Enhancement of the piezoelectric response of ZnO nanowires grown <i>via</i> PLI-MOCVD using post-deposition treatments through adjusted screening and surface effects

As a biocompatible and non-critical material, ZnO, specifically in its nanowire morphology, holds great promise for integration into highly efficient mechanical energy transducers. However, controlling the density of the free charge carriers that drive the screening effect of the piezoelectric potential under mechanical solicitations is critical for enhancing the piezoelectric properties of ZnO nanowires. Hence, herein, the effects of several post-deposition treatments, including O₂ plasma, UV ozone, and thermal annealing under O₂ atmosphere, on the properties of ZnO nanowires grown <i>via</i> pulsed-liquid injection metal-organic chemical vapor deposition are thoroughly investigated and compared. The thermal annealing treatment at high temperature shows predominance over the other post-deposition treatments for the decrease in the density of free electrons (roughly estimated from 1.8-3.3 × 10¹⁸ to about 10¹⁷ cm^-3 ), removal of carbon contamination inside the structure, and improvement in crystallinity. By proceeding with the thermal annealing treatment and increasing the temperature from 700 °C to 900 °C, time-resolved cathodoluminescence measurements further reveal the decrease in the density of surface traps from 7.7 to 3.0 × 10¹² cm^-2 owing to an increase in the amount of oxygen vacancies at the surfaces of the ZnO nanowires. The effective piezoelectric coefficient <i>d</i>eff33, as measured by piezo-response force microscopy, eventually shows a significant enhancement of 47%, from 4.5 to 6.6 pm V^-1, as the annealing temperature and duration are increased. These findings reveal the trade-off to be optimized when using the post-deposition treatments, as supported by finite element method simulations, which shows that the reduction in the densities of free electrons and surface traps acts in an opposite manner on the piezoelectric response of the ZnO nanowires.