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Structural characterization and optical properties of Co3O4 and CoO films

Domenii publicaţii > Fizica + Tipuri publicaţii > Articol în revistã ştiinţificã

Autori: R. Draşovean, S. Condurache-Bota

Editorial: J. Optoelectr. Adv. Mat., 11(12), p.2131-2134, 2009.

Rezumat:

Thin-film coatings based on cobalt oxides are obtained. For depositing cubic spinel Co3O4 and CoO films the sol-gel dipcoating technique was used. The as-deposited films were subjected to different thermal treatments in air and in reducing atmosphere (H2/N2), respectively. Optical absorption spectroscopy, X-ray diffraction and atomic force microscopy (AFM) were used to characterize the samples. The Swanepoel method was employed for the study of the films optical constants
as a function of wavelength. By XRD studies it was established that the annealed films become polycrystalline. The films exposed in air consist in Co3O4 phase, while the films annealed in reducing atmosphere (H2/N2) exhibit the CoO phase.
AFM studies showed uniform films with smooth surface. By exposing the film to H2/N2, the average rugosity increases ten times. After treatment, the cobalt oxide films transmittance values decrease. The decrease is more meaningful when the films are exposed in reducing atmosphere. The UV-VIS spectra of the films exposed in air present an absorption peak at 730 nm, which corresponds to a charge transfer ligand-metal: O2- to Co3+. The peak which corresponds to λ= 690 nm is due
to Co2+. These transitions confirm the existence of Co3O4. Concerning the films annealed in reducing atmosphere (H2/N2),
the band at 594 nm which appears in the spectra is typical for Co2+ in octahedral coordination and ascribed to the 4A2 to 4T1(P) transition. The refraction index presents a normal dispersion, the films exposed in forming gas having higher values than the films exposed in air. The films annealed in a reducing atmosphere feature only direct transitions, representing an internal oxido-reduction process Co3+ to Co2+. The films exposed in air present both direct and indirect transitions. Co3O4 films present an allowed direct interband transition of 1.4-1.5 eV and 2.18-2.23 eV, respectively, while CoO films have an optical band gap energy of 2.2-2.8 eV.

Cuvinte cheie: Sol-gel, Cobalt oxide, Optical constants, Optical band gap