Insight into the structures and properties of morphology-controlled legs of tetrapod-like ZnO nanostructures

The fine structure characterization of individual legs is essential for understanding the detailed formation mechanism and the origin of the unique properties of tetrapod-like ZnO nanostructures. We have synthesized tetrapod-like ZnO nanostructures with thin needle (TN-ZnO), uniform hexagonal prism (TU-ZnO), and hierarchical hexagonal prism (TH-ZnO) legs through oxidization of zinc vapor followed by ZnO condensation at relatively lower temperatures. The individual legs of as-synthesized ZnO nanotetrapods were characterized complementarily by scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron dispersive spectrum (EDS), and cathodoluminescence (CL). We demonstrated that the legs have wurtzite structure and prefer to grow along the [0001] direction. We found that all legs grew from similar ZnO,, nuclei, where x is about 0.3, and all of them showed a strong visible luminescent property. EDS and CL spectra obtained from different regions in an individual leg illustrated that the strong visible luminescence resulted from their surface states rather than the heavy oxygen vacancy. The possible nucleation and growth mechanisms of the legs with different morphologies are discussed.