Red and near-infrared emitting phosphors based on Eu3+- or Nd3+-doped lanthanum niobates prepared by the sol-gel route

Abstract

We have evaluated the structural and luminescence properties of Eu³⁺- or Nd³⁺-doped lanthanum niobate systems synthesized via a sol-gel route and containing different dopant contents. XRD analysis revealed that the orthorhombic La₃NbO₇ and monoclinic LaNbO₄ crystalline phases were present in all the samples, regardless of the dopant concentration. The excitation spectra of the samples displayed a broad band due to Nb⁵⁺→O^2- charge transfer; this band was quite sensitive to the increasing Eu³⁺ content. The photoluminescence emission spectra of the samples with a lower Eu³⁺ content showed that Eu³⁺ occupied both crystalline phases. However, when the Eu³⁺ content increased, these ions preferentially occupied the C₂ symmetry sites in the LaNbO₄ host lattice. There was no emission quenching for the Eu³⁺-doped samples with a Eu³⁺ content as high as 20 mol%. The emission spectra of the Nd³⁺-doped samples displayed an intense emission band in the NIR-II biological window under NIR-I excitation, at 808 nm. In the case of the samples with a lower Nd³⁺ content, Nd³⁺ occupied distinct symmetry sites in La₃NbO₇. In contrast, in the samples with a higher Nd³⁺ content, these ions preferentially occupied LaNbO₄ sites. The Nd³⁺ concentration that quenched emission in the Nd³⁺-doped samples was about 2.6 mol%, due to Nd³⁺-Nd³⁺ cross-relaxation processes. On the basis of these findings, the Eu³⁺-doped samples explored herein have promising applications in the lighting field, whereas the Nd³⁺-doped samples have potential use as solid-state lasers and biomarkers.