Face-centered-cubic (fcc) and hexagonal-close-packed (hcp) phases of cobalt monoxide (CoO) nanostructures are prepared using thermolysis route at the same reaction temperature 296 °C with synthetic approach conditions. These nanostructures show mixture of nearly spherical and nanoflake morphologies. The structural phases of these nanostructures transform to spinel-Co₃O₄ by application of heat or Raman excitation laser beam power. The absorbance spectra of fcc and hcp-CoO and Co₃O₄ nanostructures yield significantly higher values of band gap which can be explained by electron confinement. Such results provide new opportunities for optimizing and enhancing the properties and performance of cobalt oxide nanomaterial.

Electronic structure and optical band gap of CoFe₂O₄ thin films grown on (001) oriented LaAlO₃ have been investigated. Surprisingly, these films show additional Raman modes at room temperature as compared to a bulk spinel structure. The splitting of Raman modes is explained by considering the short-range ordering of Co and Fe cations in octahedral site of spinel structure. In addition, an expansion of band-gap is observed with the reduction of film thickness, which is explained by the quantum size effect and misfit dislocation. Such results provide interesting insights for the growth of spinel phases.

Cobalt monoxide (CoO) nanocrystals were synthesized by thermal decomposition of cobalt oleate precursor in a high boiling point organic solvent 1-octadecene. The X-ray diffraction pattern and transmission electron microscopy studies suggest that pure face-centered-cubic (fcc) phase of CoO can be synthesized in the temperature range of 569–575 K. Thermolysis product at higher synthesis temperature 585 K is a mixture of fcc and hexagonal-closed-packed (hcp) phases. These nanocrystals are single crystals of CoO and exhibit mixture of two types of morphologies; one is nearly spherical with 5–25 nm diameter, and other one is 5–10 nm thick flake. The pure fcc-CoO nanocrystals show enhanced, and mixture of fcc- and hcp-CoO nanocrystals show reduced antiferromagnetic ordering temperature. Such results provide new opportunities for optimizing and enhancing the properties and performance of cobalt oxide nanomaterials.