How to Prepare the Brightest Luminescent Coatings?

We address here the question of studying the parameters affecting the brightness of luminescent nanoparticulate coatings, among which are the absorption rate, the internal quantum yield of the phosphor nanoparticles, and the extraction factor of the emitted light in a solid angle perpendicular to the substrate. Experimental investigations are achieved on spray-deposited YVO₄:Eu particles, a system whose synthesis and properties are well documented so that particles of different sizes and microstructure can be considered. This allows a quantitative evaluation of the factors affecting film brightness. Considering a film made from raw colloidal particles, this work shows that its brightness is limited by a factor of 5 due to altered quantum yield of nanoparticles, a factor of 1.75 by dielectric effects and a factor of 2.4 by light extraction issues. This investigation, through providing quantitative evaluations of these different parameters, opens the way toward a possible rational design of inorganic luminescent coatings, with a possible improvement of brightness that could reach a factor of 30 as compared to simple films made directly from colloidal suspensions

Local Environments of Dilute Activator Ions in the Solid-State Lighting Phosphor Y_3–<i>x</i>Ce_<i>x</i>Al₅O₁₂

The oxide garnet Y₃Al₅O₁₂ (YAG), when substituted with a few percent of the activator ion Ce³⁺ to replace Y³⁺, is a luminescent material that is nearly ideal for phosphor-converted solid-state white lighting. The local environments of the small number of substituted Ce³⁺ ions are known to critically influence the optical properties of the phosphor. Using a combination of powerful experimental methods, the nature of these local environments is determined and is correlated with the macroscopic luminescent properties of Ce-substituted YAG. The rigidity of the garnet structure is established and is shown to play a key role in the high quantum yield and in the resistance toward thermal quenching of luminescence. Local structural probes reveal compression of the Ce³⁺ local environments by the rigid YAG structure, which gives rise to the unusually large crystal-field splitting, and hence yellow emission. Effective design rules for finding new phosphor materials inferred from the results establish that efficient phosphors require rigid, highly three-dimensionally connected host structures with simple compositions that manifest a low number of phonon modes, and low activator ion concentrations to avoid quenching

Correlating Local Compositions and Structures with the Macroscopic Optical Properties of Ce³⁺-Doped CaSc₂O₄, an Efficient Green-Emitting Phosphor

Calcium scandate (CaSc₂O₄) substituted with small amounts (<1%) of Ce³⁺ is a recently discovered bright-green-emitting phosphor with favorable light absorption and emission properties and robust temperature stability that make it well-suited for solid-state white-lighting applications. Combined analyses of scattering, solid-state nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), and photoluminescence measurements establish the compositional and structural origins of the macroscopic optical properties of this phosphor material. Simultaneous refinements of synchrotron X-ray and neutron diffraction data of Ce³⁺-doped CaSc₂O₄ enable the average crystal structure to be determined, which is shown to correspond to an exceedingly rigid host structure, as corroborated by density functional theory (DFT) calculations. Such structural rigidity leads to high quantum efficiency, which is optimized by the substitution of as little as 0.5 mol % of Ce³⁺ for Ca²⁺ ions, with higher extents of Ce³⁺ substitution leading to decreased photoluminescent quantum yields. Solid-state ⁴³Ca and ⁴⁵Sc magic-angle spinning (MAS) NMR spectra are sensitive to the effects of the paramagnetic Ce³⁺ dopant ions on nearby atoms in the host structure and yield evidence for local structural distortions. EPR measurements provide direct insights on structures of the Ce³⁺ ions, as a function of Ce³⁺ substitution. The combined scattering and spectroscopic analyses yield detailed new understanding of the local and long-range structures of Ce³⁺-doped CaSc₂O₄, which account for the sensitive composition-dependent optical properties of this important phosphor material