A wide variety of synthetic methods have been employed to prepare ZnO colloids and films, some of which are sol-gel chemistry, spray pyrolysis, metal-organic chemical vapor deposition (MOCVD), and cathodic electrodeposition. Despite the technological importance of ZnO as an optical material, and the quantity of literature devoted to its study (more than 600 scientific papers since 1989) it has so far proven to be impossible to prepare 'optically intrinsic' ZnO thin films or nanoparticles - which we define to be films or particles which produce photoluminescence (PL) spectra which are free of trap state emission.

We have recently succeeded in preparing such films using hybrid electrochemical/chemical (E/C) synthetic approach. In this case, because the desired product is a metal oxide, the E/C procedure involves just two steps (as shown schematically below): Zinc metal was first electrochemically deposited at basal-plane oriented graphite electrode surfaces from dilute aqueous solutions; then this deposit was permitted to spontaneously oxidize and dehydrate at open circuit in the pH = 1.0 plating solution.

The deposition of ZnO nanoparticles occurred size-selectively and wurtzite phase ZnO nanocrystallites having mean diameters in the range from 15 to 100 Å were obtained using this approach. Relative standard deviations of the particle diameter for ZnO particle dispersions varied from 25-50%. NC-AFM images for two samples are shown below, (and these can be compared with histograms for these surfaces):

Left - Non-contact atomic force micrographs of ZnO nanoparticles prepared using the E/C method with a deposition time of 50 ms.
Right - Non-contact atomic force micrographs of ZnO nanoparticles prepared using the E/C method with a deposition time of 3s.

Polycrystalline ZnO films of 100-400 Å in thickness were also obtained by depositing larger quantities of zinc metal in the first step of the synthesis. For ZnO particles (dia. less then 80 Å), electron diffraction analysis revealed a preferred orientation for ZnO crystallites where the c-axis of the wurtzite unit cell was oriented perpendicular to the plane of the graphite surface, but x-ray powder diffraction data indicated that this orientational preference was lost when larger quantities of zinc were deposited and ZnO films were obtained.

Luminescence spectra for the ZnO films prepared using this E/C method consisted of a single exciton band near 3.2 eV at room temperature with no deep trap state emission.

At low temperatures (20 K), this exciton band split into a cleanly resolved and fully assignable phonon loss progression, shown above.