New insights on the optical properties and upconversion fluorescence of Er-doped CoAl₂O₄ nanocrystals

In this study, Er-doped CoAl₂O₄ nanocrystals (NCs) were synthesized via co-precipitation. All the NCs were crystallized in the form of a single phase with a spinel structure and Er³⁺ ions replaced Al³⁺ ions in the formation of the CoAl_2-ErO₄ alloy structure. The optical characteristics of the Er³⁺ ion-doped CoAl₂O₄ NCs were thoroughly investigated by analyzing both the UV-VIS and photoluminescence spectra, using the Judd-Ofelt theory. The effect of Er doping content on the luminescent properties of the CoAl₂O₄ pigment (using lasers emitting at wavelengths of 413 and 978 nm) has been studied. The values of Judd-Oflet intensity parameters (Ω₂, Ω₄, and Ω₆) were determined from the absorption spectra using the least square fitting method. The J-O parameters were calculated and compared with those of other host materials; the values of the Ω₂, Ω₄, and Ω₆ parameters decreased with an increase in Er concentration. This suggests that the rigidity and local symmetry of the host materials become weaker as the concentration of Er³⁺ ions increases. The highest value of the Ω₂ parameter, when compared with Ω₄ and Ω₆, suggests that the vibrational frequencies in the given samples are relatively low. The upconversion fluorescence phenomenon was observed and explained in detail under an excitation wavelength of 978 nm when the excitation power was varied.