon temperature variations 122,127 …”

“…Recently, our group highlighted that coupled vibrational modes between the ammonium polar heads (R‐NH 3 + ) and the inorganic sublattice are the origin of this strong exciton‐phonon coupling in microcrystalline 2D lead iodide, originating this dual‐band emission profile 122 . Also, some vibrational modes in the organic layers of 2D lead iodide influence the phase transition of these materials, revealing a soft‐mode characteristic of these vibrations upon temperature variations 122,127 …”