Tuning Green to Red Color in Erbium Niobate Micro- and Nanoparticles

Tetragonal Er_0.5Nb_0.5O₂ and monoclinic ErNbO₄ micro- and nanoparticles were prepared by the citrate sol-gel method and heat-treated at temperatures between 700 and 1600 °C. ErNbO₄ revealed a spherical-shaped crystallite, whose size increased with heat treatment temperatures. To assess their optical properties at room temperature (RT), a thorough spectroscopic study was conducted. RT photoluminescence (PL) spectroscopy revealed that Er³⁺ optical activation was achieved in all samples. The photoluminescence spectra show the green/yellow ²H_11/2, ⁴S_3/2→⁴I_15/2 and red ⁴F_9/2→⁴I_15/2 intraionic transitions as the main visible recombination, with the number of the crystal field splitting Er³⁺ multiplets reflecting the ion site symmetry in the crystalline phases. PL excitation allows the identification of Er³⁺ high-energy excited multiplets as the preferential population paths of the emitting levels. Independently of the crystalline structure, the intensity ratio between the green/yellow and red intraionic transitions was found to be strongly sensitive to the excitation energy. After pumping the samples with a resonant excitation into the ⁴G_11/2 excited multiplet, a green/yellow transition stronger than the red one was observed, whereas the reverse occurred for higher excitation photon energies. Thus, a controllable selective excited tunable green to red color was achieved, which endows new opportunities for photonic and optoelectronic applications.