Surface Polarity and Shape-Controlled Synthesis of ZnO Nanostructures on GaN Thin Films Based on Catalyst-Free Metalorganic Vapor Phase Epitaxy

With the rapid developments in nanotechnology, there has been tremendous interest in ZnO-related nanomaterials because ZnO is a wide band-gap semiconductor and a piezoelectric oxide. As a result, it is useful for optoelectronics, transparent electronics, sensors, and transducers. ZnO crystals have a noncentral symmetric wurtzite structure and are composed of close packed O and Zn2þ layers piled alternatively along the c-axis, producing positively charged Zn-terminated (0001) polar surfaces and negatively charged O-terminated (0001) polar surfaces. Together with the polar surfaces, three fast growth directions of [0001], [0110], and [21 10] facilitate anisotropic growth of ZnO nanocrystals which have various one-dimensional (1D) structures including c-axis oriented nanowires and a-axis oriented nanobelts. For ZnO at the nanometer scale, meanwhile, the surface to volume ratio increases significantly with size reduction and thus, the effect of the surface, especially the surface polarity, plays an important role in determining the structures as well as the chemical and physical properties of the ZnO nanostructures. For example, electrostatic interactions driven by the polar surfaces leads to the deformation of ZnO nanostructures to form unique shapes such as seamless nanorings, nanobows, nanosprings, and nanohelices. In addition, ZnO nanocrystals with varying surface configurations and polarities display different optical and electronic properties. These phenomena make control of the surface is a crucial factor in designing nanomaterials for various applications.