The controlled growth of oxygen-deficient ZnO nanowires on ZnO films in terms of diameter, alignment, and density is achieved by controlling the ZnO films, and the structural, optical, and field emission properties related to the oxygen-deficiency are discussed. Planar (2D) and nano-sized (3D) defects due to the large amount of oxygen deficiency are characterized using a high-resolution transmission electron microscopy with the aid of simulations. It is suggested that the two-stage linearity of the Fowler-Nord-heim plot is attributable to the deep-level defects of the nonstoichiometric ZnO nanowires. (C) 2011 Elsevier B.V. All rights reserved.