Ferromagnetic mechanism of (Co, Cu)-codoped ZnO films with different Co concentrations investigated by X-ray photoelectron spectroscopy

• Cobalt/copper-codoped ZnO films show Cu valence from +2 to +1 as the Co concentration level exceeds 1% by XPS.
• The optical bandgap of Zn0.95−xCoxCu0.05O films shows a blue-shift from 3.01 to 3.13 eV with increasing Co concentration.
• The ferromagnetism of Zn0.95−xCoxCu0.05O films is correlated with the valence state of the Cu ions and oxygen vacancies.

Cobalt/copper-codoped ZnO nanoparticles, synthesized with different Co concentrations by a sol–gel method using ethanol as solvent, were studied via XPS. Hexagonal wurtzite structure was found in all samples, with no evidence of any secondary phase. The average crystallite size of the samples was around 20–30 nm, altered significantly with increasing Co concentration. Copper ions and Cobalt ions are indeed substituted into the ZnO lattice at the Zn2+ site, as shown by XRD and XPS. Further studies showed dramatic changes of Cu valence from +2 to +1 as the Co concentration level exceeds 1%, accompanied by a blue-shift of the optical bandgap from 3.01 to 3.13 eV. Ferromagnetism of the Co-doped Zn0.95Cu0.05O thin films was observed and found to be tunable – a phenomenon associated with the valence state of the Cu ions and the existence of some defects like oxygen vacancies in the films.

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