Electron-phonon coupling in two-dimensional ferroelectric: NbOI₂

Interaction between charge carriers and the polar lattice plays an important role in understanding the optoelectronic properties. While electron-phonon coupling in conventional ferroelectric semiconductors has been intensively discussed, the study of this scenario in two-dimensional (2D) ferroelectrics is rare. The reduction in dimensionality introduces a completely different dielectric screening for carriers injected into 2D materials, leading to a question about how carriers interact with the polar lattice in a less screened environment. Here, we apply coherent phonon spectroscopy to study the electron coupling to the phonon mode in a 2D ferroelectric semiconductor, NbOI₂. The van der Waals (vdW) layered structure of bulk NbOI₂ provides an ideal pseudo-2D environment. Using a broadband probe with a spectral window across the bandgap of NbOI₂, we have found the coupling between photogenerated electrons and a coherently excited phonon with a damping time as long as 30 ps at room temperature. While angle-resolved Raman measurements suggest that the coupled phonon mode is allowed only when the incident polarization is aligned with the polar axis, transient reflectance spectroscopy shows strong carrier coupling to the same phonon mode regardless of the probe's polarization if the probing energy is above the gap. Combining results from Raman and transient reflectance spectroscopy, we propose that the coupling between charge carriers and the specific phonon mode breaks the symmetry upon photoexcitation.