Color tunable up-conversion emission from Er³⁺:Y₂O₃ nanoparticles embedded in PMMA matrix

Yttria (Y₂O₃) nano-powders doped with various concentrations of Er³⁺ were synthesized using thermal decomposition method and annealed at 800 °C. Annealed powders were doped in PMMA with 2.0 weight% of MMA by free radical polymerization. The crystalline structures of the powders and those embedded in PMMA matrix were identified as the cubic (BCC) Y₂O₃ phase using X-ray diffraction technique. According to the Scherer equation, the average crystallite sizes were found to be 32 ±3 nm and 20 ±3 nm for the powders and those embedded in the PMMA matrix, respectively. The transmission electron micrographs of samples were also taken and found to be consistent with the results obtained using X-ray diffraction measurements. Upconversion emission spectra of the Y₂O₃:Er³⁺ powders were measured in the 400–850 nm wavelength range under the excitation of a diode laser operated at 975 nm and, compared with the results obtained for Y₂O₃:Er³⁺ in PMMA. Spectral profiles of the emissions remain unchanged for different Er³⁺ concentrations and excitation powers. The doping of the nanopowders into the PMMA matrix did also not change the spectral profile, but only enhanced the intensity of the upconversion emission. Measurements of the emission intensity dependence on the pumping power under atmospheric conditions were performed to provide a possible explanation for the mechanisms responsible for the upconversion luminescence. Relative intensities for all the luminescence transitions were found to vary with Er³⁺ concentration resulting in the possible control of both color tuning and the color quality parameters.