Owing to the unique mechanism of photoelectron storage and release, long persistent phosphorescence, also called long persistent luminescence or long lasting afterglow/phosphorescence, plays a pivotal role in the areas of spectroscopy, photochemistry, photonics and materials science. In recent years, more research has focused on the manipulation of the morphology, operational wavebands and persistent duration of long persistent phosphors (LPPs). These desired achievements stimulated the growing interest in designing bio-labels, photocatalysts, optical sensors, detectors and photonic devices. In this review, we present multidisciplinary research on synthetic methods, afterglow mechanisms, characterization techniques, materials system, and applications of LPPs. First, we introduce the recent developments in LPPs for the synthesis of nanoparticles from the aspects of particle sizes, monodispersity and homogeneity based on the urgent application of bio-imaging. In the later sections, we present the possible mechanisms, which involve the variation of trap distribution during the trapping and de-trapping process, complicated photo-ionization reaction of trap site levels and impurity centers together with their corresponding migration kinetics of carriers. Meanwhile, we emphasize the characterization techniques of defects, used to qualitatively or quantitatively describe the types, concentrations and depths of the traps. This review article also highlights the recent advances in suggested LPPs materials with a focus on the LPPs' hosts and optically active centers as well as their control, tuning and intrinsic links. We further discuss the classification of LPPs based on the different emission and excitation wavebands from the ultraviolet to the near-infrared region along with an overview of the activation mode of afterglow. Afterwards, we provide an exhibition of new products towards diverse application fields, including solar energy utilization, bio-imaging, diagnosis, and photocatalysts. Finally, we summarize the current achievements, discuss the problems and provide suggestions for potential future directions in the aforementioned parts.

Long persistent phosphors—from fundamentals to applications

Garnets have the general formula of A3B2C3O12 and form a wide range of inorganic compounds, occurring both naturally (gemstones) and synthetically. Their physical and chemical properties are closely related to the structure and composition. In particular, Ce3+-doped garnet phosphors have a long history and are widely applied, ranging from flying spot cameras, lasers and phosphors in fluorescent tubes to more recent applications in white light LEDs, as afterglow materials and scintillators for medical imaging. Garnet phosphors are unique in their tunability of the luminescence properties through variations in the {A}, [B] and (C) cation sublattice. The flexibility in phosphor composition and the tunable luminescence properties rely on design and synthesis strategies for new garnet compositions with tailor-made luminescence properties. It is the aim of this review to discuss the variation in luminescence properties of Ce3+-doped garnet materials in relation to the applications. This review will provide insight into the relation between crystal chemistry and luminescence for the important class of Ce3+-doped garnet phosphors. It will summarize previous research on the structural design and optical properties of garnet phosphors and also discuss future research opportunities in this field.

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Ce3+-Doped garnet phosphors: composition modification, luminescence properties and applications.

We use organic cations to template the solution-state assembly of corrugated lead halide layers in bulk crystalline materials. These layered hybrids emit radiation across the entire visible spectrum upon ultraviolet excitation. They are promising as single-source white-light phosphors for use with ultraviolet light-emitting diodes in solid-state lighting devices. The broadband emission provides high color rendition and the chromaticity coordinates of the emission can be tuned through halide substitution. We have isolated materials that emit the "warm" white light sought for many indoor lighting applications as well as "cold" white light that approximates the visible region of the solar spectrum. Material syntheses are inexpensive and scalable and binding agents are not required for film deposition, eliminating problems of binder photodegradation. These well-defined and tunable structures provide a flexible platform for studying the rare phenomenon of intrinsic broadband emission from bulk materials.

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Self-assembly of broadband white-light emitters.

Currently, the major commercial white light-emitting diode (WLED) is the phosphor-converted LED made of the InGaN blue-emitting chip and the Ce3+:Y3Al5O12 (Ce:YAG) yellow phosphor dispersed in organic epoxy resin or silicone. However, the organic binder in high-power WLED may age easily and turn yellow due to the accumulated heat emitted from the chip, which adversely affects the WLED properties such as luminous efficacy and color coordination, and therefore reduces its long-term reliability as well as lifetime. Herein, an innovative luminescent material: transparent Ce:YAG phosphor-in-glass (PiG) inorganic color converter, is developed to replace the conventional resin/silicone-based phosphor converter for the construction of high-power WLED. The PiG-based WLED exhibits not only excellent heat-resistance and humidity-resistance characteristics, but also superior optical performances with a luminous efficacy of 124 lm/W, a correlated color temperature of 6674 K and a color rendering index of 70. This easy fabrication, low-cost and long-lifetime WLED is expected to be a new-generation indoor/outdoor high-power lighting source.

A new-generation color converter for high-power white LED: transparent Ce3+:YAG phosphor-in-glass

Properties of transparent Ce:YAG ceramic phosphors for white LED

We report on long-lasting afterglow phosphorescence at around 500 nm in Ce3+-doped yttrium scandium gallium garnet (YSGG) ceramics with the composition of (Y0.995Ce0.005)3Sc2Ga3O12 prepared by solid-state reaction at 1600 °C. The afterglow luminescence was observed for 1 h after the 440 nm excitation in the Ce:YSGG ceramic prepared under vacuum. However, the afterglow decay time decreased after O2 annealing. Therefore, one of the potential traps can be oxygen vacancies. Based on the correlation between the 5d levels and the conduction band, we present an afterglow mechanism and propose a new persistent phosphor under white light emitting diode (LED) illumination.

https://iopscience.iop.org/article/10.1143/APEX.4.042602/pdf

Afterglow Luminescence in Ce3+-Doped Y3Sc2Ga3O12 Ceramics

Transparent Ce:YAG ceramics were synthesized from the Ce:YAG powder which was produced by co-preparation method of the hydroxides. The Ce:YAG ceramics exhibit a broad emission band peaked at 530 nm due to the 5d 4f transition of Ce 3+ . The transmittances of the samples obtained were 70 ~ 87 % at 800 nm. The absorption coefficient and emission intensity of Ce 3+ were increased with the increase of the thickness. With increasing thickness of the sample, the color coordinates of the Ce:YAG ceramics under 465 nm LED excitation shifted from the blue region to the yellow region with passing nearby the theoretical white point. The highest value of luminous efficacy of the white LED was 73.5 lm/W.

https://iopscience.iop.org/article/10.1088/1757-8981/1/1/012031/pdf

Preparation and Optical Properties of Transparent Ce:YAG Ceramics for High Power White LED

Transparent Ce:GdYAG ceramics were synthesized in the composition of (Gd Y0.999-Ce0.001)3Al5O12 using a co-preparation method of the hydroxides. The maximum value of parallel light transmittance of the sample was 80 % at 800 nm. The samples showed a broad emission band due to the 5d→4f transition of Ce3+. The emission peak wavelength red-shifted from 530 nm to 560 nm with increasing Gd content. The maximum QY values of the samples with different Gd concentration were 85 ~ 95 %. The color coordinates of the Ce:GdYAG ceramics based white LED located near the theoretical white point by optimizing the thickness. The Ra value improved from 65 to 81 with increasing Gd content.

https://iopscience.iop.org/article/10.1088/1757-899X/18/10/102005/pdf

Preparation of Transparent Ce3+:GdYAG Ceramics Phosphors for White LED

Eu2+ and Sm3+ co-doped silicate glasses were prepared under reducing atmosphere in a system SiO2-Al2O3-CaO and the glass-ceramics were obtained by heat-treatment at temperatures between 1000 and 1400°C. Broad emission band were observed between 400 and 550 nm due to the 5d→4f transition of Eu2+, as well as sharp emission peaks at 563, 600, 646 and 713 nm due to the 4f→4f transition of Sm3+ in the samples. Emission color became white under ultraviolet or violet excitation for the glass ceramics heat-treated at above 1100°C. Ceramming temperature dependence of the color coordinates, emission spectra were investigated and discussed with the precipitated crystal phases. We have successfully obtained the glass ceramic phosphor emitting white color, suitable for a white LED without organic resin for power illumination.

Shotaro Nishiura

Papers

Properties of transparent Ce:YAG ceramic phosphors for white LED

Afterglow Luminescence in Ce3+-Doped Y3Sc2Ga3O12 Ceramics

Preparation and Optical Properties of Transparent Ce:YAG Ceramics for High Power White LED

Preparation of Transparent Ce3+:GdYAG Ceramics Phosphors for White LED

Preparation and optical properties of Eu2+ and Sm3+ co-doped glass ceramic phosphors emitting white color by violet laser excitation