This review explains that persistent luminescence is controlled by defects that act as charge-carrier traps. The stored energy is later released when trapped electrons are thermally freed and then recombine, producing afterglow emission.

The paper reports that the ultraviolet afterglow decay curves show faster decay when exposed to stronger white light, indicating accelerated release of electrons from deep traps under illumination. It also states that the material’s luminescence is due to electrons stored in energy traps that cannot be released without thermal or light stimulation.

The study states that phosphorescence mechanisms in inorganic solids depend on the propensity of luminescent centers to be oxidized or reduced in the host lattice, and that the traps present in the lattice control the phosphorescent behavior.

“Phosphorescence, also called afterglow, refers to a luminescence with delayed radiative return that is caused by trapping of photo-generated electrons and/or holes in meta-stable states within the band gap.” This definition links the afterglow directly to trapped charge carriers that are later released and recombine radiatively, emitting light.

The article says that to achieve long-afterglow luminescence, traps must effectively capture electrons and have an appropriate trap depth. This links the afterglow directly to electron trapping in the material lattice.

In persistent luminescent phosphors, carriers (electrons and holes) are captured by defects or dopant-related trap levels within the band gap and can remain stored for extended periods until they are released (detrapped) by thermal or other external stimuli, then recombine radiatively on luminescent centers to emit light. For SrAl2O4-based hosts, Eu2+ acts as the luminescent center, while defects and co‑dopants such as Dy3+ and others introduce trap levels that store excitation energy and control the afterglow duration and intensity.

The article explains that persistent luminescence involves the temperature-controlled gradual release of trapped electrons, followed by migration of electrons and recombination to produce the persistent emission. It explicitly describes the emission as being stored in traps and released with thermal energy.

This companion PDF states that the long decay time of persistent luminescence is due to storage of excitation energy by traps, which is released with thermal energy. It says the gradual release of trapped electrons is followed by migration to the activator center through the conduction band, and recombination produces the persistent emission.

“This paper is concerned with the measurement and theory of long-period phosphorescence in solids. The delayed emission is, in such cases, due to the time taken for electrons, which have been raised to the conduction band by the exciting radiation, to reach recombination centres.” The author interprets long-period phosphorescence in terms of electron traps and their eventual recombination leading to light emission.

Persistent luminescence is generally explained by electron (or hole) trapping in defects and their subsequent release followed by radiative recombination at luminescent centers. Using an integrated approach combining charging, afterglow and thermoluminescence measurements, we show that trap filling in Eu,Dy-doped SrAl2O4 is a thermally activated process and that the distribution of filled traps after charging strongly depends on the excitation wavelength. This suggests trapping of the electron close to the ionized Eu center, without full delocalization to the conduction band during the trapping process.

Photoluminescence occurs when a material absorbs light, exciting electrons to higher energy states, which then emit lower-energy photons as they relax. In phosphorescence, the material continues to glow after illumination has stopped because the excited electrons relax only slowly.

“Luminescence in minerals is created by ions, groups of ions, or electronic defects that can absorb energy and emit it as visible light.” The article explains that electronic defects in the crystal lattice act as centers that store and later release energy as luminescence, and discusses impurity ions and defects as activators responsible for afterglow behavior.

“The released electron is then trapped at an O defect by crossing the CB… Dy3+ will enhance the influence of Eu2+ on O vacancies (acting as electron traps), thereby increasing the density and depth of the traps.” The review explains that these trap levels in the crystal lattice store excitation energy and that “the formation of a high density of trap levels by the Dy3+ ions at suitable depths in the SAO crystal lattice… ensures bright and long-lasting phosphorescence at room temperature.”

The article explains that when electrons become trapped within a defect in the atomic or molecular lattice, light is prevented from reemitting until the electron can escape. A defect such as a vacancy can trap an electron, storing its energy until random thermal energy releases it, after which the material emits a photon.

“Persistent phosphors are materials that continue to emit light for minutes to hours after the exciting source has been turned off. This phenomenon is attributed to the presence of lattice defects and impurities that create trap levels in the band gap. Electrons and/or holes are captured in these traps and are slowly released, leading to a delayed recombination and light emission.” The review specifically notes that “defects like vacancies and lattice defects create an appropriate quantity of trap levels.”

For long persistent luminescence (LPL) phosphors based on strontium aluminate, trap distributions introduced by co‑dopants such as Zr4+, Ho3+ and Er3+ regulate the energy storage ability of the materials. The addition of these ions can modify the density and depth of traps so that a significant number of photo‑excited electrons are stored in trap levels after charging and are slowly released, leading to bright yellow‑emitting long persistent luminescence from Mn2+-activated SrAl2O4.

The study finds that native point defects and dopants can act as efficient electron traps for room-temperature persistent luminescence. In particular, it identifies interstitials and substitutional impurities as electron traps, while noting that these traps govern the emission process.

In strontium aluminate glow powders, the luminescent cycle begins with absorption of excitation energy from external light sources such as sunlight or artificial light. Photons with sufficient energy promote electrons in the dopant ions from their ground states to higher excited states, creating electron–hole pairs within the phosphorescent matrix. Dysprosium co‑activator ions create discrete trap states within the band gap that function as temporary storage sites for excited electrons; these trap states of varying depths allow controlled energy release over time scales from minutes to hours, sustaining the afterglow as trapped electrons are gradually released and recombine radiatively.

At the heart of all glow-in-the-dark products lies a fascinating scientific process called photoluminescence. This phenomenon occurs when materials absorb energy from a light source—whether sunlight, LED lights, or fluorescent bulbs—and then release it gradually as visible light. The key to this effect is the use of phosphors, which are substances that can absorb and store energy before emitting it slowly over time.

“Probing the defects and trap distribution in MgAl2O4 nanocrystals through electron spin resonance and thermoluminescence… reveals a wide distribution of trapping levels associated with lattice defects. These traps capture charge carriers during excitation and release them over time, giving rise to long-lasting afterglow.” The study links specific defect types in the spinel lattice to thermoluminescence and persistent luminescence behavior.

When strontium aluminate is activated with suitable dopants such as Eu2+ (for example, Eu:SrAl2O4), it acts as a photoluminescent phosphor with long persistence of phosphorescence. The excitation wavelengths for strontium aluminate range approximately from 200 to 450 nm, and the emission wavelengths from about 420 to 520 nm, producing green and aqua hues with long glow times. The persistent phosphorescence is attributed in the literature to dopant- and defect-related traps in the crystal lattice that capture charge carriers during excitation and slowly release them, enabling emission long after the light source is removed.

When you expose phosphors that have those specific characteristics mentioned above to visible or ultraviolet wavelengths, they absorb the light, which excites the phosphors’ electrons. As the electrons are exposed to light for extended periods of time, they store the energy and move to increasingly higher energy levels. When you stop charging the phosphors (turn the lights off), they slowly drop back down to normal energy levels. That surplus of energy that they just stored is released little by little, which is what causes the glowing effect. The materials with phosphors that check both boxes are zinc sulfide and strontium aluminate.

The emission of light after the excitation source is switched off is known as phosphorescence. Phosphorescence in Eu2+,Dy3+-doped strontium aluminates such as SrAl2O4, SrAl4O7 and SrAl12O19 occurs when suitable elements with active electronic configuration are doped into host materials that can provide trap levels for charge carriers. These phosphorescent materials show long afterglow times and high quantum efficiency because the host lattice and dopants create defects and trap centers that store excitation energy and release it gradually, allowing electrons and holes to recombine and emit light over extended periods.

Tailoring the luminescence properties of strontium aluminate involves engineering the density and energy distribution of traps associated with lattice defects and dopant sites. When many traps are present, a significant portion of photo‑generated carriers detour into long‑lived trap states rather than recombining immediately, which reduces prompt emission but sustains a slower release over time and thus a persistent afterglow. By adjusting synthesis conditions and dopant concentrations, researchers can control how efficiently the material stores excitation energy and how slowly electrons are released from traps to recombine radiatively.

When something glows in the dark, it means the material can absorb light, store that energy, and then slowly release it as visible light even in complete darkness. This process is called photoluminescence, specifically a type called phosphorescence. At the heart of every glow-in-the-dark product are chemicals called phosphors. These are substances that can absorb energy from light and re-emit it over time. When light hits phosphor molecules, their electrons jump to higher energy levels. As these electrons gradually fall back to their normal state, they release the stored energy as light we can see. Phosphorescent materials charge up under normal light and continue glowing in darkness—sometimes for hours. The glow gradually fades as the stored energy depletes.

“It contains defects in its crystal structure, and when it absorbs energy from UV light, electrons become trapped in higher energy states. As these trapped electrons return to their lower energy states, they emit light in the visible spectrum, causing the characteristic glow-in-the-dark effect.” The article explains that in minerals showing phosphorescence, “the crystal lattice… must have specific defects or imperfections that can trap the excited electrons” and that their gradual return produces the afterglow.

When exposed to light, this material essentially soaks it up. Then, when it gets dark, they slowly release this light, which is why they glow. Phosphors are the key ingredients in glow-in-the-dark materials and are substances that have the ability to exhibit phosphorescence. When exposed to light, phosphors absorb energy and then slowly re-emit it over time, resulting in the characteristic glow. High-quality phosphors can store light energy more efficiently and for a longer duration, enhancing the brightness and longevity of the glow.

Strontium aluminate is one of the most efficient photoluminescent phosphors used in glow-in-the-dark materials. When exposed to light, electrons in the dopant ions are excited and some of these electrons become trapped in metastable energy states created by lattice defects or dopant sites. Over time, thermal energy gradually releases the trapped electrons, which then recombine at luminescent centers and emit visible light, producing the characteristic slow afterglow.

That is the basic idea of phosphorescence. Photons hit the material, promoting electrons to a higher energy state. The promoted [electrons] then fall back to the lowest energy state, emitting the energy difference (falling from high to low energy state) in the form of the “glow-in-the-dark” light. The electrons spend time in the electron trap before falling back to the lowest energy level and that is the reason why the material emits light long after the room lights have been turned off, hence “glow-in-the-dark”. Pan et al. (2011) believe that the longevity of the afterglow of their material is due to the many electron traps created by the addition of the Cr3+ ions that are spread throughout a wide spectrum of energy levels.

“Optical measurements reveal that the diverse family of internal lattice defects neither noticeably change the excitonic transitions of CdSe/CdS core/shell nanocrystals nor create internal electronic defects for the nanocrystals.” This work emphasises that not all lattice defects act as traps; only specific defects or dopants that introduce electronic states in the band gap can behave as trapping centers relevant for afterglow.

If you are wondering how this energy is stored, it's stored by putting the electrons of the phosphorus atoms into an excited state. The energy then slowly dissipates in the form of visible light as the electrons relax back to their ground state. Defects in the crystal lattice can act as traps for these electrons, holding them in metastable states. The slow release from these traps is what causes the long-lasting glow seen in many glow-in-the-dark materials.

These materials can absorb energy but then instead of immediately releasing it it slowly releases it over time, continuing to glow even after the energy source is removed and that is why you can sort of charge up, for lack of a better term, glow-in-the-dark materials. Materials [in phosphorescence] the excited electrons once they get pumped up to a higher energy level they get sort of trapped in that higher energy level for a little while before gradually decaying and relaxing down to their ground state. This trapping effect is what allows the phosphorescent materials to actually continue to glow even after you turn the light source off. It's crucial here to reiterate that these materials can store and release energy but they're not a perpetual energy machine; the stored energy is eventually depleted and the glow stops unless they are recharged by light.

The key to persistent glow-in-the-dark behavior is the presence of electron traps associated with defects or dopants in the crystal lattice of the phosphor (for example, in strontium aluminate or zinc sulfide pigments). Electrons excited by ultraviolet or visible light can become trapped in these defect states and are released slowly over time, recombining and emitting photons as they return to lower energy states. Such materials are commonly used in luminous paints and "glow-in-the-dark" plastics and beads, which can be charged by sunlight and show an afterglow lasting minutes to hours depending on the trap depth and concentration.