Journal of MaterialsVolume 2013 (2013), Article ID 613090, 10 pageshttp://dx.doi.org/10.1155/2013/613090

Research Article

Giant Improvement on the Afterglow of Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ Phosphor by Systematic Investigation on Various Parameters

Hom Nath Luitel, Takanori Watari, Rumi Chand, Toshio Torikai, and Mitsunori Yada

Department of Applied Chemistry, Faculty of Science and Engineering, Saga University, 1 Honjo-machi, Saga 840-8502, Japan

Received 28 November 2012; Accepted 22 January 2013

Academic Editor: Necmettin Maraşlı

Copyright © 2013 Hom Nath Luitel et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Highly intense, long persistent Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ blue-green phosphor with different B³⁺, Eu²⁺, Dy³⁺, and Ag⁺ contents was prepared by solid-phase reaction at various temperatures in reductive atmosphere of 10% H₂ in N₂. The effects of synthesis parameters like calcination temperature and time, calcination environment, effect of stoichiometry of the host composition, and additives like addition of boron and rare earth ions (Eu, Dy) were studied in detail. Results revealed that the phosphor containing ~40 mol% H₃BO₃ showed dense and pure Sr₄Al₁₄O₂₅ phase with higher emission intensity, but in the samples containing less than 20 mol% H₃BO₃ mixed phases consisting of Al₂O₃, SrAl₁₂O₁₉ and SrAl₂O₄ were observed, while in higher H₃BO₃ content, SrAl₂B₂O₇ phases predominated. When the stoichiometry of Al/Sr was 3.7, the best phosphorescence and afterglow were noted. The phosphor containing 4 at.% of Eu and 8 at.% of Dy, and 3 at.% Ag exhibited the maximum initial intensity of 5170 mcd·m⁻² and the longest persistency of greater than 30 hours over the value of 5 mcd·m⁻², higher than the commercial products and applicable for various display applications involving indoor as well as outdoor uses.

1. Introduction

Phosphor materials with long afterglow is a kind of energy storing materials that can absorb UV-visible light from sun light, and gradually release the absorbed energy in the darkness at a certain wavelength [1]. Among the various phosphors, ZnS-based materials doped with Cu or Mn ions were first prepared and applied for various fields [2]; however, the luminescent intensity of these phosphors are not bright enough and the afterglow time is short. Therefore, some radio-active elements such as Co and Pm had to be codoped into the ZnS-matrix so as to prolong the afterglow time. Compared to sulfide-based phosphors, oxide-based phosphors have been recognized as the efficient host materials because of their high quantum efficiency [3], chemical stability, no radio-active radiations, environmental capability, and long persistence of phosphorescence [4].

As far as the development of phosphor is concerned, Eu²⁺ ions were used frequently as an activator ion for various host lattices. This occurs because the 4f⁷ electron configuration of the Eu²⁺ ion shows efficient luminescence owing to the 4f → 5d transition, and the luminescence colors or wavelengths change widely from near UV to red regions depending on the nature of host lattice [5]. As a new phosphor, strontium aluminates doped with Eu divalent ion (SAE) have been investigated as an efficient phosphor that has high quantum efficiency and good stability, indicating their good practical prospects [4–8]. These phosphors have decay time ranging from nanosecond to tens of seconds and were used as important constituents of light emitting devices, fluorescent lamps, plasma display panels, and lamps for medical applications [9]. However, for specific dark vision applications involving signage, blackouts, emergency rescue guidance systems, and luminous watches, the phosphors with appreciable brightness and long persistent time of more than 5 h are fundamental requisites [7].

In order to improve the phosphorescence characteristics, Dy³⁺ is added to SAE and fabrication conditions have been examined [9–15]. For specific dark vision applications, the existing brightness and afterglow phosphorescence have been improved but are not enough and needed more improvements. Fascinated by the techno-commercial applications, many researchers started exploring the dependence of various physico-chemical parameters such as thermal parameters [10], addition of flux [1, 7, 11–15], and effect of compositions [15, 16] on the luminescence and long persistent behavior of these phosphors. But detailed study on the composition of Al/Sr, effects of flux, composition of rare earth ions, charge compensator ions, and heating temperature and time are not well revealed for Sr₄Al₁₄O₂₅ phosphor. Thus, in this paper, the systematic examination regarding composition of starting materials, rare earth ions (Eu²⁺, Dy³⁺), flux (H₃BO₃), charge compensator ions, reduction temperatures/time, and reduction atmosphere on the Sr₄Al₁₄O₂₅ phosphor have been explained in detail.

2. Experimental

Strontium aluminates doped with Eu²⁺ and Dy³⁺ were prepared by the reaction between strontium carbonate (SrCO₃; Aldrich, 99.9%), aluminium oxide (Al₂O₃, Baikalox, AR), europium oxide (Eu₂O₃; Aldrich, 99.99%), dysprosium oxide (Dy₂O₃; Aldrich, 99.99%), and boric acid (H₃BO₃; Aldrich, 99%) in a reducing atmosphere of H₂/N₂ = 1/9 (20 mL H₂/180 mL N₂) at various temperatures (1100–1400°C) for different time periods (4–6 h). The powders were weighed according to the chemical composition of (4--)SrCO₃ + 7Al₂O₃ + at.% Eu (as Eu₂O₃) + at.% Dy (as Dy₂O₃) + (0, 20, 40 and 100) mol% B (as H₃BO₃). The appropriate amounts of the starting materials according to the derived compositions were mixed thoroughly in a mortal and pestle with the help of ethanol. The mixture was allowed to dry to form powder and prepared in the form of pellets of (Ø 13 mm × 3 mm) dimension applying pressure of 650 kg·cm⁻² for 3 minutes. Then, these pellets were preheated in air atmosphere at 800, 900, and 1000°C for 4 h at heating rate 150°C/h. After pulverization, pellets (Ø 13 mm × 3 mm) were prepared. The pellets were heated in reducing atmosphere of H₂/N₂ = 1/9 flowing H₂-20 mL/min and N₂-180 mL/min. The products obtained after reduction were gritty solids which were pulverized to get fine powder.

Phase identification was carried out using a Shimadzu-610D X-ray diffractometer with Cu-Kα radiation. Thermo-gravimetric (TG) and differential thermal analyses (DTA) were carried with a SEIKO-6300TG/DTA instrument. Scanning electron microscopy (SEM) observations were carried out using a Hitachi S-3000N scanning electron microscope. EDS coupled with SEM were used to observe the elemental mapping images of Eu and Dy in various samples. Elemental mapping of all samples were carried out for elapsed time of 100 seconds. Bulk area (Ø 10 mm) EDAX measurements were carried out using Rayny EDX-800HS energy dispersive X-ray fluorescence spectrometer. Photoluminescence (PL) emission spectra were recorded using USB 4000-UV-VIS-NIR fiber optic spectrometer (Ocean optics). PL excitation spectra were measured using Hamamatsu Quantaurus QY absolute quantum yield measurement instrument. The decay curves were recorded using a brightness meter (Konica Minolta LS-100) equipped with RS232C Comm and fitted using Microsoft program solver. Before afterglow and decay curves measurements, each sample was exposed to 15 W normal D65 fluorescent lamps for 5 minutes. All measurements were carried out at room temperatures.

3. Result and Discussion

To understand the thermal behavior of the raw materials and mixtures during raising temperature, it is necessary to observe TG and DTA. The TG and DTA measurements were carried out for representative samples S₄A₇B₀ (0 mol% H₃BO₃), S₄A₇B0.2 (20 mol% H₃BO₃), and S₄A₇B0.4 (40 mol% H₃BO₃) with 4 at.% Eu and 8 at.% Dy, and the results are depicted in Figure 1. The weight loss below 300°C indicates the dehydration of boric acid and γ-alumina used as raw powder. For S₄A₇B₀ mixture, ~3 wt% loss was due to dehydration of γ-alumina. The weight loss of S₄A₇B0.2 and S₄A₇B0.4 mixture below 300°C were 3.4 and 3.8%, respectively, that came from the weight loss in γ-alumina as well as the decomposition of boric acid present in the mixture. The total weight loss of 17.26%, 17.34%, and 18.84% was analogous to the theoretical values of 17.1%, 17.4%, and 17.7% for the S₄A₇B₀, S₄A₇B0.2, and S₄A₇B0.4, respectively. The endothermic peak on TG and DTA at 700–900°C indicated the decomposition of SrCO₃ to form SrO, and it clearly suggested that SrCO₃ decomposed at relatively lower temperature in case of boric acid containing mixtures than without boric acid containing mixture.

Figure 1: TG-DTA curves of 4SrCO₃·7Al₂O₃ mixture containing different amount of boric acid as flux showing the reaction process during calcinations.

Figures 2(a)–2(d) show the XRD patterns of strontium aluminate phosphors containing 0, 20, 40, and 100 mol% H₃BO₃, respectively, prepared at 1300°C for 5 h. XRD patterns showed that in the phosphor without H₃BO₃ (S₄A₇B₀), SrAl₂O₄ (JCPDS; 34-0379), and SrAl₁₂O₁₉ (JCPDS; 26-0976) were appeared as main phases along with some residual alumina (JCPDS; 46-1212). On increasing H₃BO₃ content to 20 mol% (S₄A₇B0.2), these SrAl₂O₄ and SrAl₁₂O₁₉ phases decreased, and at the same time Sr₄Al₁₄O₂₅ phase appeared as a major phase. The phosphor obtained by adding 40 mol% H₃BO₃ (S₄A₇B0.4) showed single-phase Sr₄Al₁₄O₂₅ having orthorhombic crystal structure with Pmma space group which is in good agreement with the JCPDS data (52-1876) and the results of others [16–18]. It suggests that Sr₄Al₁₄O₂₅ phase is formed by the reaction between SrAl₁₂O₁₉ and SrAl₂O₄ in presence of boron oxide that accelerates the Sr₄Al₁₄O₂₅ phase formation acting as high temperature solvent. As boron content increased to 100 mol% (S₄A₇B1.0), there appeared some crystalline aluminoborates SrAl₂B₂O₇ (JCPDS; 47-0182) along with glassy phases. So at higher H₃BO₃ concentrations, B₂O₃ (formed by decomposition of H₃BO₃) not only acts as high temperature solvent, but actively participates in chemical reaction with the aluminates to form aluminoborates as in Figure 2(d). Thus, formation of Sr₄Al₁₄O₂₅ involves complex chemical reaction process in which SrAl₂O₄ and SrAl₁₂O₁₉ are formed as intermediate phases [18]. The whole synthesis process can be expressed by the following chemical reactions:

Figure 2: XRD patterns of strontium aluminates phosphors: (a) S₄A₇B₀, (b) S₄A₇B0.2, (c) S₄A₇B0.4, and (d) S₄A₇B1.0 sintered at 1300°C for 5 h.

The proposed reaction scheme is supported by TG results in Figure 1, and direct XRD patterns of the intermediate reaction products given in Figure 3. At very low reaction temperature of 800–900°C, there appeared SrAl₂O₄ and unreacted residual alumina. As reaction temperature increased to 1000°C, there appeared a new phase SrAl₁₂O₁₉ along with the SrAl₂O₄ phase. As in (5), SrAl₂O₄ reacts with residual alumina to form SrAl₁₂O₁₉ phase. Further, increase of temperature to 1200°C, a new metastable phase Sr₄Al₁₄O₂₅ appeared slightly. However, at reaction temperature of 1300°C, Sr₄Al₁₄O₂₅ single phase was observed (Figure 3(d)). Thus, it is generalized that at higher temperatures SrAl₂O₄ reacts with SrAl₁₂O₁₉ in the presence of liquid boron oxide flux to generate metastable Sr₄Al₁₄O₂₅ phase as in (6). However, if boron oxide flux concentration is very high (>40 mol%), at higher temperature it actively participates in reaction with the products and forms aluminoborates and other glassy phases. We also conducted experiments at higher temperature (1400°C, 1450°C, and 1500°C) keeping the flux concentration constant, but we observed the same phase Sr₄Al₁₄O₂₅. However, if temperature is increased to 1600°C, Sr₄Al₁₄O₂₅ phase decomposes to form SrAl₂O₄ and SrAl₁₂O₁₉ which are stable at higher temperatures.

Figure 3: The effect of preheating temperature on the phase formation of strontium aluminate phosphors.

The change in the grain morphology and the surface microstructure of the phosphors was examined by SEM observations. Figures 4(a)–4(d) represent the SEM micrograph of strontium aluminate phosphors containing 0, 20, 40, and 100 mol% H₃BO₃, respectively. It is clear from Figure 4(a) that in the absence of H₃BO₃ irregular to round shaped, tinny (Ø 0.2 to 0.5 μm) grains connected with each other were formed. Due to spherical particle shape, micropores were observed in the microstructure of the ceramics. When the H₃BO₃ concentration was increased to 20 mol% (Figure 4(b)), hexagonal to angular particles with increased particle size (~1 × 1 μm) were observed. The microstructure was not smooth throughout indicating the presence of other phases or glasses on the surface [1]. It is well known that H₃BO₃ is a good glass former at higher temperature, the rough microstructure seen in the SEM images in Figures 4(b) and 4(d) was glass which was also supported by the XRD profile in Figure 2(b). In XRD patterns, the mountain like part from value of 18° to 30° is due to the presence of glassy phase. Due to the crystal growth, the micropores as observed in the ceramics without H₃BO₃　were reduced. As H₃BO₃ concentration increased to 40　mol%, fine and regular particles grew with increased particles size of ~2 × 2 μm (Figure 4(c)). The particles distribution was almost homogeneous throughout the surface. These angular shaped phosphor particles might have been formed by dissolution precipitation process from SrAl₂O₄ in the presence of molten B₂O₃. But in Figure 4(d), at high B₂O₃ content, very big and irregular shaped particles with smooth corner and round holes were observed. At higher B content, large amount of glass was formed and completely covered the surface of the phosphor particles making the ceramics rough. The particle size distribution was wide ranging between 5 and 10 μm. These results are also supported by the previously published works [11–15]. Thus, it might be suggested that at high temperature, the B₂O₃ acts as liquid medium flux and accelerates the grain growth of the particles generating highly crystalline and dense Sr₄Al₁₄O₂₅ phase at optimum concentration but at higher concentration it generates the glasses and crystalline aluminoborates.

(a)

(b)

(c)

(d)

Figure 4: SEM microstructures of the Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ phosphors prepared by adding different H₃BO₃ contents: (a) 0 mol%, (b) 20 mol%, (c) 40 mol%, and (d) 100 mol%.

Figure 5 shows the excitation and emission spectra of Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ phosphors prepared at 1300°C. These samples can be excited by a wide range of UV to visible light (<250 nm to 450 nm wave length,  nm) showing wide applications. All the samples exhibited emission centered at 497 nm with blue-green color　except S₄A₇B₀　sample, but their intensity varied significantly with B contents. In the case of S₄A₇B₀　sample, a significant amount of SrAl₂O₄　phase appeared which exhibits emission centered at 520 nm [13]. As a result, the emission peak is shifted to about 505 nm. All other samples contain Sr₄Al₁₄O₂₅ as a major phase which exhibits emission centered at about 495 nm. At the same time, increasing H₃BO₃　concentrations the phosphorescence intensity increased　and reached the maximum at ~40 mol% H₃BO₃ and above 40 mol% again decreased as in Figure 6. It may be explained as, in S₄A₇B₀ phosphor, mixed phases of SrAl₂O₄ and SrAl₁₂O₁₉ were existed, the later is nonweak emissive [13]. But increasing B concentration to ~40 mol%, highly crystalline, dense, single phase Sr₄Al₁₄O₂₅ was formed which produced higher PL intensity. It is the flux that can properly facilitate the entry of activator ions into the crystal lattice and aids in the formation of luminescent centers [19]. Due to liquid boron oxide as high temperature flux, distribution of the rare ions becomes random and deep throughout the lattice, as well as, the flux enforce the Eu and Dy ions to replace the Sr site in the lattice. This increases the Eu and Dy contents in the lattice and causes steep increase in the PL properties. Further, to prove the increment of activator in the strontium aluminates host, EDAX experiments on samples containing 0, 20, 40, and 100 mol% B with same amount of Eu (4 at.%) and Dy (8 at.%) in the starting mixture were carried out, and the results were presented in Table 1. As shown in Table 1, the incorporated amount of Eu and Dy in the strontium aluminates host increased with the increase of the B content as flux, consequently increasing the PL and AGL intensity. The decrease in the PL intensity at higher content of B (S₄A₇B1.0) might be due to the formation of nonemitting phase SrAl₂B₂O₇ and glasses as observed in XRD profiles that covers the emitting surface. Further, the EDS elemental mapping images of Eu and Dy in the phosphor containing 0, 40, and 100 mol% H₃BO₃ were shown in Figure 7. From the mapping images, it was clear that the surface concentration of Eu and Dy increased on increasing the H₃BO₃ concentrations.

Table 1: The list of samples with various B contents and Eu²⁺ and Dy³⁺ values obtained from EDAX studies correlated with emission intensities.

Samples
Emission maximum (nm)
Phases
Amount of Eu²⁺ wt%
Amount of Dy³⁺ wt%
AGL intensity(cd·m⁻²)

S₄A₇B₀ (boron free)
505
SrAl₂O₄ and SrAl₁₂O₁₉
0.125%
0.5%
0.18

S₄A₇B0.2 (20 mol% B)
498
SrAl₂O₄ and Sr₄Al₁₄O₂₅
0.134%
0.69%
3.09

S₄A₇B0.4 (40 mol% B)
495
Sr₄Al₁₄O₂₅
0.146%
0.89%
3.31

S₄A₇B1.0 (100 mol% B)
493
Sr₄Al₁₄O₂₅ and SrAl₂B₂O₇
0.156%
0.92%
1.81

Figure 5: Excitation (dotted lines) and emission (solid lines) spectra of Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺, mol% H₃BO₃ phosphor. Inset shows the digital micrograph of 40 mol% B contained phosphor in black light.

Figure 6: Variation of afterglow intensities of strontium aluminate phosphor with different flux (H₃BO₃) concentrations.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 7: EDS mapping of Eu ((a), (c), and (e)) and Dy ((b), (d), and (f)) in strontium aluminate phosphors prepared with different H₃BO₃ concentrations: ((a), (b)) 0 mol%, ((c), (d)) 40 mol%, and ((e), (f)) 100 mol%.

Figure 6 depicts the persistent afterglow pattern of the phosphor samples containing 0, 20, 40, and 100 mol% H₃BO₃ with 4 at.% Eu and 8 at.% Dy. The afterglow data were recorded 10 seconds after the excitation source was switched off. It was found that the persistent time from B free phosphor was the least. As the amount of B increased, the initial phosphorescence intensity and the afterglow duration increased and reached the maximum at 40 mol% (S₄A₇B0.4), above that it again decreased. The liquid phase sintering aided by H₃BO₃ facilitates the grain growth of the strontium aluminates and increases the penetration of trap centers in the phosphor that enhances the phosphorescence properties [20]. Further, the dissolved B³⁺ ions substitute Al³⁺ ions at tetrahedral positions [21]. This substitution result in the shrinkage of Sr₄Al₁₄O₂₅ lattice and enhance the hole trapping ability of Dy³⁺ ions that further increases the phosphorescence afterglow [22, 23]. The decrease of afterglow on increasing the B content (S₄A₇B1.0) might be due to presence of glasses which covers the emissive surface of the phosphor.

The phosphorescence intensity and durability increased on increasing the preheating temperature from 800°C to 1000°C. The 800°C and 900°C preheated mixture showed some peaks of alumina and SrAl₁₂O₁₉ phase in XRD profiles (Figure 3). As temperature increased to 1000°C, alumina peaks completely disappeared, and SrAl₁₂O₁₉ peaks increased. As in reaction described above these two phases, namely, SrAl₁₂O₁₉ and SrAl₂O₄ reacts with each other in presence of boron oxide to generate Sr₄Al₁₄O₂₅ phase. The higher PL intensity in case of 1000°C preheated and 1300°C reduced sample might be due to existence of dense, highly crystalline, single phase Sr₄Al₁₄O₂₅ as described earlier.

Figure 8 shows the variation of PL properties as a function of heating temperature and time. We observed that the phosphor prepared at 1300°C for 5 h in reducing atmosphere showed higher PL intensity and persistency. As reduction temperature increased above 1300°C and reduction time increased more than 5 h, the increase in emission intensity was incremental, indicating that 1300°C heating for 5 h was the sufficient temperature and time to get good PL properties. To get better PL properties, the phosphor should be prepared under slightly reduced atmosphere so as to reduce Eu³⁺ to Eu²⁺. When the phosphor was prepared on air atmosphere, the PL intensity as well as durability was negligible. The negligible Eu²⁺ emission in the sample prepared in air comes from the self reduced Eu²⁺ in the Sr²⁺ sites as explained by Peng et al. [24].

Figure 8: Variation of phosphorescence intensities as a function of heating temperature and time.

Figure 9 shows the decay curves of afterglow phosphorescence of the phosphor specimens with various Al/Sr ratios (). Keeping Al concentration constant, we monitored the Sr concentrations. While increasing the value of from 3.4 to 4.0, the initial intensity (inserted in Figure 9) and the afterglow duration of the phosphors first increased, attained the maximum at equals 3.7, and then decreased when the value was increased above 3.7. Similar results were observed by Yuan et al. [17] and explained on the basis of decay time values obtained by curve fitting and thermoluminescence measurements using TSL parameters. According to them, slightly Sr deficit samples exhibits large number of traps which is suitable for the brighter afterglow and longer afterglow time.

Figure 9: Afterglow decay curves of Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ phosphor with different Al/Sr ratio. Inset shows the Phosphorescence intensity of the corresponding phosphors just after switching off excitation source.

All the phosphors doped with different amount of Eu and Dy showed the main emission peak centered at ~497 nm as shown in the inserted curve in Figure 10. Further, the phosphor samples with lower concentration of Eu showed two emission peaks; one pronounced emission peak at 497 nm and another weak peak at about 407 nm. The weak peak at 407 nm decreased continuously on increasing Eu concentration while at the same time the main peak at 497 nm increased till it reached saturation. According to Li et al. [25] there are two Eu²⁺ sites in the Sr₄Al₁₄O₂₅ matrix due to existence of two different types of Sr²⁺ ions; one surrounded by 6 oxygen atoms and another surrounded by eight oxygen atoms in the oxygen polyhedra. However, with the increase of Eu concentration energy transfer from higher energetic Eu site to lower energy Eu site predominates, and emission band at 407 nm gradually decreased while at 497 nm increased. On increasing the Eu concentration, the initial phosphorescence intensity goes on increasing, attain maximum at 4 at.% Eu, and above it starts decreasing. It means 4 at.% Eu may be the critical quenching concentration [26]. When the doped Eu concentration is above 4 at.%, the nearby Eu ions in the lattice interacts with each other because the critical distance, assumed to be 40 nm [9], overcame above 4 at.% Eu concentration. So, the optimum doped amount of Eu²⁺ is 4 at.% in the Sr₄Al₁₄O₂₅ phosphor.

Figure 10: Phosphorescence decay curves of Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ phosphors with various Eu concentrations. Inserted figure shows the effect of Eu at.% on the PL intensity.

From the decay curves as shown in Figure 10, it is clear that the decay speed of the persistency of phosphors changed with the amount of Dy³⁺ concentration too. The codoped Dy acts as trap centers [2, 27] that trap the holes generated during the exposure of phosphor to excitation source. To monitor the variation of phosphorescence intensity according to Dy concentration, we kept the doped amount of Eu at 4 at.% and changed the codoped amount of Dy. On increasing Dy concentrations, the phosphorescence intensity increased as shown in Figure 10. When the codoped amount of Dy was 8 at.%, the phosphor showed the longest persistency of greater than 20 h with intensity greater than 5 mcd·m⁻². But as the concentration of Dy was above 8 at.%, the phosphorescence intensity and persistency decreased sharply. It can be assumed that small amount of Dy is insufficient to form enough trap defects in the Sr₄Al₁₄O₂₅ matrix to trap sufficient holes. However, if the amount of doped Dy is greater than 8 at.%, it may cause the concentration quenching and reduce the emission intensity.

The mechanism of phosphorescence in aluminates phosphor has been explained by many researchers [1, 2, 4, 11, 13, 28–30] and might be explained in detail as: upon exposure to excitation source, an electron of Eu²⁺ (4f⁷) is promoted to the 4f⁶5d¹ band followed by either direct or phonon assisted escape of the electron from Eu²⁺ to the host conduction band. The lattice defects (especially, oxygen vacancies) close to the bottom of the host conduction band traps the electrons. Large numbers of electrons are trapped by the Dy³⁺ or Dy³⁺-borate complex assisted traps at various depths [28–30] making an important role of Dy³⁺ during the persistent afterglow. When the excitation source is removed, the captured electrons near the host conduction band released to the conduction band with thermal energy and consequent recombination of them with the emitting Eu²⁺/Eu³⁺ centers lead the persistent afterglow. Thus, the important point for the persistent afterglow is the number of electrons captured in the traps and its depths from the bottom of the host conduction band. The afterglow intensity of the persistent phosphor, thus, depends on the densities of the traps/captured electrons, while the afterglow duration depends on the depth of the captured electrons. The number of captured electrons, in turn, depends on the concentration of oxygen vacancies/Dy³⁺/Dy-borate complexes and the trap depth of it. Under the suitable trap depth, the trap concentration is directly proportional to the Dy ions or the concentration of Dy-borate complex. Hence, optimum Dy concentration produces the longer afterglow duration with higher intensity. Further, the factors which stabilize the traps or increase its density definitely increased the afterglow intensity and its duration.

Figure 11 shows the effect of various charge compensators on the PL emission intensity of the Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ phosphor. The Li⁺, Na⁺, Ca²⁺ and Ag⁺ ions doping increased the PL intensity while the Si⁴⁺, W⁶⁺, Ba²⁺, and Mg²⁺ doping decreased the PL intensity on the increasing order. Since the Ag⁺ doping increased the PL intensity the most, the effect of concentration of the Ag⁺ ions on the PL emission of the phosphor was studied and expressed in Figure 12. It was observed that doping 3 mol% Ag⁺ on the Sr₄Al₁₄O₂₅:4 at.% Eu²⁺, 8 at.% Dy³⁺ phosphor increased both the PL and AGL intensity more than 1.5-fold exhibiting most intense PL and longest afterglow duration. Similar results were observed by Suriyamurthy and Panigrahi [31]. According to them the enhanced afterglow intensity and lifetime by Ag⁺ doping is due to the increased number of traps in the Ag⁺　doped sample. It is speculated that the addition of Dy³⁺ ions into the Sr²⁺ sites produces stress due to charge imbalance which is reduced by the Ag⁺ ions doping due to charge neutrality. We measured the TSL parameters of samples with and without Ag⁺ ions doping as shown in Figure 13, and various TSL parameters are presented in Table 2. It is clear that addition B³⁺ not only increased the trap density but also deepened the traps making the phosphor persist for longer periods. But the addition of Ag⁺ ions did not shift the peak positions, but the trap densities were drastically increased. The increased number of traps is responsible for the enhancement of the afterglow intensity.

Table 2: TSL parameters of Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ phosphors.

Samples
Glow peak (°C)
Glow peak shape parameters
TSL parameters

S₄A₇B₀
97
70
130
200
0.35
0.446
1.34 × 10⁶

S₄A₇B0.4
111
79
154
233
0.34
0.641
1.87 × 10⁶

S₄A₇B0.4, Ag0.03
113
78
158
236
0.33
0.645
2.77 × 10⁶

Figure 11: Variation of phosphorescence intensity of Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ phosphor with some of the charge compensating ions, each additives being 4 at.%.

Figure 12: Effect of Ag⁺ ions doping on the afterglow emission of the Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ phosphor.

Figure 13: TL glow curves of Sr₄Al₁₄O₂₅: 4 at.% Eu, 8 at.% Dy with and without B and Ag doping.

The phosphor was also compared with the commercially available product from Nemoto and Co. LTD (BGL300M) in term of PL and decay parameters and expressed in Figure 14. It clearly suggests that our product is much more superior to BGL300M (the best one among commercially available blue-green persistent phosphor) and possess superior applications.

Figure 14: Comparison of PL excitation and PL emission intensity of Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺,Ag⁺ phosphor with the commercial Nemoto BGL300M.

4. Conclusions

Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺,Ag⁺　phosphor with high brightness and long persistent luminescence was prepared by the solid-phase reaction method at 1300°C. The broadband UV-visible excited luminescence of the orthorhombic　Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺,Ag⁺　was observed in the blue-green region ( nm) due to transitions from 4f⁶5d¹ to 4f⁷ configuration of the Eu²⁺ ion. Well crystallized, regular shaped phosphor particles were obtained by the addition of small amount of H₃BO₃ as flux. The phosphorescent characteristics are highly influenced by the addition of small amount of H₃BO₃　in the starting mixture, and the maximum PL intensity was observed by adding 40 mol% H₃BO₃. When the ratio of Al/Sr was 3.7, and the Eu and Dy concentrations were 4 at.% and 8 at.%, respectively, the phosphor exhibited a higher emission intensity and longer afterglow duration. The addition of Ag⁺ ions has better effect on the PL emission and AGL intensity and duration of the phosphor due to the stabilization of the Dy³⁺ into the host and consequently increasing the traps responsible for the afterglow. The phosphor was compared with the commercial product and found to be much superior than the existing one.

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