Large Vertical Piezoelectricity in a Janus Cr₂I₃F₃ Monolayer

Two-dimensional (2D) materials have potential applications in nanoscale sensors and spintronic devices. Herein, motivated by experimental synthesis of a CrI3 monolayer possessing intrinsic magnetism and a Janus MoSSe monolayer with piezoelectricity, we propose a 2D Janus Cr2I3F3 monolayer as a multifunctional material exhibiting both piezoelectricity and ferromagnetism. Using density functional theory calculations, we systematically investigated the structural stability and the electronic, magnetic, and piezoelectric properties of the Janus Cr2I3F3 monolayer. We predicted that a vertical polarization of up to -0.155 × 10-10 C/m is induced in the Cr2I3F3 monolayer due to the breaking of symmetry. The origination mechanism of polarization was demonstrated in terms of a local dipole moment calculated by maximally localized Wannier functions. Meanwhile, it was found that a remarkable piezoelectric response can be produced under a uniaxial strain in the basal plane. The calculated piezoelectric coefficients of the Cr2I3F3 monolayer compare favorably with those of the frequently used bulk piezoelectric materials such as α-quartz and wurtzite AlN. Particularly, the e31 and d31 values of the Cr2I3F3 monolayer are nearly 10 times as large as that of Mo-based transition metal dichalcogenides. We also found that the magnitude of e31 mainly arises from the ionic contribution, while the electronic contribution can be nearly neglected. The considerable piezoelectric response combined with the intrinsic magnetism make the Janus Cr2I3F3 monolayer a potential candidate for novel multifunctional devices integrating both piezoelectric and spintronic applications.