Magnetoelectric Multiferroic Janus Monolayers VOXY (X/Y = F, Cl, Br, or I, and X ≠ Y) with In-Plane Ferroelectricity and Out-of-Plane Piezoelectricity

Multiferroicity in two-dimensional (2D) materials results from a broken space-inversion and time-reversal symmetry, effecting spontaneous electric polarization and magnetization in the same phase. Recently discovered Janus 2D materials are based on a similar principle, where out-of-plane symmetry is broken to engineer new exciting functionalities, such as an out-of-plane polarization. In this work, a new group of 2D multiferroic Janus monolayers VOXY (X/Y = F, Cl, Br, or I, and X ≠ Y) is derived by breaking the out-of-plane symmetry in the parent multiferroics VOX2. Ferroelectric and magnetic properties of Janus VOXY monolayers are found to be comparable to their parent VOX2 monolayers. In addition to large in-plane piezoelectric coefficients (e11 from 11.46 × 10–10 to 13.97 × 10–10 C/m, e12 from −2.54 × 10–10 to −3.32 × 10–10 C/m), most of the Janus VOXY monolayers have significant out-of-plane piezoelectric coefficient. Our theoretical work predicts a new series of 2D multiferroic materials and encourages their further investigation for high-performance nanoelectronic devices.