1. Introduction

Monoclinic SrAl₂O₄:Eu²⁺,Dy³⁺, is a persistent luminescent phosphor that was primarily developed for applications such as safety signs or emergency displays and luminescent paints [1]. The commercial production of strontium aluminate is generally done in bulk, using conventional solid state synthesis that involves combining metal oxide reagents, typically in the presence of a flux like boric acid, and heating the mixture to high temperatures (1300°C to 1500°C) for several hours in a mildly reducing atmosphere [2]. These long reaction times and high temperatures tend to produce highly agglomerated, relatively large (20 μm to 100 μm) [3], and polydispersed particles. Size reduction by milling and sieving is then required to obtain smaller particles, although does not often results in a monodisperse product. Additionally, the mechanical milling is shown to negatively impact the phosphor’s optical properties by oxidizing Eu²⁺ to Eu³⁺ as well as the possibility of producing amorphous phases, which both influence the optical properties [4]. Recent research suggests that if monoclinic SrAl₂O₄:Eu²⁺,Dy³⁺ can be prepared as submicron particles, it may be of use for bioanalytical applications such as in vitro diagnostics [5, 6]. These applications require a small particle size to minimize gravitational sedimentation [7] and an increase in surface-area-to-volume ratio for optimal binding of the recognition molecules [8, 9]. Moreover, a small particle size allows effective transport through porous media [10] like membranes in lateral-flow assays [11], dipstick tests, and flow-through assays [12], all of which could use monoclinic SrAl₂O₄:Eu²⁺,Dy³⁺ as an optical reporter for analyte detection [6]. As a result, a process for targeting small particles of SrAl₂O₄:Eu²⁺,Dy³⁺ is extremely desirable [5].

Various synthetic strategies have been investigated for producing alkaline earth aluminate persistent phosphors with particle sizes less than 10 μm. Solution based synthesis methods like; sol-gel [13–15], reverse micelle [2, 16, 17], solvothermal [18, 19], combustion [3, 20–24], spray pyrolysis [25–27], and molten salt methods [28], are often studied because they provide a considerable reduction in particle size of the phosphor precursor materials and a significant decrease in the required reaction temperature, both of which can limit particle size. Laser synthesis has also been shown to reduce particle size by limiting reaction time [29]. Sol-gel [30] and co-precipitation [31] methods, in combination with microwave-assisted heating, also limit crystal growth by using lower reaction temperatures and decreased reaction time. Unfortunately, many of these methods yield significant impurities of hexagonal SrAl₂O₄:Eu²⁺,Dy³⁺ and Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ in the final products [26]. Furthermore, it remains unclear if particles prepared using these alternative synthetic routes have comparable optical properties to the materials made by the more common high temperature solid state methods.

Here, a process is reported that combines reverse micelle synthesis with microwave-assisted heating to achieve nearly phase-pure monoclinic SrAl₂O₄:Eu²⁺,Dy³⁺. This route establishes a solution-based method to prepare persistent luminescent materials where the rapid microwave-based heating and reverse micelle precursors facilitates the reaction of the starting materials to desired product in less than 20 min while also limiting particle size. Comparing the photon excitation and photon emission spectra, the time-gated luminescence, and the thermoluminescence of the solution-based persistent phosphors to an all solid state route indicates only minor changes to the optical properties. Finally, this report highlights the capability of using a low-cost consumer microwave oven that can be adapted to virtually any laboratory enabling the broader adoption of SrAl₂O₄:Eu²⁺,Dy³⁺ or other persistent phosphors as reporter compounds in laboratories that may not otherwise have access to high-temperature solid state synthesis equipment.

2. Experimental

2.1 Precursor synthesis

2.1.1 Reverse micelle preparation

The reverse micelle synthesis used europium (III) nitrate pentahydrate (99.9%), dysprosium (III) nitrate hydrate (99.9%), strontium nitrate (≥99%), aluminum nitrate nonahydrate (≥98%), hexadecyltrimethylammonium bromide (CTAB; ≥98%), heptane, anhydrous 1-butanol, ammonium carbonate, and acetone were purchased from Sigma-Aldrich. Ammonium hydroxide (28%-30%) was obtained from EM Science (Gibbstown, N.J.), and 200 proof anhydrous ethanol from PHARMCO-AAPER. Deionized (DI) water was obtained from a Millipore Milli-Q system.

The precursor particles were prepared with a nominal stoichiometry of (Sr_0.95Eu_0.01Dy_0.04)Al₂O₄ by dissolving the starting material salts in DI water. _[2] The total molar concentration of the aqueous metal salt solution was kept at ≈475 mM, with 150.4 mM Sr(NO₃)₂, 316.7 mM Al(NO₃)₃, 1.7 mM Eu(NO₃)₃, and 7.0 mM Dy(NO₃)₃. An aqueous solution of precipitating agents was prepared with 170 mM ammonium carbonate dissolved in stock 28% to 30% ammonium hydroxide. Two different flasks containing the organic components for the emulsions (n-heptane, CTAB, and 1-butanol) were prepared with weight percentages of 50% n-heptane, 24% 1-butanol, and 26% CTAB.

Two separate microemulsions were then prepared: one by pouring the metal salt solution into a flask containing CTAB/n-heptane/1-butanol, and the other by adding the precipitating agent solution to the remaining flask with CTAB/n-heptane/1-butanol [2, 17]. The ratio of the aqueous solution to the organic CTAB/n-heptane/1-butanol mixture that was used to form a stable emulsion was approximately 1:4 by mass, giving an approximate [H₂O]/[CTAB] molar ratio of ≈15. The two different emulsions were stirred for at least 20 min until no clumps of CTAB remained and each emulsion appeared to be a homogenous, transparent solution. Formation of these emulsions is apparently endothermic, and placing the suspensions on a hot plate at 37°C helped dissolve the CTAB. The metal salt emulsion and precipitating agent emulsion were combined into a single flask and placed on a magnetic stir plate at room temperature. The solution gradually turned from transparent to cloudy as the reaction proceeded.

After reacting for 24 h, the solution was poured into a separatory funnel. The microemulsion was disrupted by addition of a 50% v/v ethanol/water solution at a volume ratio of 1:1 of 50% ethanol to microemulsion solution. The contents of the separatory funnel was stirred and allowed to sit for at least 10 min to allow phase separation to occur. After isolating the precursor particles, they were washed at least three times with 50% ethanol/water to remove excess ammonia, dissolved salts, and organics. The particles were washed a final time in acetone, and dried at 100°C for 24 h. Before reaction of the precursor powders in the microwave, a 4 wt% boric acid (Sigma-Aldrich, 99.99%) flux was added to the starting material [17].

2.1.2 All solid state preparation

For comparison, homogeneous polycrystalline powders were synthesized via an all solid state synthesis with starting reagents of SrCO₃ (Alfa Aesar, 99.9%), Al₂O₃ (Sigma-Aldrich, 99.99%), Eu₂O₃ (Sigma-Aldrich, 99.99%), and Dy₂O₃ (Sigma-Aldrich, 99.99%) and ground thoroughly with an agate mortar and pestle for approximately 30 min. A 4 wt% flux of boric acid (Sigma-Aldrich, 99.99%) was added prior to heating.

2.2 Synthesis procedure

2.2.1 Microwave-assisted heating

The reverse micelle and all solid state starting materials were both reacted using microwave-assisted heating. The samples were loaded into a 5 mL alumina crucible (AdValue Tech) that was centered in a larger 50 mL alumina crucible (AdValue Tech) with 6.5 g of activated carbon (Darco, 12 mesh-20 mesh, Sigma-Aldrich) packed into the annular space and covered by an alumina disk (AdValue Tech) to maintain a reducing atmosphere produced by the hot carbon. Heating by a commercial microwave (Panasonic NN-SN651B, 1200 W) produced phase-pure materials with optimal luminescence intensity by using a two-step heating process comprised of an initial heating step at 960 W for 9 min and a second step at 480 W for 5 min. The resulting phosphor powders were first ground by hand with an agate mortar and pestle and then sieved using a 3 in. sieve shaker to a 325 mesh size (Cole-Parmer; Performer III) to break up any agglomerates prior to characterization.

2.2.2 High temperature furnace heating

A series of different compounds were prepared using the traditional high temperature furnace heating route as a standard. Separate samples that used started materials from the reverse micelle preparation as well as from the all solid state preparation were heated at 1300°C for 5 hours with a heating and cooling ramp rate of 3°C/min in a reducing atmosphere of 5% H₂/95% N₂. The resulting phosphor powders were first ground by hand with an agate mortar and pestle and then sieved using a 3 in. sieve shaker to a 325 mesh size (Cole-Parmer; Performer III) to break up any agglomerates prior to characterization.

2.3 Characterization

The composition of the reverse micelle precursors was analyzed prior to the microwave-assisted reaction using Inductively Coupled Plasma-Optical Emission Spectroscopy (Agilent 725 ICP-OES) to determine the exact ratios of Sr to Al. Samples were prepared by digestion in HF and HNO₃ for analysis. The phase purity of the reacted powders was then verified via powder X-ray diffraction using a PANalytical X’Pert PRO diffractometer at room temperature with an average wavelength of λ = 1.5406 Å. Phase purity and crystallographic information were determined by Rietveld refinement, which was completed using the General Structure Analysis System (GSAS) [32, 33]. Particle size analysis was carried out by mixing 50 mg of sample in 5 mL of ethanol and sonicating for 2 h; laser diffraction spectroscopy (Malvern Mastersizer 2000) was used to determine equivalent spherical diameter. Measurements were collected five times with a 30 s integration and averaged together to provide the 50% diameter volume percentage (d_0.5). A JOEL JSM-6330F field emission Scanning Electron Microscope (SEM) was used to visualize particle size distribution at 200 × magnification with a beam focus and accelerating voltage of 15 eV and an emission current of 12 μA. An AMETEK EDAX Octane Pro energy dispersive X-ray spectroscopy (EDS) established the semi-quantitative compositions of the reverse micelle and solid state materials. Samples were coated with 25 nm of carbon to limit charging in the SEM. All crystal structure drawings were created using VESTA [34].

2.4 Optical properties

Photon excitation and emission spectra were collected using a PTI QuantaMaster 400 equipped with a 75 W Xe steady state lamp (PTI Instruments). Temperature dependent photoluminescence, luminescence lifetime decays, and thermoluminescence (TL) were carried out using a Janis cryostat (VPF-100) to control the temperature. Temperature dependent photoluminescence was measured between 80 K and 500 K in 30 K intervals. Temperature dependent luminescence lifetime decays were measured by first irradiating the sample for 10 min at 365 nm followed by a 15 sec delay after shutting off irradiation. TL measurements were collected between 100 K to 500 K. Samples were first heated to 500 K and then cooled to 100 K prior to being irradiated for 10 min at 365 nm using the Xe lamp. A 3 min delay was employed after turning off the lamp before the temperature of the cryostat was ramped at a rate of 5 K/min. The TL emission was collected at λ_em = 520 nm.

3. Results and discussion

3.1. Microwave-assisted, reverse micelle synthesis of persistent phosphors

Reverse micelle synthesis affords the advantages of a sol-gel route where the elements for the phosphor host and the luminescent centers start as ions in aqueous solution and are then precipitated to give a precursor material with a more homogenous distribution of the component elements at small scales. Instead of allowing precipitation to occur freely throughout the solution, as in a conventional sol-gel approach, the reverse micelle synthesis continues to confine the reactions within aqueous nanodroplets formed by micelles [24, 35]. The reaction works by preparing two separate emulsions. One solution is an aqueous solution of dissolved metal salts of the elements needed for the phosphor host. A separate solution is prepared containing an aqueous solution of precipitating agents such as ammonium carbonate or ammonium hydroxide. After combining the emulsions, the micelles collide in solution and exchange reactants, causing precipitation. The precipitation reactions used for preparing strontium carbonate and aluminum hydroxide as precursors for the strontium aluminate host are given by Eq. (1) and Eq. (2). Figure 10 shows the resulting X-ray diffraction pattern confirming the formation of SrCO₃.

A significant volume of work has been published on synthesizing materials and inorganic nanoparticles with reverse micelles [35], examining the effect of surfactants and co-surfactants on the resulting particle size distribution, and modeling the complexities of the reaction kinetics and dynamics. Reverse micelle synthesis of nanoparticles has also been demonstrated to be highly scalable with the successful scale-up of manganese zinc ferrite nanoparticle synthesis from a bench-top setup to a 30 L pilot plant [36]. One established system for reverse micelle synthesis is the use of cetyltrimethylammonium bromide (CTAB) as a surfactant with 1-butanol as a co-surfactant, and an organic such as n-heptane or a similar solvent for the organic component [24, 37, 38]. The CTAB/butanol/heptane system was previously described for the synthesis of Sr₄Al₁₄O₂₅ and other strontium aluminates with a reaction temperature of 1200°C [2, 17]. Another study reported the synthesis of nano-sized SrAl₂O₄:Eu²⁺,Dy³⁺ using a related microemulsion route and high temperature (1000°C) calcination to obtain a mixture of polymorphs, i.e., the product contained both hexagonal and monoclinic space groups [16].

One limitation of these solution-based synthetic methods is that the stoichiometric ratio of metals in the final precursor material is determined by the extent of precipitation of the metal salts, and so it is not necessarily equal to the initial molar concentration of the metal salts in solution. Therefore, measuring the molar concentrations of Sr and Al in the resulting precipitates (precursor powders) is essential to ensure that the reverse micelle protocol results in the correct stoichiometry for SrAl₂O₄. Analysis by ICP-OES (Table 1) revealed that there was an excess amount of Al present in the precursor material when the initial metal salt solution started with an exact 1:2 mole ratio of Sr to Al salts. Hence, two additional solutions were prepared with excess Sr(NO₃)₂ to account for the presence of the additional Al. As shown in Table 1, using 5% excess Sr(NO₃)₂ in the reverse micelle synthesis improved the molar ratio while 10% excess Sr(NO₃)₂ produced the exact, desired stoichiometry. Therefore, all subsequent reactions used samples that started with 10% excess Sr(NO₃)₂ to ensure the formation of the stoichiometric SrAl₂O₄:Eu²⁺,Dy³⁺.

Table 1. ICP-OES of reverse micelle precursors establishing the amount of excess Sr(NO₃)₂ required to produce the desired stoichiometric ratios of Sr:Al.

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Once the starting reverse micelle precursor powders are prepared, they must be reacted to achieve the final product. The most common method for reaction is to perform high temperature calcination or sintering (1300 °C to 1500 °C) under a reducing atmosphere (5%H₂/95%N₂). This heating environment can require reaction times up to 24 hours and may produce large crystal growth and particle agglomeration. Instead, rapid microwave-assisted heating has also been shown as an effective method to produce high quality inorganic phosphors [23]. The advantage of microwave heating is that the time required to produce a pure phase product can be reduced to less than 20 minutes, limiting crystal growth and agglomerate formation. In this approach, microwave heating programs must first be optimized to ensure ideal distribution of heat throughout the material. A program that is too long may melt the starting materials, while heating for too little time will produce inhomogeneous, impure products. Once a program is determined, in this case, a two-step process with an initial heating step and then a holding step as described in the experimental section, numerous batches from the same starting precursors can be prepared with identical results. To illustrate the reliability and consistency of the microwave-assisted heating, five batches, each consisting of ≈50 mg of the reverse micelle precursor, were reacted and examined via powder X-ray diffraction. Figure 1 shows the phase forms in the desired monoclinic space group P2₁ (No. 4), and was produced as a major phase with a minor Al₂O₃ impurity, likely from the alumina crucible. This impurity can be disregarded because it lacks any optical properties that could have an impact on the optical properties of strontium aluminate. The diffraction patterns show that the microwave-assisted synthetic route is consistent in converting the precursor material to the desired product repeatedly, without any indication of hexagonal SrAl₂O₄:Eu²⁺,Dy³⁺ or other related strontium aluminates.

 

Fig. 1 Powder X-ray diffraction patterns of microwave-assisted reverse micelle SrAl₂O₄:Eu²⁺,Dy³⁺ showing batch-to-batch consistency of the microwave heating process. Black is the calculated pattern. [39] “+” is a minor Al₂O₃ impurity.

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To compare the reverse micelle synthesis pathway with conventionally prepared materials, a sample was also made using the all solid state method by weighing out stoichiometric amounts of the respective oxide reagents and mixing them by hand grinding with a mortar and pestle. This all solid state sample was reacted following the same microwave-assisted heating protocol as the reverse micelle sample to also yield nearly phase-pure material. Elemental composition was analyzed by SEM-EDS to determine that no elemental impurities were present in either product. A semi-quantitative EDS analysis of the reverse micelle sample estimates the mole ratio between Sr and Al to be approximately 1:2.6 while the solid state sample is 1:2.3. The excess Al measured in the EDS is likely from the Al₂O₃ impurity also detected by powder X-ray diffraction.

Because impurities like hexagonal SrAl₂O₄ and Sr₄Al₁₄O₂₅ are often present in the products of these solution based methods, Rietveld refinements of the reverse micelle (Fig. 2(a)) and the all solid state (Fig. 2(b)) prepared materials via microwave-assisted heating were conducted. The refinement data and associated crystallographic parameters are listed in Table 2 and Table 3. The refinements show excellent agreement with the observed data, supporting there are no impurities beyond Al₂O₃.

 

Fig. 2 (a) Reverse micelle and (b) solid state synthesis Rietveld refinements show the reverse micelle is comparable to the traditional solid state synthesis. Black circles are for observed data, solid lines are the refined pattern, and “+” is the Al₂O₃ impurity.

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Table 2. Rietveld refinement results for reverse micelle and solid state synthesis of SrAl₂O₄:Eu^2+,Dy³⁺ using powder X-ray diffraction.

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Table 3. Crystallographic results as determined by Rietveld refinement of powder X-ray diffraction

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SrAl₂O₄:Eu²⁺,Dy³⁺ crystallizes in monoclinic space group P2₁ with two independent crystallographic Sr²⁺ sites, both at Wyckoff position 2a, that are 7-coordinated to oxygen (six of which are crystallographically independent) [40]. There are also four crystallographically independent Al³⁺ forming [AlO₄] tetrahedra connected in a three-dimensional, corning-sharing framework [40]. The refined unit cell of the reverse micelle product is visualized in Fig. 3 with the [AlO₄] tetrahedra and the two crystallographically independent [SrO₇] polyhedral highlighted. The similarity between the crystal structures is confirmed by comparing the refined unit cell parameters and atomic positions, which are nearly identical regardless of synthesis method. Further, the two synthesis methods produce nearly identical polyhedral unit volumes indicating a consistent local composition. In fact, the difference in polyhedral volumes between the two synthetic pathways for [Sr(1)O₇] is only ≈0.348 ų with the reverse micelle being slightly larger whereas the polyhedral volumes of [Sr(2)O₇] is even closer, differing by only 0.017 ų. Combining the information obtained from ICP-OES, SEM-EDS, and powder X-ray diffraction, it can be concluded that the reverse micelle synthesis produces a nearly (structurally) identical monoclinic SrAl₂O₄:Eu²⁺,Dy³⁺. Finally, to substantiate that microwave heating generates equivalent samples to conventionally prepared products, a separate set of compounds were reacted using high temperature furnace heating and analyzed using the same techniques (Fig. 11 and Table 5 and Table 6). These data reveal that the traditionally prepared samples are in excellent agreement with the compounds prepared using microwave-assisted heating.

 

Fig. 3 The two independent polyhedra (a) [Sr(1)O₇] and (b) [Sr(2)O₇] are shown to the left of the unit cell (c) of SrAl₂O₄ in the monoclinic space group P2₁ with the [AlO₄] tetrahedra highlighted.

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3.2 Particle size analysis

The use of solution-based routes can generate nano-sized precursor powders and allow shorter reaction times leading to a decrease in product’s particle size [41]. This is advantageous for materials used in bioanalytical applications where particle sizes <500 nm are often targeted because of the increased surface-to-volume ratios [5]. Therefore, targeting particles with a diameter between 250 nm and 1μm is of scientific importance. To achieve this size range in SrAl₂O₄:Eu²⁺,Dy³⁺, extensive post processing is often required, which can cause changes in the optical properties due to the oxidation of Eu²⁺ to Eu³⁺ [4]. Therefore, it is necessary to identify a procedure for preventing destructive post processing. Here, SEM micrographs of the samples synthesized using both approaches were collected at 200 × magnification to demonstrate the difference in overall particle size between the two routes. The reverse micelle method (Fig. 4(a)) clearly produced significantly smaller particles than the all solid state synthesis method (Fig. 4(b)); both remain relatively polydispered

 

Fig. 4 SEM with a 200 × magnification and the scale bar is 50 μm. (a) Reverse micelle synthesis visualizes the overall particle sizes are much smaller than the (b) solid state synthesis. Both starting materials were reacted using microwave-assisted heating.

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Because the SEM micrographs revealed a significant difference in particle size, laser diffraction spectroscopy was used to measure the distribution of particle volume/mass of all particles in a sample. The equivalent sphere diameter is reported for the 50th-percentile (d_0.5), or the percentage of particle sizes at or below that diameter. As shown in Fig. 5(a), the reverse micelle synthesis produces a much greater fraction of small particles compared to the solid state synthesis (Fig. 5(b)). In fact, combining the microwave-assisted heating with the reverse micelle route leads to a nearly 70% decrease in the particle size relative to the all solid state powders reacted in the microwave. Comparing these data to the samples prepared via high temperature furnace shows that conventional heating produces significantly larger particles for both the reverse micelle starting materials (d_0.5 = 14.3 μm) and even larger particles (d_0.5 = 15.2 μm) for the all solid state method. Interestingly, the use of reverse micelle powders in conjunction with reacting in a conventional high temperature furnace shows only a 6% decrease in particle size compared to starting from oxide powders. These data are shown in Fig. 12 in the Appendix. Hence, for SrAl₂O₄:Eu²⁺,Dy³⁺ the advantage of the solution-based route is only fully realized when coupled with microwave-assisted heating. This substantial decrease in particle size limits the need for extensive mechanical milling to reduce the particle size of SrAl₂O₄:Eu²⁺,Dy³⁺ powders, which is ideal because the milling process can generate impurities and ultimately the loss of any optical response [42].

 

Fig. 5 Particle size analysis of the (a) reverse micelle synthesis showing that 50% of the equivalent sphere diameters (d_0.5) are 4.2 μm or smaller and (b) the all solid state synthesis gives a d_0.5 = 14.3 μm.

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3.3 Optical characterization: photoluminescence, persistent luminescence, and thermoluminescence

In light of the significant decrease in particle size measured for the reverse micelle and all solid state produced via microwave-assisted heating method, these two samples are the focus of the ensuing optical property measurements. The excitation spectra of the two syntheses, Fig. 6(a), indicated a nearly identical peak shape. Subsequently exciting the samples at λ_ex = 365 nm produced identical emission peaks with a λ_em,max = 520 nm. The substitution of Eu²⁺ for the two crystallographically independent positions of Sr²⁺ in SrAl₂O₄ should produce two emission peaks, both of which follow the customary 5d→4f transition of Eu²⁺ from the first excited state, ⁷H_j, to the ground state, ⁸S_7/2 [1]. However, in SrAl₂O₄:Eu²⁺ the second emission peak at 450 nm is not observed at room temperature [1]. Calculating the color coordinates for the emission spectra using the 1931 Commission Internationale de l’Eclairage (CIE) diagram validated the reverse micelle (0.3218, 0.5605) and solid state routes (0.3107, 0.5552) are nearly identical, further confirming that photon emission is not affected by the synthesis method (Fig. 6(b)).

 

Fig. 6 (a) Excitation and emission spectra of solid state (SS) and reverse micelle (RM) showing λ_em,max = 520 nm for both synthesis pathways. (b) CIE diagram illustrating the calculated color coordinates have nearly identical visible emission.

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Although the observed steady state photoluminescence is nearly identical, further inquiry into potential optical differences was necessary. Typically, temperature-dependent luminescence is conducted on phosphors to determine their ability to maintain emission intensity when being heated to high temperatures [43, 44]. In the case of strontium aluminate, determining quenching temperature provides insight into the thermal stability of persistent lifetime behavior [1, 45, 46]. Fig. 7 visualizes the temperature dependence of both the reverse micelle (Fig. 7(a)) and the all solid state (Fig. 7(b)). Each sample was heated to 500 K and subsequently cooled to 80 K to ensure all trap states were emptied prior to beginning the measurement. Emission spectra were then taken every 30 K. Interestingly, the reverse micelle prepared sample exhibits an improved quenching temperature (T₅₀), the temperature at which the emission intensity has reached 50% of the initial intensity, by 68 K. This improved T₅₀ could be due to additional trap states being occupied or an increase in the trapping and retrapping of electrons; however, additional research into the origin of this difference is required to identify the exact mechanism. Moreover, the emission seen at approximately λ_em = 450 nm does completely quench prior to room temperature for both the reverse micelle and the all solid state sample [1]. Finally, the revers micelle sample shows a noticable blue shift of the main emission peak and a red shift of the minor emission peak as a function of temperature. This suggests there will be a longer deviation of the emission color between the two samples at low temperature.

 

Fig. 7 Temperature-dependent measurement of (a) RM and (b) SS emission spectra (top) and the relative integrated intensity of the quenching temperature (T₅₀) of the combined emission peaks (bottom)

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Additionally, investigating the temperature dependence of the two synthesis pathways was completed by performing temperature-dependent long lifetime measurements on the reverse micelle sample (Fig. 8(a)) and the all solid state sample (Fig. 8(b)). Each lifetime was measured starting at room temperature and then at 25 K intervals until 423 K with an irradiation time of 10 min and a 15 sec delay before beginning the measurement. This temperature range is of great importance for use in bioanalytical systems, both in vivo and in situ, because cells need to be maintained at a constant temperature of 310 K [47, 48]. Therefore, maintaining a sufficiently long lifetime at temperatures at or above 310 K is largely desirable. Each sample was prepared by first ensuring all trap states were emptied. The lifetime decay was then collected for one hour (3600 sec). The resulting data was fit to a tri-exponential, Eq. (3) where; I is the intensity; A₁, A₂, and A₃ are pre-exponential values; t is time; and τ₁, τ_2, and τ₃ are the lifetime decay components.

 

Fig. 8 Temperature-dependent luminescent decay of the (a) reverse micelle and (b) solid state observed for 3600 seconds. The data fit to a tri-exponential (c) reverse micelle and (d) solid state show the reverse micelle has a longer lifetime then the all solid state sample.

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The three lifetime components are compared to temperature in Fig. 8(c) (reverse micelle) and Fig. 8(d) (solid state). A decrease in lifetime is expected as temperature increases due to the detrapping of deeper traps, and is clearly observed in both samples. However, the reverse micelle exhibits a longer lifetime than the all solid state sample, even at room temperature. Furthermore, the all solid state sample only exhibited two decay components at 423 K, which is consistent with its T₅₀ observed in Fig. 7(b). This interesting result indicates that particle size has a slight influence on the long luminescence lifetime of SrAl₂O₄:Eu²⁺,Dy³⁺.

The origin for the change in lifetimes is most reliably determined by conducting TL measurements to identify and quantify the trap depths, which control the long lifetime in persistent phosphors. These measurements are particularly important because a change in TL emission is known to occur with a change in particle size for nanophosphors such as ZnS and Y₂O₃:Eu³⁺ [49, 50]. Therefore, the reverse micelle and all solid state samples were examined using TL measurements between 100 K and 500 K. The overall shape of the TL spectra (Fig. 8(a)) clearly differ, with a widening of the TL emission spectra and presence of multiple new features in the reverse micelle sample compared to the all solid state sample.

Deconvoluting the TL emission spectra with multiple Gaussian functions indicated there is not only one additional peak in the reverse micelle sample (Fig. 9(b)) relative to the all solid state sample (Fig. 9(c)) but also that the peaks occur in different positions. Each peak location, intensity, and shape is strongly dependent on the irradiation wavelength, heating rate, and time delay between initiating heating and termination of the excitation source [1, 51]. A semi-quantitative comparison between the two materials is best achieved by calculating the trap depth energies (within 5% of the exact trap depth [52]) using the deconvoluted TL spectra following the “peak shape method” [53] (Eq. (4), where E_A is the activation energy (of the trap) T_m is the temperature maximum of the TL emission, and k_B is the Boltzmann constant. The geometric factor (μ_g) is found according to Eq. (5), where σ = T₂ – T_m, T₂ is the low temperature half maximum and ω is the full width at half maximum of TL emission [53, 54]. Because deconvolution provides ω, it is directly calculated to be μ_g = 0.50, which is in agreement with second order kinetics as previously reported for monoclinic SrAl₂O₄:Eu²⁺,Dy³⁺ [54]. The calculated values are provided in Table 4.

 

Fig. 9 (a) TL emission curve cumulative fit of solid state (black) and reverse micelle (red), grey is the observed data. Peaks determined by deconvolution for (b) reverse micelle and (c) solid state. Peaks with traps < 0.4 eV or > 1 eV are dashed lines and peaks between 0.4 eV and 1 eV are solid lines.

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Table 4. Calculated trap depths of reverse micelle and solid state synthesis from deconvolution of TL emission spectra.

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Based on the E_A calculated from the data in Fig. 8 and reported in Table 4, it is possible to determine if these new traps affect the luminescence lifetimes. It is understood that the optimal depth for long persistent lifetime is ≈0.65 eV [55–57] and that materials with trap depths greater than 1 eV are too deep for electrons to be detrapped at room temperature. Conversely, trap depths <0.4 eV are too shallow and will instantaneously detrap at room temperature. Therefore, trap depths calculated to have values 0.4 eV < E_A > 1.0 eV are considered to contribute to a persistent luminescent lifetime [51, 56].

The increase in the number of traps observed likely arises from the presence of surface defects due to the smaller particle size. This new trap has a depth of 0.84 eV should increase the luminescence lifetime; however, the reverse micelle sample also has a significantly deeper trap (trap 5) than in the solid state sample. Moreover, the changes in trap depth observed at 333 K (RM) and 331 K (SS) could be explained by a retrapping and detrapping behavior rather than a deeper trap. It is likely that the combination of these changes negate any potential impact on the persistent luminescence lifetimes. Therefore, synthesizing smaller particles using the reverse micelle microwave-assisted synthesis will not affect most applications of monoclinic SrAl₂O₄:Eu²⁺,Dy³⁺.

4. Conclusions

A synthetic approach combining reverse micelle synthesis with microwave-assisted heating was developed to produce small particles of SrAl₂O₄:Eu²⁺,Dy³⁺. Because this solution-based route does not guarantee the desired starting stoichiometry, ICP-OES was employed to identify that 10% excess Sr(NO₃)₂ is necessary to acquire the correct Sr:Al stoichiometry for SrAl₂O₄. Subsequently, reacting multiple batches of these powders using microwave-assisted heating showed excellent reproducibility, which, in combination with the solution-based reverse micelle synthesis, provides an opportunity to scale-up this synthesis for mass production of these materials. Moreover, a direct comparison of the reverse micelle and an all solid state prepared materials show the crystal structures are nearly identical. The major difference between the two syntheses is in the particle size, with the reverse micelle synthesis producing a 70% decrease in the particle size compared to the all-solid state route. Even with the reduction of the particle size, the observed photon excitation and photon emission spectra were not affected. The temperature dependence of the photoluminescence and long lifetimes were considered and the reverse micelle showed both an improved thermal quenching temperature and longer lifetimes. Finally, thermoluminescence revealed the reverse micelle pathway created an additional trap state that could contribute to a persistent luminescence, which may be due to surface defects because of the smaller particle size. The combination of these results highlights that future persistent luminescent materials like SrAl₂O₄:Eu²⁺,Dy³⁺ can be synthesized by employing soft chemical synthetic routes, like the reverse micelle approach demonstrated here, coupled with microwave-assisted heating to reduce particle size with an enhanced overall optical performance.

Disclosures

The authors declare the following competing financial interest(s): A. Paterson is co-founder of Luminostics, Inc., a company aiming to commercialize smartphone-based diagnostics enabled by persistent luminescent phosphors. A. Paterson and R.C. Willson are the inventors on patent applications involving the use of persistent luminescent phosphors in bioanalytical methods, including a lateral flow assay read-out system.

Appendix A

Powder X-ray diffraction of precursors show formation of SrCO₃ as predicted by balanced chemical Eqs. 1 and 2. Rietveld refinements and resulting crystallographic information of the reverse micelle and solid state synthesis for reactions performed via high temperature furnace heating, Particle size analysis for reactions performed via high temperature furnace heating for reverse micelle and solid state synthesis routes.

 

Fig. 10 Powder X-ray diffraction of precursor material from reverse micelle synthesis. Black is the observed data and blue is the calculated pattern from ICSD [58]

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Fig. 11 (a) Reverse micelle and (b) solid state synthesis performed by high temperature furnace heating of Rietveld refinements. Show the reverse micelle is again comparable to the solid state sample. Black circles are for observed data, solid lines are the refined pattern. And “+” is the Al₂O₃ impurity. An additional unidentified impurity.

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Table 5. Rietveld refinement results for reverse micelle and solid state synthesis of SrAl₂O₄:Eu²⁺,Dy³⁺ using powder X-ray diffraction of synthesis performed using a high temperature furnace

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Table 6. Crystallographic results as determined by Rietveld refinement of powder X-ray diffraction. The samples were reacted using a high temperature furnace.

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Fig. 12 Particle size analysis of samples prepared via high temperature furnace heating (a) reverse micelle synthesis and (b) solid state synthesis have comparable particle sizes.

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Funding

NIAID/NIH (Grants No. U54 AI057156 and 1R43AI118180-01A1); the Welch Foundation (Grant No. E-1264); the Huffington-Woestemeyer professorship; the Department of Chemistry at the University of Houston; the R. A. Welch Foundation through the TcSUH Robert A. Welch Professorship in High Temperature Superconducting (HTSg) and Chemical Materials (E-0001).

Acknowledgments

The contents of this manuscript are solely the responsibility of the authors and do not necessarily represent the official views of the RCE Programs Office, NIAID, or NIH. J. Brgoch acknowledges support through the Grants to Enhance and Advance Research (GEAR) at UH. The authors thank the staff at the Texas Center for Superconductivity at the University of Houston for assistance with EDS and SEM and the staff at the University of Houston ICP Analytical Laboratory and Agilent Facility Center for their assistance in preparing and collecting the ICP-OES data.

References and links

1. J. Botterman, J. J. Joos, and P. F. Smet, “Trapping and detrapping in SrAl₂O₄:Eu²⁺,Dy³⁺ persistent phosphors: Influence of excitation wavelength and temperature,” Phys. Rev. B 90(8), 085147 (2014). [CrossRef]  

2. N. Thompson, “An approach to the synthesis of strontium aluminate based nanophosphors,” (RMIT University, 2012).

3. R. E. Rojas-Hernandez, M. A. Rodriguez, F. Rubio-Marcos, A. Serrano, and J. F. Fernandez, “Designing nanostructured strontium aluminate particles with high luminescence properties,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3(6), 1268–1276 (2015). [CrossRef]  

4. R. E. Rojas-Hernandez, F. Rubio-Marcos, E. Enriquez, M. A. De La Rubia, and J. F. Fernandez, “A low-energy milling approach to reduce particle size maintains the luminescence of strontium aluminates,” RSC Advances 5(53), 42559–42567 (2015). [CrossRef]  

5. T. Lécuyer, E. Teston, G. Ramirez-Garcia, T. Maldiney, B. Viana, J. Seguin, N. Mignet, D. Scherman, and C. Richard, “Chemically engineered persistent luminescence nanoprobes for bioimaging,” Theranostics 6(13), 2488–2523 (2016). [CrossRef]   [PubMed]  

6. A. S. Paterson, B. Raja, G. Garvey, A. Kolhatkar, A. E. V. Hagström, K. Kourentzi, T. R. Lee, and R. C. Willson, “Persistent luminescence strontium aluminate nanoparticles as reporters in lateral flow assays,” Anal. Chem. 86(19), 9481–9488 (2014). [CrossRef]   [PubMed]  

7. G. T. Hermanson, “Chapter 14 - Microparticles and Nanoparticles,” in Bioconjugate Techniques (Second Edition) (Academic Press, 2008).

8. T. Riuttamäki, “Upconverting phosphor technology: exceptional photoluminescent properties light up homogeneous bioanalytical assays,” (Annales Universitatis Turkuensis, Tuku, 2011).

9. N. L. Rosi and C. A. Mirkin, “Nanostructures in biodiagnostics,” Chem. Rev. 105(4), 1547–1562 (2005). [CrossRef]   [PubMed]  

10. R. May and Y. Li, “The effects of particle size on the deposition of fluorescent nanoparticles in porous media: Direct observation using laser scanning cytometry,” Colloids Surf. A Physicochem. Eng. Asp. 418, 84–91 (2013). [CrossRef]  

11. G. A. Posthuma-Trumpie, J. Korf, and A. van Amerongen, “Lateral flow (immuno)assay: its strengths, weaknesses, opportunities and threats. A literature survey,” Anal. Bioanal. Chem. 393(2), 569–582 (2009). [CrossRef]   [PubMed]  

12. A. K. Yetisen, M. S. Akram, and C. R. Lowe, “Paper-based microfluidic point-of-care diagnostic devices,” Lab Chip 13(12), 2210–2251 (2013). [CrossRef]   [PubMed]  

13. T. Aitasalo, J. Hölsä, H. Jungner, M. Lastusaari, and J. Niittykoski, “Sol-gel processed Eu²⁺-doped alkaline earth aluminates,” J. Alloys Compd. 341(1-2), 76–78 (2002). [CrossRef]  

14. P. Zhang, M. Xu, Z. Zheng, L. Liu, and L. Li, “Synthesis and characterization of europium-doped Sr₃Al₂O₆ phosphors by sol–gel technique,” J. Sol-Gel Sci. Technol. 43(1), 59–64 (2007). [CrossRef]  

15. I.-C. Chen and T.-M. Chen, “Effect of host compositions on the afterglow properties of phosphorescent strontium aluminate phosphors derived from the sol-gel method,” J. Mater. Res. 16(05), 1293–1300 (2001). [CrossRef]  

16. C.-H. Lu, S.-Y. Chen, and C.-H. Hsu, “Nanosized strontium aluminate phosphors prepared via a reverse microemulsion route,” J. Mater. Sci. Eng. B 140(3), 218–221 (2007). [CrossRef]  

17. N. Thompson, P. Murugaraj, C. Rix, and D. E. Mainwaring, “Role of oxidative pre-calcination in extending blue emission of Sr₄Al₁₄O₂₅ nanophosphors formed with microemulsions,” J. Alloys Compd. 537, 147–153 (2012). [CrossRef]  

18. M. Karmaoui, M.-G. Willinger, L. Mafra, T. Herntrich, and N. Pinna, “A general nonaqueous route to crystalline alkaline earth aluminate nanostructures,” Nanoscale 1(3), 360–365 (2009). [CrossRef]   [PubMed]  

19. D. Si, B. Geng, and S. Wang, “One-step synthesis and morphology evolution of luminescent Eu²⁺ doped strontium aluminate nanostructures,” CrystEngComm 12(10), 2722–2727 (2010). [CrossRef]  

20. H. Song, D. Chen, W. Tang, and Y. Peng, “Synthesis of SrAl₂O₄: Eu²⁺, Dy³⁺, Gd³⁺ phosphor by combustion method and its phosphorescence properties,” Displays 29(1), 41–44 (2008). [CrossRef]  

21. H. Tanaka, A. V. Gubarevich, H. Wada, and O. Odawara, “Process stages during solution combustion synthesis of strontium aluminates,” Int. J. Self-Propag. High-Temp. Synth. 22(3), 151–156 (2013). [CrossRef]  

22. R. Zhang, G. Han, L. Zhang, and B. Yang, “Gel combustion synthesis and luminescence properties of nanoparticles of monoclinic SrAl₂O₄: Eu²⁺, Dy³⁺,” Mater. Chem. Phys. 113(1), 255–259 (2009). [CrossRef]  

23. A. Birkel, K. A. Denault, N. C. George, C. E. Doll, B. Héry, A. A. Mikhailovsky, C. S. Birkel, B.-C. Hong, and R. Seshadri, “Rapid microwave preparation of highly efficient Ce³⁺-substituted garnet phosphors for solid state white lighting,” Chem. Mater. 24(6), 1198–1204 (2012). [CrossRef]  

24. R. Ranjan, S. Vaidya, P. Thaplyal, M. Qamar, J. Ahmed, and A. K. Ganguli, “Controlling the size, morphology, and aspect ratio of nanostructures using reverse micelles: a case study of copper oxalate monohydrate,” Langmuir 25(11), 6469–6475 (2009). [CrossRef]   [PubMed]  

25. C. Li, Y. Imai, Y. Adachi, H. Yamada, K. Nishikubo, and C.-N. Xu, “One-step synthesis of luminescent nanoparticles of complex oxide, strontium aluminate,” J. Am. Ceram. Soc. 90(7), 2273–2275 (2007). [CrossRef]  

26. W. S. Shi, H. Yamada, K. Nishikubo, H. Kusaba, and C. N. Xu, “Novel structural behavior of strontium aluminate doped with europium,” J. Electrochem. Soc. 151(5), H97–H100 (2004). [CrossRef]  

27. Y. Imai, R. Momoda, Y. Adachi, K. Nishikubo, Y. Kaida, H. Yamada, and C.-N. Xu, “Water-resistant surface-coating on europium-doped strontium aluminate nanoparticles,” J. Electrochem. Soc. 154(3), J77–J80 (2007). [CrossRef]  

28. R. E. Rojas-Hernandez, F. Rubio-Marcos, R. H. Gonçalves, M. Á. Rodriguez, E. Véron, M. Allix, C. Bessada, and J. F. Fernandez, “Original synthetic route to obtain a SrAl₂O₄ phosphor by the molten salt method: Insights into the reaction mechanism and enhancement of the persistent luminescence,” lnorg. Inorg. Chem. 54(20), 9896–9907 (2015). [CrossRef]   [PubMed]  

29. R. Aroz, V. Lennikov, R. Cases, M. L. Sanjuán, G. F. de la Fuente, and E. Muñoz, “Laser synthesis and luminescence properties of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphors,” J. Eur. Ceram. Soc. 32(16), 4363–4369 (2012). [CrossRef]  

30. S. Wu, S. Zhang, and J. Yang, “Influence of microwave process on photoluminescence of europium-doped strontium aluminate phosphor prepared by a novel sol–gel-microwave process,” Mater. Chem. Phys. 102(1), 80–85 (2007). [CrossRef]  

31. W. Shan, L. Wu, N. Tao, Y. Chen, and D. Guo, “Optimization method for green SrAl₂O₄:Eu²⁺,Dy³⁺ phosphors synthesized via co-precipitation route assisted by microwave irradiation using orthogonal experimental design,” Ceram. Int. 41(10), 15034–15040 (2015). [CrossRef]  

32. A. C. V. D. Larson, R.B., “General Structure Analysis System (GSAS),” (2004).

33. B. H. Toby, “EXPGUI, a graphical user interface for GSAS,” J. Appl. Cryst. 34(2), 210–213 (2001). [CrossRef]  

34. K. Momma and F. Izumi, “VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data,” J. Appl. Cryst. 44(6), 1272–1276 (2011). [CrossRef]  

35. L. Qi, “Synthesis of inorganic nanostructures in reverse micelles,” in Encyclopedia of Surface and Colloid Science, 2nd ed., P. Somasundaran, ed. (Taylor & Francis, 2006).

36. S. A. Morrison, C. L. Cahill, E. E. Carpenter, and V. G. Harris, “Production scaleup of reverse micelle synthesis,” Ind. Eng. Chem. Res. 45(3), 1217–1220 (2006). [CrossRef]  

37. M. A. López-Quintela, C. Tojo, M. C. Blanco, L. García Rio, and J. R. Leis, “Microemulsion dynamics and reactions in microemulsions,” Curr. Opin. Colloid Interface Sci. 9(3-4), 264–278 (2004). [CrossRef]  

38. S. Sharma and A. K. Ganguli, “Spherical-to-cylindrical transformation of reverse micelles and their templating effect on the growth of nanostructures,” J. Phys. Chem. B 118(15), 4122–4131 (2014). [CrossRef]   [PubMed]  

39. A. R. Schulze and H. M. Buschbaum, “Zur Verbindungsbildung von MeO: M₂O₃. IV. Zur Struktur von monoklinem SrAl₂O₄,” Z. Anorg. Allg. Chem. 475(4), 205–210 (1981). [CrossRef]  

40. F. Clabau, X. Rocquefelte, S. Jobic, P. Deniard, M. H. Whangbo, A. Garcia, and T. Le Mercier, “Mechanism of phosphorescence appropriate for the long-lasting phosphors Eu²⁺-Doped SrAl₂O₄ with codopants Dy³⁺ and B³⁺,” Chem. Mater. 17(15), 3904–3912 (2005). [CrossRef]  

41. B. B. Srivastava, A. Kuang, and Y. Mao, “Persistent luminescent sub-10 nm Cr doped ZnGa₂O₄ nanoparticles by a biphasic synthesis route,” Chem. Commun. (Camb.) 51(34), 7372–7375 (2015). [CrossRef]   [PubMed]  

42. S. K. Kandpal, B. Goundie, J. Wright, R. A. Pollock, M. D. Mason, and R. W. Meulenberg, “Investigation of the emission mechanism in milled SrAl₂O₄:Eu, Dy using optical and synchrotron X-ray spectroscopy,” ACS Appl. Mater. Interfaces 3(9), 3482–3486 (2011). [CrossRef]   [PubMed]  

43. P. F. Smet, A. B. Parmentier, and D. Poelman, “Selecting conversion phosphors for white light-emitting diodes,” J. Electrochem. Soc. 158(6), R37–R54 (2011). [CrossRef]  

44. N. C. George, K. A. Denault, and R. Seshadri, “Phosphors for solid-state white lighting,” Annu. Rev. Mater. Res. 43(1), 481–501 (2013). [CrossRef]  

45. J. Bierwagen, S. Yoon, N. Gartmann, B. Walfort, and H. Hagemann, “Thermal and concentration dependent energy transfer of Eu²⁺ in SrAl₂O₄,” Opt. Mater. Express 6(3), 793–803 (2016). [CrossRef]  

46. S. H. M. Poort, W. P. Blokpoel, and G. Blasse, “Luminescence of Eu2+ in barium and strontium aluminate and gallate,” Chem. Mater. 7(8), 1547–1551 (1995). [CrossRef]  

47. T. Maldiney, M. U. Kaikkonen, J. Seguin, Q. le Masne de Chermont, M. Bessodes, K. J. Airenne, S. Ylä-Herttuala, D. Scherman, and C. Richard, “In vitro targeting of avidin-expressing glioma cells with biotinylated persistent luminescence nanoparticles,” Bioconjug. Chem. 23(3), 472–478 (2012). [CrossRef]   [PubMed]  

48. M. Sun, Z.-J. Li, C.-L. Liu, H.-X. Fu, J.-S. Shen, and H.-W. Zhang, “Persistent luminescent nanoparticles for super-long time in vivo and in situ imaging with repeatable excitation,” J. Lumin. 145, 838–842 (2014). [CrossRef]  

49. W. Chen, Z. Wang, Z. Lin, and L. Lin, “Thermoluminescence of ZnS nanoparticles,” Appl. Phys. Lett. 70(11), 1465–1467 (1997). [CrossRef]  

50. R. H. Krishna, B. M. Nagabhushana, H. Nagabhushana, N. S. Murthy, S. C. Sharma, C. Shivakumara, and R. P. S. Chakradhar, “Effect of calcination temperature on structural, photoluminescence, and thermoluminescence properties of Y₂O₃:Eu³⁺ nanophosphor,” J. Phys. Chem. C 117(4), 1915–1924 (2013). [CrossRef]  

51. K. Van den Eeckhout, A. J. J. Bos, D. Poelman, and P. F. Smet, “Revealing trap depth distributions in persistent phosphors,” Phys. Rev. B 87(4), 045126 (2013). [CrossRef]  

52. S. W. S. McKeever, Thermoluminescence of Solids (Cambridge University Press, 1985).

53. R. Chen, “Glow Curves with General Order Kinetics,” J. Electrochem. Soc. 116(9), 1254–1257 (1969). [CrossRef]  

54. I. P. Sahu, D. P. Bisen, N. Brahme, R. K. Tamrakar, and R. Shrivastava, “Luminescence studies of dysprosium doped strontium aluminate white light emitting phosphor by combustion route,” J. Mater. Sci. Mater. Electron. 26(11), 8824–8839 (2015). [CrossRef]  

55. T. Matsuzawa, Y. Aoki, N. Takeuchi, and Y. Murayama, “A new long phosphorescent phosphor with high brightness, SrAl₂O ₄ : Eu^2 +, Dy^3 +,” J. Electrochem. Soc. 143(8), 2670–2673 (1996). [CrossRef]  

56. R. Chen and S. W. S. McKeever, Theory of Thermoluminescence and Related Phenomena (World Scientific, 1997).

57. P. Dorenbos, “Mechanism of persistent luminescence in Eu^2 + and Dy^3 + codoped aluminate and silicate compounds,” J. Electrochem. Soc. 152(7), H107–H110 (2005). [CrossRef]  

58. W. Pannhorst and J. L. Ö. Hn, “Zur Kristallstruktur von Strontianit, SrCO₃,” in Zeitschrift für Kristallographie - Crystalline Materials, (1970), p. 455.