Visualizing the valence states of europium ions in Eu-doped BaAl2O4 using X-ray nanoprobe mapping

This study develops and successfully demonstrates visualization methods for the characterization of europium (Eu)-doped BaAl₂O₄ phosphors using X-ray nanoprobe techniques. X-ray fluorescence (XRF) mapping not only gives information on the elemental distributions but also clearly reveals the valence state distributions of the Eu²⁺ and Eu³⁺ ions. The accuracy of the estimated valence state distributions was examined by performing X-ray absorption spectroscopy (XAS) across the Eu L₃-edge (6.977 keV). The X-ray excited optical luminescence (XEOL) spectra exhibit different emission lines in the selected local areas. Their corresponding emission distributions can be obtained via XEOL mapping. The emission properties can be understood through correlation analysis. The results demonstrate that the main contribution to the luminescence intensity of the Eu-doped BaAl₂O₄ comes from the Eu²⁺ activator and the emission intensity will not be influenced by the concentration of Eu²⁺ or Eu³⁺ ions. It is anticipated that X-ray nanoprobes will open new avenues with significant characterization ability for unravelling the emission mechanisms of phosphor materials.

Highly Efficient Transparent Nanophosphor Films for Tunable White-Light-Emitting Layered Coatings

Bright luminescence in rare-earth (RE) nanocrystals, the so-called nanophosphors, is generally achieved by choosing a host that enables an effective excitation of the RE activator through charge or energy transfer. Although tungstate, molybdate, or vanadate compounds provide the aforementioned transfer, a comparative analysis of the efficiency of such emitters remains elusive. Herein, we perform a combined structural and optical analysis, which reveals that the tetragonal GdVO₄ matrix gives rise to the highest efficiency among the different transparent nanophosphor films compared. Then, we demonstrate that by a sequential stacking of optical quality layers made of Eu³⁺- and Dy³⁺-doped nanocrystals, it is possible to attain highly transparent white-light-emitting coatings of tunable shade with photoluminescence quantum yields above 35%. Layering provides a precise dynamic tuning of the chromaticity based on the photoexcitation wavelength dependence of the emission of the nanophosphor ensemble without altering the chemical composition of the emitters or degrading their efficiency. The total extinction of the incoming radiation along with the high quantum yields achieved makes these thin-layered phosphors one of the most efficient transparent white converter coatings ever developed.