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

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)(2)PbI4 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.