Interface engineering of ferroelectricity in thin films of thiophosphate ABP2X6(A=Cu,Ag;B=In,Bi,Cr,V;X=S,Se)

Two-dimensional ferroelectrics (FEs) are promising in the miniaturization of memory devices with ultra-high-density data storage and low power consumption. However, many thiophosphate monolayers, i.e., analogs of ${\mathrm{CuInP}}_{2}{\mathrm{S}}_{6}$ and referred to as ${\mathrm{ABP}}_{2}{\mathrm{X}}_{6}$, lose ferroelectricity and instead exhibit an antiferroelectric (AFE) or paraelectric ordering. We propose to tune the AFE ${\mathrm{ABP}}_{2}{\mathrm{X}}_{6}$ monolayers into the FE ordering through interface engineering. The mechanism is that there are couplings between the charge polarizations of the ${\mathrm{ABP}}_{2}{\mathrm{X}}_{6}$ monolayers and the local dipoles as well as the induced electronic polarizations in the substrate which have a tendency to stabilize the FE ordering. We further perform first-principles calculations for ${\mathrm{CuInP}}_{2}{\mathrm{Se}}_{6}$ and ${\mathrm{CuCrP}}_{2}{\mathrm{S}}_{6}$ monolayers and their van der Waals heterostructures. We find that an AFE ${\mathrm{CuInP}}_{2}{\mathrm{Se}}_{6}$ monolayer becomes FE as interfaced with graphene, ${\mathrm{MoS}}_{2}$, and h-BN monolayers. In contrast, the ${\mathrm{CuCrP}}_{2}{\mathrm{S}}_{6}$ monolayer remains AFE since there is a large energy difference between the AFE and FE phases. Interfacing it with a ${\mathrm{MoTe}}_{2}$ monolayer induces a metal-insulator transition for the heterostructure, whereas interfacing with a polar surface MgO(111) can drive it into FE. The interfacing effect can also be used to manipulate the FE properties of ${\mathrm{ABP}}_{2}{\mathrm{X}}_{6}$ multilayers. We further find that the AFE-to-FE transition is electrically switchable in these systems. In particular, it is accompanied by an indirect-direct band-gap transition for the ${\mathrm{CuInP}}_{2}{\mathrm{Se}}_{6}$ monolayer. Our study offers an effective approach to tune the FE and electronic properties of ${\mathrm{ABP}}_{2}{\mathrm{X}}_{6}$ thin films for applications in electronics and optoelectronics.