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

In this study, Er-doped CoAlO nanocrystals (NCs) were synthesized co-precipitation. All the NCs were crystallized in the form of a single phase with a spinel structure and Er ions replaced Al ions in the formation of the CoAlErO alloy structure. The optical characteristics of the Er ion-doped CoAlO 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 CoAlO pigment (using lasers emitting at wavelengths of 413 and 978 nm) has been studied. The values of Judd-Oflet intensity parameters (, , and ) 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 , , and 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 Er ions increases. The highest value of the parameter, when compared with and , 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.