Evolution of the morphology and optical properties of ZnO nanowires during catalyst-free growth by thermal evaporation

ZnO nanowires were grown on Si(001) substrates by catalyst-free thermal evaporation of ZnO powders in a lateral quartz tube without using a carrier gas. In the first phase of the vapour deposition process, film-like structures were formed on the substrate surface in three-dimensional island growth mode and then rod-like nanostructures nucleated at the vertices of these ZnO islands. The rod-like structures grew further into slender needle-like structures that finally evolved into straight hexagonal prismatic ZnO nanowires having c-axis oriented single-crystalline wurtzite structure. Morphological evolution of the ZnO nanostructures accompanied improvement in the crystal qualities as reflected in the emission characteristics investigated by photoluminescence (PL) spectroscopy, where intensified near-band-edge (NBE) emission was observed as compared with previously reported cases.