Effect of Cu²⁺ doping on the structural, electronic and optical properties of ZnAl₂O₄: A combined experimental and DFT+U study

Using the sol-gel technique, undoped and Cu²⁺ doped ZnAl₂O₄ samples were prepared at 0≤x≤1.24 Cu²⁺ percentages. X-ray diffraction (XRD) analysis confirmed that the prepared samples were cubic and that there was no phase segregation. Energy dispersive X-rays (EDS) was then used to investigate and confirm the purity of Zn, Al, O and Cu samples with no other characteristic peaks. From the XRD spectra of Cu²⁺ at different concentrations, Cu²⁺ doping was found not to lead to significant lattice distortion an observation that was corroborated with density functional theory (DFT) calculations. As the Cu²⁺ concentration was increased a slight shift to higher angles was recorded in XRD analysis, suggesting a larger ionic radius atom substituted by a smaller ionic radius atom. At 283 nm excitation, the doped samples were found to emit in the blue region (at 425 nm and 480 nm) of the color spectrum. Theoretically using DFT with the Hubbard correction term U, the doped system was found to emit at 435 nm which is within the emission range observed experimentally. From the calculated substitutional energies, O_vac was found to have negative formation energies indicating ease of formation form in ZnAl₂O₄. The binding energy for Cu_Zn+O_vac was negative indicating that these two point defects don't exist as pair inside ZnAl₂O₄.