Dimensionality Effect on Surface States and Piezoelectric Behavior in ZnO Nanowires Grown by Pulsed-Liquid Injection Metal–Organic Chemical Vapor Deposition

ZnO nanowires are known as promising candidates for the development of highly efficient mechanical energy transducers using biocompatible and noncritical materials. However, the decoupling of the different contributions to the piezoelectric response of ZnO nanowires coming from the dimensionality effect, the nature and amount of defects, the density of free charge carriers, and the nature and density of surface traps is still lacking. The growing attractivity of mechanical energy transducers thus necessitates a thorough study on the interplay among the piezoelectricity process, free charge carrier screening, and surface depletion effects in these semiconducting nanowires as well as their relationship to the dimensions of ZnO nanowires. To this day, the growth of ZnO nanowires by pulsed-liquid injection metal–organic chemical vapor deposition has been less explored despite the excellent structural and optical properties of ZnO nanowires. By fixing all growth conditions except for the growth time, the impact of the dimensions of ZnO nanowires on their piezoelectric and physical properties is carefully assessed. Based on electrical characterization and cathodoluminescence measurements, the densities of free electrons and of surface traps are determined in the ranges of 1.8 × 1018 to 3.3 × 1018 cm–3 and 5 × 1012 to 1 × 1013 cm–2, respectively, and are shown to vary with the radius of ZnO nanowires. The incorporation of AlZn and carbon species in ZnO nanowires is further responsible for their high electrical conductivity. Furthermore, the piezoelectric response of ZnO nanowires is experimentally determined by piezoresponse force microscopy and compared with theoretical computations based on finite element method simulations, showing that the piezoelectric coefficient in the range of 4.5–5.5 pm/V strongly depends on the dimensions of ZnO nanowires as an ultimate tool to enhance the performance of the related mechanical energy transducers.