Formation Mechanisms of ZnO Nanowires: The Crucial Role of Crystal Orientation and Polarity

Abstract

ZnO nanowires grown in liquid phase are considered as promising building blocks for a wide variety of optical and electrical devices. However, their structural morphology is still limited by the lack of understanding of their growth mechanisms. We have systematically investigated the effects of orientation and polarity of ZnO monocrystals acting as substrates on the formation mechanisms of ZnO by chemical bath deposition. Under identical growth conditions, two-dimensional layers develop on nonpolar m- and a-plane ZnO monocrystals. In contrast, nanowires form on O-polar c-plane ZnO monocrystals, while more complex nanostructures including nanowires grow on Zn-polar c-plane ZnO monocrystals. All of the structures have homoepitaxially nucleated. Very specifically to chemical bath deposition, both O- and Zn-polar c-planes are found to be active, and no polarity inversion domain boundary is observed on O-polar c-plane ZnO monocrystals, allowing the growth of O-polar ZnO nanowires. These findings reveal the crucial role of crystal orientation and polarity in the growth of ZnO nanowires in liquid phase similarly to their growth in vapor phase. They further cast a new light on the general understanding of the growth of ZnO nanowires and enable the revisiting of their formation mechanisms in liquid phase on seed layers consisting of ZnO nanoparticles.