Fabrication of ZnO Nanospikes and Nanopillars on ITO Glass by Templateless Seed-Layer-Free Electrodeposition and Their Field-Emission Properties

A simple and direct electrodeposition technique is employed to fabricate ZnO nanospikes and nanopillars on indium-tin oxide glass substrates at 70 degrees C without using any template, catalyst, or seed layer. Both ZnO nanospikes and nanopillars exhibit highly crystalline ZnO wurtzite structure with a preferred (0001) plane orientation in their high-resolution transmission electron microscopic images and X-ray diffraction patterns. The corresponding Raman spectra provide evidence for the presence of defects and oxygen vacancies in these nanostructures, which could produce the photoluminescence observed in the visible region. X-ray photoelectron spectroscopy further indicates the presence of a Zn(OH)2-rich surface region in these ZnO nanostructures and that a higher Zn(OH)2 surface moiety is found for nanospikes than nanopillars. In contrast to the nanopillars with flat tops, the nanospikes with tapered tips of 20-50 nm diameter provide a favorable geometry to facilitate excellent field-emission performance, with a low turn-on electric field of 3.2 V/microm for 1.0 microA/cm(2) and a threshold field of 6.6 V/microm for 1.0 mA/cm(2). The superior field-emission property makes the nanospikes among the best ZnO field emitters fabricated on a glass substrate at low temperature.