Large out-of-plane piezoelectricity of oxygen functionalized MXenes for ultrathin piezoelectric cantilevers and diaphragms

MXenes have currently drawn considerable attention in functional materials and energy storages, because of their versatile and excellent properties. However, few works have been made on Mxenes' piezoelectricity. Applying the density-functional theory, we show that M2CO2 (M = Sc, Y, La) MXenes possess large Poisson's ratio and in-plane piezoelectricity comparable to that of 2H-MoS2. Furthermore, M2CO2 MXenes have strong out-of-plane piezoelectricity, which is highly desirable for ultrathin piezoelectric devices (cantilever and diaphragm) with the d(31) operating mode. For a MXene diaphragm the piezoelectricity-generated charges mainly concentrate in six areas, which are determined by the lattice symmetry. In particular, the in-plane piezoelectric charges are localized near the circular clamped boundary, whereas the out-of-plane piezoelectric charges are inside the pressure-induced bubble. Our observations propose a realistic way to collect the piezoelectricity-induced charges, making these systems very promising for energy harvesting and piezoelectric sensing.