Ferroelastic Zr2P2XY (X/Y = I, Br, Cl or F; X ≠ Y) monolayers with tunable in-plane electronic anisotropy and remarkable out-of-plane piezoelectricity

Two-dimensional materials with ferroelasticity, directional control of electronic behaviors and strong piezoelectricity are highly desirable for developing advanced multifunctional nanodevices. Here, we report the new tetragonal Zr2P2XY (X/Y = I, Br, Cl or F; X ≠ Y) monolayers with intrinsic ferroelasticity and remarkable piezoelectricity, based on first-principles calculation. These novel monolayers with semiconductor properties are dynamically, thermally and mechanically stable. The promising anisotropic properties of both mechanics and electron can be perpendicularly controlled based on the intrinsic ferroelasticity. Moderate switching barriers and reversible ferroelastic strains demonstrate their excellent ferroelastic performances along in-plane direction. Additionally, the inhomogeneous charge distribution caused by mirror asymmetry endows these monolayers with strong out-of-plane piezoelectricity. The remarkable piezoelectric strain coefficient d33 = 129.705 pm/V found in Zr2P2BrCl monolayer is two orders of magnitude higher than that of MoSTe multilayer. The Zr2P2BrCl monolayer has such excellent ferroelasticity, controllable anisotropic electronic properties along in-plane directions, and strong piezoelectricity along out-of-plane direction. It provides a unique platform for memory devices, robot bionic skin, or multipurpose nanodevices that require the special electronic and piezoelectric properties along different directions at same time.