Shape evolution and optical response of ZnO nanostructures grown on thermally evaporated ZnO and Au thin films

Abstract

ZnO thin films of 100–400 nm thickness and 100 nm thickness Au thin films were deposited on glass substrates by thermal evaporation. Shape control of ZnO nanostructures on these films is achieved by microwave assisted hydrothermal synthesis at 90 °C for 30 min using zinc nitrate and zinc acetate as the precursors. There is a hierarchy of shapes comprising star shaped tetrapods and ball-shapes at the microscale made up of rice grains at the nanoscale on the ZnO and Au thin films respectively, when the precursor is zinc nitrate. The typical dimensions of the nano rice grains are 1 μm length, 100 nm width and tips of the order of 10 nm. In contrast, when the precursor is zinc acetate the nanostructures grow as hexagonal rods on the ZnO films and hexagonal tetrapods on the Au films with dimensions of 1–3 μm length and hexagon side of the order of 100–150 nm. Raman spectroscopy studies indicate the presence of defect related peaks in the nanostructures that are suppressed with increase in thickness of the ZnO films. The intensity of the defect peaks is much higher in the nanostructures grown on the Au films and is dependent on the precursor used to produce the nanostructures. Photoluminescence studies indicate that the ultraviolet-near band edge emission is the strongest on ZnO while defect related emission is prominent on the Au films. Shape control by microwave assisted hydrothermal synthesis is, thus, a facile low temperature method to produce nanostructures with different properties by simply changing the precursor and substrate on which they are grown.