Anharmonicity in 2D hybride halide perovskites

2D hybrid halide perovskites have been the focus of extensive study due to their easy tuneability and potential in optoelectronic applications such as solar cells and LEDs. The electronic properties of these materials are highly affected by thermal fluctuations, due to a change in potential acting on the electrons, during atomic displacements (electron-phonon interaction). Usually, these atomic displacements are treated within the harmonic approximation. Anharmonic fluctuations lead to the formation of local, transient dipoles that affect charge screening and electron-phonon interaction.

In order to study the influence of anharmonicity on electron-phonon interaction, we measured low-frequency Raman crystallography and emission of prototypical 2D perovskite (C₄H₉NH₃)₂PbI₄ as a function of temperature. Raman scattering results indicate large anharmonic structural dynamics even at cryogenic temperatures. With increasing temperature some modes lose their polarization dependence, suggesting large symmetry breaking and phonon-phonon scattering.