Morphological transformations induced by Co impurity in ZnO nanostructures prepared by rf-sputtering and their physical properties

Growth of uniform and vertically well aligned nanorods is a difficult process and becomes more complicated in case of ZnO nanorods on silicon (Si) substrate due to thermal instability of the Si substrate and large lattice mismatch (~ 40%) between the substrate and the ZnO nanorods array. Growth of ZnO nanorods assisted by metal ion via rf-sputtering is a good technique; however, it needs many parameters to be controlled for desired growth and morphology of nanostructures. In this work, we report the morphological transformations of ZnO nanostructured thin film by simply controlling the concentration of Cobalt (Co) impurity in sputtering target. With the introduction of Co ions in ZnO matrix, the initial coalescence grain structure (pyramidal morphology) changes into columnar grains and as the concentration of Co ions increases further, a highly oriented ZnO nanorods array is obtained. The possible mechanism with the help of schematic diagram is also proposed for the morphological transformation of ZnO nanostructures. The vertically aligned nanorods show good optical properties as well as robust ferromagnetism at room temperatures. It has also been observed that with the dopant conc. increasing there was a significant decrease in the band gap energy. The structure and morphology of rf-sputtered nanostructured thin films were investigated by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy and selected area electron diffraction. Interestingly, with Co conc. increasing in ZnO matrix results in decreasing LO modes in Raman spectroscopy. It can have strong influence on the magnetic properties of the material. The good optical and strong ferromagnetic properties of the ZnO nanorods, suggest its possible applications in the fields of lasers, spintronics and medical applications.