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Measured luminance and visual appearance of multi-color persistent phosphors

In the past decade, several persistent phosphors were intensively studied, such as CaAl(2)O(4):Eu,Nd (blue) and SrAl(2)O(4):Eu,Dy (green). Yellow-to-red emitting persistent phosphors have also been identified. All these phosphors are normally evaluated by measuring their afterglow as a function of time in photometric units. However, the eye sensitivity curve shifts from photopic vision to scotopic vision upon decreasing light intensity, with a corresponding decrease in red-sensitivity. Performance of red-emitting persistent phosphors thus cannot be described with usual photometry. An alternative description is presented, based on recent vision research in the intermediate (mesopic) region and using a unified system of photometry. It turns out that this effect seriously compromises the development of red-emitting persistent phosphors. This is illustrated on a number of commercially available persistent phosphors.

Microencapsulation of Moisture-Sensitive CaS : Eu2 + Particles with Aluminum Oxide

Single-crystal, submicrometer-sized CaS:Eu luminescent particles were synthesized via a solvothermal route, and these moisture-sensitive particles were coated with aluminum oxide using atomic layer deposition (ALD). Photoluminescence (PL) spectra of coated and uncoated particles were compared. They both showed a broad-band PL emission with a maximum of 650 nm. Microencapsulation by aluminum oxide layers did not have a pronounced effect on the intensity of the emission. In situ luminescence measurements during the accelerated aging (80 degrees C, 80% relative humidity) of coated and uncoated CaS: Eu particles were performed. While the uncoated phosphor was largely degraded within 30 h of aging, it was observed that a 20 nm thick aluminum oxide coating dramatically increased the resistance of the luminescent material against moisture, showing the conformity of the Al2O3 coating by the ALD process. Upon degradation, CaCO3 was formed, leading to Eu3+ emission as observed in cathodoluminescence. Finally, the use of these coated particles as a wavelength conversion material in light-emitting diodes was evaluated.

Valence states of europium in CaAl 2 O 4 :Eu phosphors

Persistent luminescent CaAl2O4:Eu2+,Nd3+ powders were prepared by a non-aqueous sol-gel technique. The crystallization of calcium aluminate by heat-treatment of the sols is described in detail. After heat treatment in air, the europium dopant ions are mainly in a trivalent state. For the reduction to the divalent state post-annealing in a reducing nitrogen-hydrogen atmosphere is used. The reduction of europium ions is monitored by photoluminescence and x-ray absorption (XANES) spectroscopy. The degree of reduction is strongly dependent on the annealing temperature. Although for high temperature a strong enhancement of the Eu2+ emission is observed, this also leads to powders with a gray body color.

Red Persistent Luminescence in Ca2SiS4 : Eu , Nd

We studied the persistent luminescence in europium-doped calcium thiosilicate upon codoping with the rare-earth elements neodymium, dysprosium, samarium, and thulium. The strongest afterglow was observed for neodymium codoping. The persistence emission spectrum is dominated by a band at 660 nm, which makes the afterglow color redshifted compared to the steady-state luminescence. Thermoluminescence measurements revealed different glow curves for the two europium lattice sites in the Ca 2 SiS 4 host, suggesting that transfer of charge carriers via the conduction band is of minor importance. Finally, we conclude that the Nd 3+ ions occupy lattice sites in close proximity of the Eu 2+ ions, based on site-selective luminescence measurements on Ca 2 SiS 4 :Eu,Nd.

Extending the afterglow in CaAl 2 O 4 :Eu,Nd persistent phosphors by electron beam annealing

White, well-crystallized and strongly persistent luminescent CaAl2O4:Eu2+,Nd3+ powders were obtained by electron-beam annealing. The electron-beam annealing resulted in a full reduction of Eu3+ to Eu2+, based on the luminescence spectra. Considerable grain growth was observed and the annealed powder crystallized in the monoclinic phase of calcium aluminate. The afterglow intensity was about three times this of commercially available powder and the afterglow duration extended to 10 hours, considering the 0.32mcd/m2 photopic threshold level.