Polarization switching induced by domain wall sliding in two-dimensional ferroelectric monochalcogenides

Monolayer ferroelectric materials have emerged as a promising avenue for miniaturization of ferroelectric devices, and as potential candidate materials for future photovoltaics or thermoelectrics. The properties of ferroelectrics are strongly influenced by the presence of domain walls, but a detailed theoretical understanding of their role in polarization switching and influence on optical properties is still lacking in two-dimensional materials. In this work we study structural, dynamic, electronic and optical properties of 90 and 180 degree DWs in monolayer GeS, GeSe, SnS and SnSe, by means of DFT calculations. We find that, in all cases, 180 degree walls impact the properties stronger than 90 degree walls and exhibit higher formation energies. In particular, calculations of the optical absorption show that 180 degree walls may induce a red shift of the optical absorption peak of the materials, significantly enhancing their solar light absorption at high DW densities. We then calculate the formation energies and migration barriers of DWs and find that these are strongly dependent on lattice anisotropy. The barriers allow us to extract the electric field required to induce DW migration and we find much lower values of the coercive fields compared to coherent bulk polarization switching. Our results are in agreement with experimental coercive field measured for SnS and thus elucidate the crucial role of domain walls in polarization switching.