Ferroelectric domain wall in two-dimensional GeS

Two-dimensional (2D) ferroelectrics have attracted extensive attention due to their rich variety of exquisite functionalities in novel nanoscale electronic devices. As domain walls (DWs) in ferroelectrics are topological defects separating domains with different orientations of the electric polarization, a detailed understanding of the energetic and atomistic characteristics of 2D ferroelectric DWs is a crucial issue due to its theoretical and technological importance. In the current study, using first-principles calculations, we provided a detailed investigation on the energy, variation of the atomic structure with applied strain, and the electronic properties of 180° and 90° DWs in 2D GeS including the uncharged and charged DWs. All types of DWs in 2D GeS were found to be atomically sharp. In addition, the 90° uncharged DW was more energetically favorable than the 180° DW, which is similar to DWs of perovskites. However, due to the effect of adverse electrostatic energy, the charged DW possessed higher energy than that of the uncharged DW. On the other hand, the polarization distortion of the domain region in all DWs is significantly strengthened by the biaxial strain. In addition, the density of states showed that the charged DW is conductive relative to the uncharged domain wall, because the uncompensated positive or negative charges exist at the charged domain wall. Our results provide necessary theoretical guidance to the future exploration and application of 2D ferroelectric materials.