ZnO nanostructures prepared by different methods

ZnO is of great interest for photonic applications due to its wide band gap (3.37 eV) and large exciton binding energy (60 meV). Variety of preparation methods and obtained morphologies (such as nanorods, tetrapod nanorods, nanowires, nanoribbons, hierarchical structures, nanobridges, and nanonails) were reported for this material. In this work, the morphology and optical properties of ZnO nanostructures prepared by three different methods were studied. ZnO nanostructures were prepared by oxidation of Zn (no catalyst) at 950°C, heating of a mixture of ZnO:graphite (1:1) at 1100°C, and chemical method (from solution of zinc nitrate hydrate and hexamethylenetetramine at 90°C). The properties of obtained products were examined using scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, X-ray diffraction, room temperature photoluminescence and electron paramagnetic resonance spectroscopy. Chemical synthesis method produced different morphology compared to heating of Zn and ZnO:graphite. In the former case, straight rods are obtained, while in the latter case ZnO tetrapod structures are formed. The ZnO tetrapods, both from Zn and ZnO:graphite, exhibit similar photoluminescence spectra with UV peak and characteristic broad green emission but they have different EPR spectra. The EPR signal g≈1.96 is clearly visible in ZnO tetrapods synthesized from ZnO:graphite, while it is at noise level in ZnO tetrapods synthesized from Zn. Therefore, it can be concluded that the type of intrinsic defects in ZnO nanostructures is strongly dependent on the fabrication conditions, and that the green photoluminescence is not necessarily related to g≈1.96 EPR peak which is commonly assigned to shallow donors.