Cation ordering induced two-dimensional vertical ferroelectricity in tungsten and molybdenum trioxides

Two-dimensional (2D) materials that exhibit vertical polarization might provide ground-breaking device applications, compatible with electric-field switching even in the presence of a surface-depolarizing field. Here, using first-principles calculations, we focus on vertical cationic off-plane displacements in transition metal trioxides MO3 (M=W, Mo), with a configuration similar to CrI3-like hexagonal trihalides. We further design a WMoO6 monolayer, with W and Mo arranged in an energetically stable honeycomb-checkerboard pattern, showing a net vertical ferroelectric polarization and a wide band gap. Moreover, the structural, electronic, and ferroelectric properties of WMoO6 monolayers are found to be significantly strain tunable. Our results put forward a transition metal trioxide monolayer with switchable vertical ferroelectricity, thus broadening the range of 2D functional ferroelectrics.