Persistent luminescence phosphors with long duration and high emitting intensity have attracted considerable attention for applications in safety signage and energy storage. Herein, we successfully introduce non-equivalent ions Si4+ into Al3+ sites in the garnet phosphor Y3Al2Ga3O12:Ce3+,Yb3+,B3+ by conventional solid-state reaction. The persistent luminescence duration has been dramatically enhanced over 40 h at the 0.32 mcd/m2 threshold value after visible light radiation, almost twice longer than the sample without Si4+. Moreover, the afterglow emission intensity of the persistent luminescence is also improved. We confirm that the synthesized phosphors possess not only deeper trap depth but also wider trap distribution and higher trap density after the cooperation of Si4+. The initial rise approach is used by performing a series of thermoluminescence analyses at various temperatures after 432 nm excitation, which demonstrates the exact trap distribution from 0.47 to 1.11 eV. At the end, the mechanism of the persistent luminescence is depicted using a schematic energy diagram of the vacuum referred binding energy of Y3Al2Ga3O12.
(Sr,Ca)AlSiN3:Eu²⁺ phosphors have been widely used in phosphor-converted white light emitting diodes. Herein, we reported the strong red persistent luminescence in (Sr,Ca)AlSiN3:Eu²⁺ under UV light excitation. The Sr0.8Ca0.2AlSiN3:0.15% Eu²⁺ shows the strongest red persistent luminescence with a peak emission wavelength at ~628 nm and a persistent time of ~9600 s at the 0.32 mcd/m² threshold value. A new persistent luminescence mechanism, which is different to that of SrAl2O4:Eu²⁺,Dy³⁺, was proposed by comparing the thermoluminescence excitation spectrum (TLES) and the photoluminescence excitation spectrum (PLES) of Sr0.8Ca0.2AlSiN3:0.15% Eu²⁺. The electrons are directly excited from 4f ground states to the conduction band or from valence band to conduction band in (Sr,Ca)AlSiN3:Eu²⁺; while, in SrAl2O4:Eu²⁺,Dy³⁺, they are first excited to 5d level and then stimulated thermal process to the conduction band. The effect of Eu²⁺ concentration on red persistent luminescence in (Sr,Ca)AlSiN3:Eu²⁺ were discussed. The proposed mechanism of persistent luminescence can help us to design and find new persistent luminescence materials.
To fabricate white-light-emitting diodes (white LEDs) with high color-rendering index or full light spectrum emission, the discovery of more efficient deep-red emitting phosphor materials is essential. In this paper, we have synthesized a series of Sr2-2xEu2xSi5N8 (0 ≤ x ≤ 1) solid-solution compounds, and have systemically investigated effects of full-range Eu concentration on their luminescence. Their emission band maximum can be largely tuned from 610 to 725 nm by increasing Eu content. Reabsorption at low Eu2+ concentration while both the energy transfer and Stocks shift at high Eu2+ concentration account for this large spectral red-shift. Luminescent thermal quenching performance gets worse with Eu2+ concentration increasing. The compound with x = 0.15 possesses the best crystallinity and the highest luminescence intensity with the peak position around 660 nm, and still maintains 88.5% room-temperature intensity at 400 K, indicating that great potential for the application as a deep-red phosphor.
Yttrium aluminum-gallium garnets with cerium doped is most widely used as green-yellow phosphor in solid state lighting. Extensive research has been performed on this material concerning the luminescent thermal quenching resistance and persistent luminescence. In this paper we find that a negative correlation exists between temperature-dependent luminescence and persistent luminescence with gallium content varying. The correlation originates from the electronic structures which influence both the thermal quenching of luminescence and persistent luminescence. A detailed crystal-field calculation has been performed to understand the peak shifts. In addition, theoretical calculations reveal that oxygen vacancies provide trap levels which implement the persistent luminescence. This material could be used as potential blue-light excited persistent luminescent material, with the after-glow time up to about 1. h with only cerium as the dopant, which is expected to be prolonged by co-doping other elements. This work may be helpful in guiding the discovery of other after-glow materials.
The SrLiAl3N4:Eu2+ phosphor has attracted considerable attention owing to its highly efficient narrow-band red emission. Herein, we report for the first time its red persistent luminescence (PersL) and photostimulated luminescence (PSL). After 254 nm light pre-irradiation, the SrLiAl3N4:0.1%Eu2+ phosphor shows 395 s red PersL at a 0.32 mcd m−2 threshold value and its PSL can still be detected under 980 nm light after 15 days. The thermoluminescence spectra evidence that the shallow trap (0.47 eV) plays a major role in PersL and the deep trap (0.81 eV) is responsible for PSL. The charging process for PersL and PSL is clarified by the thermoluminescence excitation (TLE) spectrum. By the aid of density functional theory (DFT) calculations, we verify that the trap levels are due to N vacancies. The electronic structure diagram (HRBE diagram) of SrLiAl3N4:Eu2+ with traps is constructed to illustrate the mechanism of PersL and PSL. The special feature that PersL and PSL both exist makes SrLiAl3N4:Eu2+ a potential candidate for applications such as anti-counterfeiting and optical information storage.
Warm-color persistent luminescent materials are strongly desired for signage markings and medical imaging in comparison with green or blue counterparts. Herein we report a novel yellow long-persistent phosphor, Nb-doped Sr3SiO5:Eu2+, with a peak wavelength of ∼580 nm and persistence time of more than 14 hours at the 0.32 mcd m−2 threshold value after UV radiation. A combination of thermoluminescence (TL), thermoluminescence excitation (TLE), electron paramagnetic resonance (EPR) measurements and density functional theory (DFT) calculations reveals that the persistent luminescence enhancement is attributed to a significant Nb-induced increase of oxygen vacancies that act as electron trapping centers with appropriate trap depths. Groups of time-dependent color-change images are realized with this material, which has potential applications as anti-counterfeit and indicator marks. This investigation also expands the application of transition metal (TM) ions to the field of persistent luminescence and would motivate further exploration of TM substitutions to design and improve silicate or aluminosilicate persistent phosphors with superior performance.
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