Strain engineering to facilitate the occurrence of 2D ferroelectricity in CuInP2S6 monolayer

Off-plane polarization in two dimensional materials is significant during the designation of functional nano-devices. As a prototype, the metal chalcogen-diphosphate CuInP2S6 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.