Two-dimensional ferroelectric (2D-FE) materials that characterize the spontaneous ferroelectricity down to monolayer limit and rich ferroic properties arising from FE orderings, have been extensively explored as low-dimensional sensor, electric and memory devices. In current work, group-IV transition metal oxide dihalide MOX2 (M = Zr and Hf, X = Cl, Br and I) monolayers have been identified as a new group of 2D-FE materials. Using the comprehensive first-principles calculations combined with finite temperature Monte Carlo (MC) and ab initio molecular dynamics (MD) simulations, we investigate the temperature stability of FE polarization and further uncover the unique properties associated with spontaneous ferroelectricity of MOX2 monolayers. In particular, ZrOI2 monolayer, a promising 2D-FE material with room temperature stable ferroelectricity, semiconducting electronic structure and optoelectronic response under visible light, offers an ideal material platform to investigate the coupling of intrinsic anisotropy, optical absorption selectivity and spin degree of freedom with 2D ferroelectricity. Typically, significant optical absorption anisotropy and giant linear dichroism effect are predicted for a 2D optical polarizer device based on ZrOI2 monolayer, where the adsorption of incident monochromatic linearly polarized light (hv = 3.23 eV) along two planar directions with a nearly 100% optical selectivity can be achieved. Moreover, the spin—orbit coupling (SOC) induced spin splitting of valence band edges and out-of-plane textured spin configuration occur in ZrOI2 monolayer. In the meanwhile, the unidirectional spin-orbit field protected by C2v wave-vector point group can further create the persistent spin helix (PSH) state, leading to the extraordinarily long spin carrier lifetime. More importantly, the nonvolatile control of PSH state via the electric field induced polarization reversal has also been demonstrated for FE-ZrOI2 monolayer, which manifests as a great advantage for applications of ZrOI2 as the low-dimensional spin-field effect transistor and all-electric spintronics devices.
