Off-plane polarization in two dimensional materials is significant during the designation of functional nano-devices. As a prototype, the metal chalcogen-diphosphate CuInP 2 S 6 monolayer is selected, and the structure transition from the initial antiferroelectric (AFE) state to off-plane ferroelectric (FE) state is realized under the combined action of normal strain and electric field. With the introduce of compressed strain, the critical driven field for the transition is remarkably lowered, which is favorable for actual device operation and avoids the possible breakdown of film. As the compressed strain increases, the FE state changes to ground state, and the height of barrier between AFE state and FE state is lowered simultaneously. It is confirmed that the FE state keeps stable at room temperature even if the strain and the electric field are removed. Furthermore, the band gap of the monolayer changes from indirect to direct as the structure transition occurs, which will have potential photovoltaic applications. We hope our theoretical investigation can supply an alternative avenue to modify and improve the functionality of low-dimensional materials.

Sulfur vacancy in MoS 2 has been found to have an important in uence on the performance of optoelectronic devices. Here, we study the effect of sulfur vacancy and O 2 adsorption on the electronic and optical properties in the two-dimensional ferroelectric CuInP 2 S 6 . It is revealed that a defect state appears at the top of valence band with the presence of sulfur vacancy. However, when O 2 is chemisorbed at sulfur vacancy, the defect state disappears. The variation of charge state and charge transfer are calculated and discussed. Although the ferroelectricity is greatly suppressed with the presence of sulfur vacancy, the ferroelectric state can be recovered when the O 2 is adsorbed. Within the framework of GW + BSE method, the optical absorption edge of CuInP 2 S 6 monolayer exhibits a red-shift for the presence of sulfur vacancy and further O 2 adsorption gives rise to a blue-shift of the spectrum. Our ndings have shown an effective way to improve the functionality of two-dimensional ferroelectrics via defect engineering.

BaTiO₃ (BTO) is a typical studying object both in ferroelectrics and in material science. By the GW method, optical property of BTO is investigated, and its volume effect under the case of iso-strain is also studied. It is found that the results of excited states are closer to the experimental results with the consideration of electron-hole interaction in the framework of GW method. Considering the volume effect, we obtain that the red shift of the peaks of optical absorption occurs under the expansion of volume, and the blue shift appears when the BTO is compressed. At the same time, the polarization and the hybridization between d orbital of Ti atom and p orbital of O atom are enhanced for the case of volume expansion, however, things will be opposite under the compression of volume. Furthermore, the volume effect in the iso-strain case is less dramatic than in the iso-stress case.