Phase-Dependent Control of Trap Depth and Persistent Luminescence in Strontium Aluminate Phosphors

Summary

This study investigates the standard optical measurements of two persistent luminescent ceramics: SAO-B (blue) and SAO-G (green), such as thermoluminescence (TL) and persistent luminescence (PersL). By controlling electron trap depth, this work innovates 'temperature-resolved' and 'time-resolved' dynamic anti-counterfeiting methods.

Abstract

Persistent luminescent (PersL) materials have garnered significant attention for applications in visualization, signaling, bioimaging, and anti-counterfeiting due to their long-lasting emission after pre-excitation. However, achieving both extended emission durations and stable luminescence remains challenging. In this study, we propose two previously lab-synthesized ceramics: Sr₄Al₁₄O₂₅:Eu²⁺, Dy³⁺, B³⁺ (blue-emitting@490 nm, SAO-B) and SrAl₂O₄: Eu²⁺, Dy³⁺, B³⁺ (green-emitting@520 nm, SAO-G) belonging to strontium aluminate phosphors. This work demonstrates a comprehensive experimental approach involving detailed thermoluminescence (TL) and PersL characterizations. Under various excitation sources (UV and visible light), we identified that SAO-B exhibits deeper traps and longer decay times, whereas SAO-G shows stronger initial emission intensity. Furthermore, the video records two novel 'temperature-resolved' and 'time-resolved' dynamic anti-counterfeiting technologies within a wide time scale from a few seconds to minutes. The provided methodology serves as a valuable reference for researchers aiming to optimize persistent phosphors for security, safety lighting, and dynamic display applications.

Introduction

High-quality anti-counterfeiting applications typically benefit from multicolor emissions generated by mixing several distinct luminescent materials¹, leading to solving challenges such as interface incompatibility, higher costs^2,3, inconsistent quality, which may even increase susceptibility to counterfeiting⁴. Indeed, employing the same matrices/composition suggests simplified fabrication, enhances chemical and physical stability, which provides potential multi-channel of optical signals through wavelength tuning or structural design⁵, thereby improving anti-counterfeiting efficiency⁶. Despite this potential, there has been limited investigation into leveraging the unique phase-dependent luminescent properties of blue and green-emitting strontium aluminate (SAO) materials to create dynamic, time-resolved anti-counterfeiting patterns^7,8.

Blue and green SAO phosphors exhibit characteristic persistent luminescence (PersL) and thermoluminescence (TL) properties governed by Eu²⁺ emission and defect-related trap structures^5,9. Green SrAl₂O₄: Eu²⁺,Dy³⁺ shows an intense, long-lasting afterglow peaking at 510-520 nm, where Eu²⁺ acts as the luminescent center and Dy³⁺ (along with intrinsic defects) provides a broad distribution of electron traps that gradually release charge carriers at room temperature, producing strong PersL. TL studies of this material consistently reveal multiple glow peaks corresponding to shallow and deep traps, confirming that its exceptional persistence arises from thermally activated carrier release over a range of trap depths^9,10. In contrast, blue Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ emits in the 440-490 nm range and typically exhibits a long afterglow when deep traps are present. TL measurements show distinct glow peaks at higher temperatures (350-440 K), indicating deeper trap levels compared with the green phase^9,11. Together, PersL and TL analyses demonstrate that the spectral color, decay behavior, and afterglow duration of these SAO phosphors are controlled by the interplay between Eu²⁺ emission and the trap depth distribution created by co-dopants and lattice defects. Although SAO-B and SAO-G are individually well studied, their phase-dependent pairing has not been explicitly leveraged for dynamic anti-counterfeiting with a single-family chemistry. Addressing this research gap can significantly advance the development of SAO materials for high-performance, security-oriented applications.

This study specifically investigated persistent luminescence (PersL) properties of ceramics: Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺,B³⁺ (blue-emitting@490 nm, SAO-B) and SrAl₂O₄: Eu²⁺,Dy³⁺,B³⁺ (green-emitting@520 nm, SAO-G). Crucially, through detailed thermoluminescence (TL) experiments, one could systematically evaluate the trap depth distributions and luminescent stability of these materials under ultraviolet (UV)^11,12, and visible pre-excitation (e.g., 450 nm)^13,14. Note that B³⁺ promotes phase formation/sintering and modulates defect chemistry/trap distribution in aluminates (often via glass-former/flux behavior), improving PersL stability at moderate heat^5,15. SAO phosphors' signal can be measured between 50-450 K. TL characterizations provide critical insights into the mechanisms underpinning the persistent luminescent behavior¹⁶, revealing deeper electron traps and longer decay durations in SAO-B, alongside stronger initial emission intensities in SAO-G.

Building upon these fundamental insights, we fill the research gap by (i) side-by-side PersL/TL characterization under identical conditions and (ii) protocolized demonstrations of time/temperature-encoded optical tags. We propose novel strategies for 'temperature-resolved' and 'time-resolved' anti-counterfeiting applications. This approach leverages the distinctive, phase-dependent PersL characteristics of the studied materials, demonstrating dynamic emission patterns that evolve predictably over time and temperature. Thus, the comprehensive investigation not only contributes to the essential knowledge on trap-controlled luminescence but also provides a practical framework for optimizing SAO-based materials for advanced technological applications in security labeling, safety illumination, and dynamic displays.

Protocol

NOTE: A schematic overview of the materials and workflow is shown in Figure 1, and the measurement system is shown in Figure 2. Following is the detailed, step-by-step standard protocol for measuring the persistent luminescence (PersL) and thermoluminescence (TL) properties of SAO-B and SAO-G ceramics pre-excited by UV, based on the clinical experimental procedure.

1. Sample preparation and preprocessing

1. Prepare the sample.
   1. PersL samples can be powders or large-sized ceramics and crystals. Measure the powders using the following program: press the pellets of SAO-B and SAO-G under a uniaxial pressure of 3.5 tons for 5 min.
      ​NOTE: Each specimen had a typical mass of ~120 mg for a uniform shape of 10 mm in diameter and 1.0 mm thickness¹⁷.
   2. Remove all traps of the PersL samples by heating the samples. Heating up at 350 K for 10 min clears shallow/surface traps only and leaves deeper traps that do not impact the PersL at room temperature (RT). For full pre-cleaning, heat up at 480 K for 10 min before measurements. Note that this temperature may differ from one material to another.
2. Storage occurs after trap emptying.
   1. Apply a thin, uniform layer of silver paste on the sample holder.
   2. Wait for 10-15 min until the paste is fully dry and the powder is firmly attached.

2. Mounting the sample

1. Mount the holder.
   1. Always use the same screws for the sample holder, matching the positions exactly.
   2. Tighten the screws in a crosswise pattern, going around three times to ensure even pressure.
2. Install the inner chamber.
   1. Insert the inner chamber (with open holes on all sides) onto the sample mount.
3. Attach the outer chamber.
   1. Place the external chamber with the blocked window facing up (for ultraviolet- visible [UV-Vis] excitation).
   2. Keep the valve closed during this process.

3. System preparation and pumping

1. Switch on the pump: Start the vacuum pump with the valve closed.
2. Monitor pump status: When the pump indicator changes from yellow (blinking) to green, open the valve.
3. Pre-pumping: Keep the valve open and wait for 15-20 min to ensure adequate vacuum.
4. Monitor pressure.
   1. During pumping, check the manometer: Red blinking = gas pressure (black scale); Green blinking = pressure (red scale).
   2. Ensure the final pressure is lower than 1 × 10^-3 (mbar) before opening the valve to the chamber.

4. Optical setup and light shielding

1. Install fiber optic and shielding.
   1. Adjust the fiber to optimize emitted light collection.
   2. Cover the whole setup with a black blanket to avoid stray light and background excitation.

5. Cooling and temperature control (for TL)

1. Water cooling: Open the water supply before using the cryostat and compressor.
2. Temperature controller: Switch on the controller. Set heater range (e.g., 50 mW) and target temperature (e.g., 10 K). Then, use the indicated menu, and confirm settings twice.
3. Allow the system to reach the target temperature.
4. Wait for the temperature to drop below 150 K before closing the valve again.
5. Keep pumping to ensure the sample holder stays under vacuum.
   NOTE: At the end of the experiment, the sample may reach a high temperature (~500 K). Always reset the heater to 10 K before finishing. Never turn off the compressor until the temperature is below 350 K to avoid apparatus damage.

6. Excitation and measurement

1. Option A: For persistent luminescence (PersL):
   1. Pre-excitation: Illuminate the sample with UV (e.g., 275 nm UV LED excitation, 2 mW·cm^-2, and for other detailed information, refer to Supplementary Table 1) or visible light for the desired time (typically 5 min)¹⁸.
      ​NOTE: The camera and spectrometer were operated with an integration (exposure) time of 2 s. Detector gain was set to the default sensitivity (not user-adjustable). A gating window of approximately 3 s was applied. The collection distance was 5 cm, with the entrance slit fully open and no grating installed. Background subtraction and dark-frame correction were performed. After excitation, emission intensity was recorded immediately and continuously over time (from seconds to several minutes or hours) using an ICCD camera controlled by the WinSpec32 software package. Other detection and acquisition parameters can be found in Supplementary Table 1. These parameters were constant for all measurements unless otherwise specified in individual figure captions.
   2. Collect PersL: Run PersL measurements on a pre-excited sample. The emission light is collected via an optical fiber by intensified charge coupled device (ICCD) camera cooled at -65 °C coupled with a spectrometer for spectral analysis.
   3. After excitation, record the emission intensity immediately and over time (from seconds to several minutes or hours), using the ICCD camera and detection software system.
2. Option: B For thermoluminescence (TL):
   1. Pre-excitation: As described above, excite the sample with UV or visible light.
   2. Temperature ramp: After excitation, ramp the sample temperature at a controlled rate (e.g., 10 K·min^-1) up to the target temperature (e.g., 470 K).
   3. Continuously record emission (luminescence) spectra as the temperature increases.

7. Finishing and safety

1. Ensure the heater is set to the low temperature set point (e.g., 10 K).
2. Let the temperature drop below 350 K before switching off the compressor. Allow the chamber to reach atmospheric pressure before opening.
3. Disassemble the chamber and sample holder after the system reaches RT.

8. Data handling

1. Analyze PersL decay curves and TL glow curves using detection software.
2. Ensure data is labeled with excitation type, sample, date, and relevant experimental conditions.
   NOTE: Necessarily, before starting, empty all traps by heating the samples at 350 K (the detrapping temperature is related to the materials). Always shield the system from ambient light during sensitive measurements. Label and track all samples for repeatability. Maintain logs of pressure, temperature, and system status for troubleshooting and reproducibility. For maximum reproducibility, always use the same procedure for all samples and note any deviations. For reproducibility, excitation and detection should be controlled, such as intensity, detection distance, and time.

Results

The persistent luminescence (PersL) properties of SAO-B and SAO-G were systematically compared at room temperature after 275 nm UV excitation, as shown in the emission spectra of both compounds (Figure 3A). SAO-B emits blue light peaking at 490 nm; on the contrary, SAO-G emits green, peaking at 520 nm. Both compounds presented outstanding PersL properties and long-lasting emission corresponding to the 5d  4f transition of Eu²⁺. Notably, SAO-B showed a longer decay than SAO-G (Figure 3B).

TL peak corresponds to the 5d4f transition of Eu²⁺ for the two samples. Strong TL signals appear above 300 K for SAO-B and above 250 K for SAO-G (Figure 4). The maximum TL intensity of SAO-B is comparable to that of SAO-G, indicating that UV excitation is efficient for the electronic transition of both samples. In detail, TL glow curves reveal a broad peak centered at 350 K for SAO-B (blue curve), while SAO-G exhibits two distinct peaks at 290 K and 320 K (green curve), which means a broader trap depth distribution. Using Urbach's formula¹⁹, one can obtain :

(E = T_max / 500)

Calculated trap depths are 0.78 eV for SAO-B and 0.62 eV for SAO-G⁵. In general, SAO-B displays deeper electronic traps, resulting in a significantly longer afterglow than SAO-G.

Ultra-long PersL observed at room temperature (RT, ~300 K) is presented in Supplementary Video 1. These SAO ceramic materials have potential applications in anti-counterfeiting applications. A dual-color anti-counterfeiting mark of 'PSL' was made of the SAO-B and SAO-G PersL ceramics (Figure 5, Supplementary Video 1). After UV pre-excitation, this anti-counterfeiting pattern emitted dual color for more than 1 h.

In addition, the video also presents a novel 'temperature-resolved anti-counterfeiting' strategy, utilizing the PersL SAO-G and SAO-B (Figure 6, Supplementary Video 2). These phosphors exhibit distinctive luminescence responses upon thermal stimulation, enabling their integration into security labeling. The application relies on the TL behavior of SAO materials. At room temperature ~300 K, strong PersL is observed. As the temperature slowly increases up to 330 K, systematic changes in luminescence intensity and emission characteristics occur. At 370 K, a significant reduction in PersL intensity is recorded from the green marked 'CHIMIE' compared to the blue marked 'PARIS'. Finally, at 420 K, the persistent emission of green marked 'CHIMIE' is indistinguishable. Only the blue marked 'PARIS' is emitting. With a controlled rising temperature rate, both time and temperature impact the luminescence behavior. This 'time/temperature dependence of optical information storage' property offers a highly secure anti-counterfeiting mechanism.

Figure 1: Scheme of persistent luminescence (PersL) of SAO doped with Eu²⁺, Dy³⁺, B³⁺ ceramic materials. Schematic of SAO-B vs SAO-G showing Eu²⁺ emission centers and the conceptual trap landscape (Dy-related electron traps; B-assisted lattice/defect modulation). The dynamic anti-counterfeiting concept combines time- and temperature-dependent responses within one material family. Please click here to view a larger version of this figure.

Figure 2: Setup of the measurement of the PersL and thermoluminescence (TL) properties of SAO-B and SAO-G ceramics, pre-excited by UV light. Overview of the vacuum chamber/cryostat, UV/visible excitation path, fiber-coupled collection, and detection chain used for PersL imaging, PersL spectra, and TL acquisition. Key checkpoints are indicated: pump indicator green; pressure ≤ 1 × 10^-3 mbar; enclosure light-tight. Please click here to view a larger version of this figure.

Figure 3: Persistent luminescence spectra and decay curves. (A) Persistent luminescence spectra and (B) decay curves of SAO-B and SAO-G phosphors. Emission spectra and decay curves of SAO-B (blue) and SAO-G (green) recorded after 5 min UV pre-excitation at 275 nm (LED; ≈ 2 mW cm^-2). Integration time = 2 s; gating ≈ 3 s; detector gain = default sensitivity; collection distance = 5 cm; slit fully open; no grating; dark-frame subtraction applied. Mean ± SD (n = 3) with 95 % CI shading. Acquisition via ICCD camera and Winspec32-Princeton software. Please click here to view a larger version of this figure.

Figure 4: Thermoluminescence (TL) of SAO-B and SAO-G phosphors. TL intensity vs. temperature (β = 10 K min^-1; 50-450 K) obtained under identical detection settings as in Figure 3. Integration = 2 s; collection distance = 5 cm; background subtraction applied. Curves represent mean ± SD (n = 3) with shaded 95% confidence intervals. Full acquisition parameters are provided in Supplementary Table 1. Please click here to view a larger version of this figure.

Figure 5: Time-resolved anti-counterfeiting using the SAO-B and SAO-G phosphors. The original time-resolved anti-counterfeiting video is shown in Supplementary Video 1. Please click here to view a larger version of this figure.

Figure 6: Temperature-resolved anti-counterfeiting using the SAO-B and SAO-G phosphors. The original time-resolved anti-counterfeiting video is shown in Supplementary Video 2. Please click here to view a larger version of this figure.

Supplementary Table 1: Detection and Acquisition Parameters. Please click here to download this file.

Supplementary Video 1: Time-resolved anticounterfeiting.mp4 Please click here to download this file.

Supplementary Video 2: Temperature-resolved anticounterfeiting.mp4 Please click here to download this file.

Discussion

Successful reproduction of this protocol relies on precise control of several experimental parameters. Uniform sample geometry (pellet thickness ~1 mm and diameter 8-10 mm) ensures consistent excitation and thermal gradients. The vacuum level must be verified at or below 1 × 10^-3 mbar with the pump indicator in the "green" range before excitation begins. Immediate data acquisition (delay < 0.5 s after shutter closure) is essential to accurately capture the early-stage decay kinetics of persistent luminescence (PersL). Constant irradiance at the sample plane (~2 mW·cm^-2 for 275 nm excitation) should be maintained to guarantee reproducible trap filling. The temperature ramp rate for thermoluminescence (TL) measurements must remain stable at 10 K·min^-1 to allow quantitative comparison of glow peaks between different samples. Finally, uniform dark-frame subtraction and normalization across all runs are critical for valid intensity comparison and kinetic fitting.

If early-time PersL signals approach detector saturation, one should reduce either the excitation irradiance or the exposure time while maintaining constant gain. Weak emission after repeated cycling may indicate incomplete trap emptying; in that case, preheat the sample to 420 K for 10 min before the next run to remove residual charges from deep traps. Inconsistent baseline levels between runs typically arise from temperature drift in the sample holder. Then allowing a 5 min equilibration period before excitation minimizes this variation. When low signal-to-noise ratios persist, narrow the collection aperture or add an optical bandpass filter centered at 520 nm to suppress stray UV light.

If visible excitation (450-470 nm) yields negligible emission, increase the exposure time to 5-10 s or return to UV excitation, as Eu²⁺ absorption tails in this range are significantly weaker. The intensity is also related to the kind and amount of the materials in the measurements. Always verify that background correction is enabled in WinSpec32 or other detection software before saving spectra to avoid residual dark noise.

In both SAO PersL phosphors, Eu²⁺ acts as the emitting center via the 4f⁶5d¹ 4f⁷ transition, while Dy³⁺ introduces electron traps that capture and gradually release charge carriers at room temperature⁵. The incorporation of B³⁺ assists lattice densification and modifies oxygen-vacancy environments, leading to slightly deeper trap states and prolonged PersL duration⁵. However, the differences in persistent luminescence (PersL) and thermoluminescence (TL) behaviors between SAO-G and SAO-B are due to their different crystal structure. SrAl₂O₄ crystallizes in monoclinic polymorph lattice parameters: a = 8.447 Å; b = 8.816 Å; c = 5.163 Å; β = 93.42°; space group P21, Z = 4. Sr₄Al₁₄O₂₅ crystallizes in the orthorhombic Pmma space group with a = 24.745 Å, b = 8.473 Å, c = 4.881 Å, Z = 2⁵. This structural differences modulate the local crystal field around Eu²⁺, shifting the 5d energy level relative to the conduction band. In SAO-B with tighter Sr-O coordination, the Eu²⁺ 5d state lies closer to the conduction band minimum, facilitating the generation of deeper, thermally more stable traps (at 0.78 eV) that sustain emission over longer timescales. Conversely, in SAO-G, a slightly larger 5d-CB gap results in shallower traps (at 0.62 eV) that release charges more rapidly, producing intense but shorter-lived afterglow⁵.

This study compares two representative Sr-aluminate phases without systematic variation of dopant concentrations. Future work should include undoped SAO, Eu-only, and Dy/B co-doping series to clarify the individual contributions of each dopant to trap formation and persistent luminescence effect. Thermal quenching above 420 K reduces emission intensity and limits high-temperature readability, suggesting the need for co-dopants with higher trap depths for elevated-temperature operation. Additionally, the optical readout demonstrated here relies on laboratory-grade detectors; portable detectors may require fine calibration to maintain comparable sensitivity.

Traditional dynamic anti-counterfeiting strategies often depend on mixed-material composites that combine multiple phosphors of different colors or decay times⁴. These approaches can suffer from uneven dispersion, optical crosstalk, and degradation of individual components under repeated excitation. In contrast, the phase-dependent SAO approach employed here leverages intrinsic structural differences within a single chemical family, providing reproducible and chemically stable time- and temperature-dependent responses without complex blending. This single-family strategy simplifies ink formulation, enhances compatibility in multilayer devices, and maintains predictable emission dynamics across fabrication batches. While the achievable color range is narrower than that of multi-phosphor systems, the gain in reproducibility and tunable persistence behavior offers clear advantages for practical optical tagging and security applications.

This protocol establishes a reproducible framework for evaluating PersL and TL in SAO-based ceramics, emphasizing critical steps such as vacuum integrity, timing precision, and parameter reporting. The combination of quantitative acquisition parameters and mechanistic discussion provides a foundation for extending this phase-dependent luminescent system to temperature-responsive sensors and dynamic anti-counterfeiting devices. As a perspective, SAO PersL ceramics not only can be embedded in ink for fast proceeding anti-counterfeiting patterns but also can be mixed with other commercial materials for bioimaging^20,21,22 and 3D printing^23,24. The color-tunable PersL strontium aluminate phosphors are expected to contribute to a new generation of optical information labelling and anti-counterfeiting technologies.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
405 nm LED	Thorlabs	N/A	Excitation source for TL measurements
455 nm LED	Thorlabs	N/A	Excitation source for persistent luminescence
Acton SpectraPro 2150i spectrometer	Princeton Instruments	Acton Spectra Pro	Visible monochromator; 300 grooves/mm, centered at 500 nm; coupled with ICCD for TL
Boric acid (H3BO3)	Sigma-Aldrich	10043-35-3	Precursor for B3+ doping in solid-state reaction
Closed-cycle He-flow cryostat	Sumitomo Cryogenics	HC-4E	Used for TL experiments, down to 15 K
Heating plate	Thermo Scientifi	88880004	Used to pre-heat samples for thermal detrapping before QY measurements
Lightfield	Princeton Inc.	N/A	Detection software
Potassium Bromide (KBr)	Sigma-Aldrich	7758-02-3	Used as matrix for pellets (50 mg sample + 300 mg KBr → pressed into 1 cm diameter × 2 mm thickness pellets)
Scientific Pixis 100 ICCD camera	Princeton Instruments (Roper Scientific)	Pixis 100	ICCD Camera; Used for TL, PersL, RL detection (cooled to −65 °C for X-ray measurements)
Sr4Al14O25:Eu2+,Dy3+,B3+ sample	BREVALOR Sàrl	Daniel Rytz prepared	Synthesized in lab by solid-state reaction at 1400 °C using oxides (4 N purity); Eu2+ ~1.0 mol%, Dy3+ ~1.5 mol%, B3+ ~17.5 mol%.
SrAl2O4:Eu2+,Dy3+,B3+ sample	BREVALOR Sàrl	Daniel Rytz prepared	Commercial powder; Eu2+ ~1.5 mol%, Dy3+ ~5.0 mol%, B3+ ~0.9 wt%.
Temperature controller	Lakeshore	Model 340	Attached to cryostat for TL control
Winspec32	Winspec33	N/A	Detection software