Paper NS-TuP27
Comparative Study on the Properties of ZnO Nanowires and Nanocrystalline Thin Films

Tuesday, October 19, 2010, 6:00 pm, Room Southwest Exhibit Hall

Nanostructures made of wurtzite ZnO, such as dots, nanobelts, nanowires, and nanocrystals, have recently attracted attention due to their proposed applications in electronic and optoelectronic devices. Here we present a comparative study on the properties of ZnO thin films containing nanocrystals and nanowires.

ZnO nanocrystalline (NC) films, obtained by the sol-gel process, were deposited from a precursor solution using zinc acetate dehydrate in ethyl alcohol. After a four-layer spin-coating on crystalline substrates, the films were transformed into nanocrystalline films by a thermal treatment at 370 ^oC during 3 hours. ZnO nanowires (NW) were electrochemically grown onto a ZnO seed layer sol-gel spin-coated using a conventional three electrode cell, with the substrate as the cathode, a Zn sheet as the counter electrode and a saturated calomel electrode (SCE) as the reference one. The electrolyte was an aqueous solution of the Zn⁺² precursor (1 mM zinc acetate) and a supporting electrolyte (0.1 M sodium acetate), saturated with bubbling oxygen. The electrodeposition was carried out at 70 ^oC under potentiostatic conditions at two different potential values (-0.900 and -1.000 V vs. SCE) and during 70 min. The initial pH was adjusted to 6.76. Both, NC and NW samples, were deposited onto crystalline quartz substrates covered by a Au or Ag electrode, and ready to use in a quartz crystal microbalance (QCM).

Samples microstructure was characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Optical Diffuse Reflectance Spectroscopy (DRS). XRD measurements show in both cases a typical diffraction pattern of ZnO wurtzite structure. SEM micrographs of NC films have a smooth surface, while the NW sample reveal the presence of nanowires with hexagonal section and diameters ranging from 40 to 100 nm. No specific structure was observed by the DRS measurements on the seed layer, probably due to the fact that these films were not thick enough as required by the technique. For the NW onto seed layer samples, the optical characterization reveals the presence of ZnO with bandgap energy between 3.22 eV (for the ones grown onto Au metallic contacts) and 3.29 eV (for the Ag metallic contacts).

A QCM placed in a vacuum chamber was used to measure the water adsorption of the samples. Water vapour was introduced through a leak valve while a capacitance manometer was used to measure the partial pressure of water in the range 10¹-10⁵ Pa. The mass of water adsorbed on the surface of the quartz crystal was calculated using the Sauerbrey equation. The NW drastic increase of the surface area was revealed through a higher amount of water adsorption.