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

Tetragonal ErNbO and monoclinic ErNbO micro- and nanoparticles were prepared by the citrate sol-gel method and heat-treated at temperatures between 700 and 1600 °C. ErNbO 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 Er optical activation was achieved in all samples. The photoluminescence spectra show the green/yellow H, S→I and red F→I intraionic transitions as the main visible recombination, with the number of the crystal field splitting Er multiplets reflecting the ion site symmetry in the crystalline phases. PL excitation allows the identification of Er 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 G 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.