New insights on the optical properties and upconversion fluorescence of Er-doped CoAl 2 O 4 nanocrystals.

In this study, Er-doped CoAl 2 O 4 nanocrystals (NCs) were synthesized via co-precipitation. All the NCs were crystallized in the form of a single phase with a spinel structure and Er 3+ ions replaced Al 3+ ions in the formation of the CoAl 2- x Er x O 4 alloy structure. The optical characteristics of the Er 3+ ion-doped CoAl 2 O 4 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 CoAl 2 O 4 pigment (using lasers emitting at wavelengths of 413 and 978 nm) has been studied. The values of Judd-Oflet intensity parameters ( Ω 2 , Ω 4 , and Ω 6 ) 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 Ω 2 , Ω 4 , and Ω 6 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 3+ ions increases. The highest value of the Ω 2 parameter, when compared with Ω 4 and Ω 6 , 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.