Strain engineering of Bi2OS2 ultrathin films: electronic and ferroelectric properties

Abstract

Two-dimensional (2D) ferroelectric semiconductors are of fundamental importance due to their exotic physical properties and potential applications in many devices such as high-density nonvolatile random-access memories. Based on the density functional theory method, we find that Bi₂OS₂ ultrathin films are such semiconductors of which the electronic and ferroelectric properties can be tuned significantly via strain engineering. The band gaps of Bi₂OS₂ ultrathin films increase markedly under compressive strain, while varying slightly with tensile strain. This originates from a non-monotonous relationship between the valence band maximum (VBM) and strain. The reason for this non-monotonous trend lies in distinct response of two kinds of Bi–S bonds in 2D Bi₂OS₂ to strain stimuli. Furthermore, giant ferroelectric spontaneous polarization can be induced in 2D Bi₂OS₂ under tensile strain conditions. Interestingly, ferroelectric polarizations exhibit a non-monotonous correlation with ferroelectricity-stabilized energy, which is significantly different from a positive correlation in common displacement-type ferroelectric materials such as PbTiO₃ and BaTiO₃. The origin of this exotic behavior can be attributed to that the cancellations of ion displacements lead to small relative cation–anion displacements and thus low polarization values, although large magnitudes of ion displacements exist. We also reveal the vital role of lone-pair electrons in determining ferroelectric ion displacements. Our work not only demonstrates exotic electronic and ferroelectric properties in Bi₂OS₂ ultrathin films but also reveals the possibility of designing novel functional materials, which benefits from the distinct atomic coordination environments (either for Bi or S) in Bi₂OS₂.