Effect of the Scaling of the Mechanical Properties on the Performances of ZnO Piezo-Semiconductive Nanowires

Zinc Oxide piezoelectric nanowires may offer unprecedented functional properties for piezotronics and piezoelectrics due to their electrical properties combined to the increased strength. Conventional "bulk" modeling used so far has suggested that "the smaller, the better" is the design tenet to obtain stronger and more reliable systems with higher piezopotential and efficacy. However, prior work has never addressed the interaction between electrical and mechanical size-effects of ZnO NWs, particularly the effect of the mechanical stiffening on the piezoelectric potentials and mechanical-to-electrical conversion efficacy of a NW. In this paper we propose a refined "size-dependent" modeling based on a "double power-law scaling" for the mechanical size-effects, which reveals, though consistent simulations based on the Finite Element Method, that stiffening effects can indeed collide with the electrical size-effects in very small NWs, thus meaning that minimizing the NW diameter does not yield always the best piezo-performance, as purported by standard bulk modeling. Our computations support that the elastic size-effects need to be carefully accounted in the design of ZnO NWs piezo-technology.