Strain-tunable out-of-plane polarization in two-dimensional materials

Two-dimensional (2D) ferroelectrics are a special class of materials that fit the need of miniaturization of devices, which can lead to a plethora of applications. Up to now, albeit the breakthrough in the discovery of a handful of 2D ferroelectrics, the mechanism of which still requires better understanding. In this work, we clarify that the ferroelectric ground state in monolayer (ML) In2Se3 is strongly affected by in-plane strain, and the strain-induced phase transition is of a first-order transition. We further propose that strain is an effective approach to tune the ferroelectric ground state in 2D materials, which can even intrinsically introduce out-of-plane polarizations. By performing first-principles calculations, we show that the epitaxial strain is able to tune the structural transition between polar and nonpolar layer groups. According to this, the epitaxial strain can not only fully suppress the polarization in ML-In2Se3, but also cause out-of-plane polarizations in a group of 2D nonpolar materials, i.e., ML-Bi2Se3, ML-Bi2Te3, ML-Sb2Te3, and ML-Sb2Se3. The coupling mechanism between strain and polarization in these compounds is further illustrated.