Development of a Blue Emitting Calcium-Aluminate Phosphor

Abstract

We report methodological advances that enhance the phosphorescence efficiency of a blue-emitting calcium aluminate phosphor (CaAl₂O₄: Eu²⁺, Nd³⁺). The investigation of long-persistence blue-emitting phosphors is highly desirable due to their promising applications, such as white LEDs; however, the development of highly efficient blue-emitting phosphors is still challenging. Here, we have quantitatively characterized the phosphorescence properties of the blue-emitting phosphor CaAl₂O₄:Eu²⁺, Nd³⁺ with various compositions and directly related these properties to the quality of its luminescence. We optimized the composition of the activator Eu²⁺ and the co-activator Nd³⁺, the doping conditions with alkaline earth metals, alkali metals, and Si to create crystallographic distortions and, finally, the flux conditions to find the best parameters for bright and persistent blue-emitting phosphors. Our research has identified several doping compositions with good to excellent performance, with which we have demonstrated bright and persistent phosphors with afterglow characteristics superior to those of conventional phosphors.

Introduction

Phosphor materials have attracted much attention in applications such as electroluminescent displays, particularly white light emitting diodes (LEDs), and a large number of new phosphorescent materials have been developed in the last decade[1–7]. Among them, green-emitting ZnS:Cu phosphors have been used as long-lasting phosphorescent phosphors and applied in a variety of areas, including watches, clocks, traffic signs, emergency signage, and textile printing for signaling in the darkness[8, 9]. However, the applications of ZnS have been limited due to their short intrinsic decay time (1 h) and low emission intensity. Although doping with Co²⁺ enhances the emission intensity of ZnS:Cu phosphors, the incorporation of a large amount of a dopant into the host degrades the mechanical and physical properties of the host, particularly in the presence of moisture, which then becomes chemically unstable, limiting its application[5–7]. Another phosphor, YAG(yttrium aluminium garnet):Ce³⁺, is also a green- or yellow-emitting material; however, it also suffers from poor color rendition and high thermal quenching[10, 11]. To overcome this problem, Matsuzawa et al. have recently developed long lasting green-emitting phosphors of polycrystalline SrAl₂O₄ co-doped with Eu²⁺ and Dy³⁺, which resulted in improved lifetimes, intensity, and chemical stability over the previous phosphors[7]. Due to their higher chemical stability, the duration of the phosphorescence, and its high intensity, these materials could potentially replace the traditional ZnS-based phosphors, and further improvements have been recently achieved by many researchers[1–5].

Recently, white light emitting diodes (w-LEDs) are replacing conventional lighting products due to high efficiency, good material stability and long operation lifetime and phosphors are widely used as white LED sources[12]. A typical w-LEDs are fabricated by the combination of blue-emitting InGaN LEDs and yellow-emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) phosphor. However, this strategy has a number of disadvantages, such as high correlated color temperature (CCT > 4500 K) and low color rendering (CRI) index (R_a < 75)[13, 14]. An alternative strategy to generate white light is to coat near UV emitting LEDs with a mixture of high efficiency red, green and blue emitting phosphors which produces excellent CRI values and better color stability[15, 16]. However, it suffers from poor efficiency due to the large Stokes shift between emission and excitation of the near UV excitable phosphor. The eventual performance and efficiency of the w-LEDs strongly depends on the luminescence properties of the phosphors[17]. Therefore, it is highly desirable to develop highly efficient phosphors which can be excited by near UV LEDs. However, most of the improvements have been limited to green-emitting phosphors and the development of highly efficient blue-emitting phosphors is still challenging. The investigation of long-persistence blue-emitting phosphors is hence highly desirable, since the luminescence properties of blue-emitting phosphors can significantly affect the eventual performance of white LEDs[17].

Therefore, in this work, we have studied the effect of various doping compositions and impurities on the phosphorescence of blue-emitting calcium earth aluminate phosphor CaAl₂O₄:Eu²⁺, Nd³⁺ and improved its phosphorescence characteristics. The CaAl₂O₄:Eu²⁺ system is a blue emitting phosphor with an emission peak at 448 nm, analogous to the SrAl₂O₄:Eu²⁺ system[18–20]. It is known that the introduction of Nd³⁺ into the CaAl₂O₄:Eu²⁺ system boosts its phosphorescence intensity and lifetime, in a similar way as Dy³⁺ doping in the SrAl₂O₄:Eu²⁺ system[20]. However, the effects of Eu²⁺ and Nd³⁺ ions and/or native defects in the phosphorescence of CaAl₂O₄:Eu²⁺, Nd³⁺ remain unclear. In this paper, we present in detail the phosphorescence characteristics of CaAl₂O₄:Eu²⁺, Nd³⁺ crystals grown with various compositions by systematic investigations on the preparation, composition, structure, and luminescence properties, aiming at improving its phosphorescence characteristics.

Results

Composition of the activator and co-activator

CaAl₂O₄ is known to have a monoclinic structure with three non-equivalent Ca sites[4]. When doped with Eu²⁺, those atoms occupy the only type of Ca sites that meets their size requirements[4]. Such Eu²⁺ ion doping boosts the intensity of the phosphorescence, and Eu²⁺ ions are typically used as the luminescence center and activator in the phosphor[20, 21]. Therefore, if the amount of Eu²⁺ incorporated in the host lattice is varied, the luminescence properties can be tuned owing to the significant changes in the local surroundings around a substituted site, such as the bond length and angle, as well as the point symmetry. In order to find the best conditions of Eu²⁺ doping for the development of a bright CaAl₂O₄:Eu²⁺, Nd³⁺ phosphor, we characterized the effect of the activator composition on the phosphorescence properties in an initial optimization. The samples were irradiated with 365 nm light for 5 min and the persistent luminescence was measured at room temperature for various Eu²⁺ concentrations ranging 0.006–0.014 mol. The phosphorescence spectra show the typical broadband emission resulting from the 5d → 4f transition in Eu²⁺, which is the transition between the crystal field components of the 4f⁶ 5d excited state configuration and the 4f⁷ ground state. These f–d transitions are known to be very sensitive to distortions in the crystal field[22]. The band position, shape, and width do not vary with the Eu²⁺ composition, while the persistence times vary greatly with the composition, indicating the same luminescent Eu²⁺ center at different Eu²⁺ compositions. An emission band is observed at 448 nm, at a shorter wavelength than that of SrAl₂O₄:Eu²⁺. As the radius of the alkaline earth ion (Ca²⁺, Sr²⁺, and Ba²⁺) increases, narrow emission bands are known to appear at progressively shorter wavelengths, which is consistent with our observations. The afterglow intensity was monitored for the emission peak at ~448 nm, which corresponds to the 5d → 4f transition in Eu²⁺. The decay curves of the afterglow intensity are shown in Fig 1. The curves are very similar for the different Eu²⁺ compositions and the only difference is the intensity. We further computed the lifetimes and decay rates of the emission at 448 nm by fitting them with three exponential components and different decay times[23]. The fitting results are presented in Table 1. The initial afterglow intensity measured after 5 s changes significantly with the Eu²⁺ concentration, while the decay times do not vary greatly with the composition. The decay curves show an initial rapid decay followed by long-persistence luminescence after removal of the UV light, consistent with the decay characteristics of phosphorescence. The observations suggest that the intensity decreases as the Eu²⁺ concentration increases, ~0.006 mol Eu²⁺ (per mol CaAl₂O₄:Eu²⁺, Nd³⁺) resulting in the brightest and longest emission, which is significantly lower than those for the optimized Eu²⁺ concentration of green-emitting phosphor SrAl₂O₄:Eu²⁺,Dy³⁺ (~0.935 mol Eu²⁺ per mol SrAl₂O₄:Eu²⁺,Dy³⁺). Owing to the larger size of Eu²⁺ (131 pm), it is relatively harder to introduce it in the Ca²⁺ sites of CaAl₂O₄ than in the similar-size Sr²⁺ sites (132 pm) of green-emitting SrAl₂O₄ crystals. Therefore, the afterglow of the phosphors is mainly influenced by the Eu²⁺ concentration and the host structure even when Eu²⁺ is used as both the activator and the luminescent center in those crystals. In addition, the increasing Eu²⁺ concentration could cause significant changes in the local surroundings, such as point symmetry, bond length, bond angle around a substituted site, interrupting the phosphorescence mechanism in CaAl₂O₄: Eu²⁺, Nd³⁺.

Fig 1.

(A) Emission spectrum of CaAl₂O₄:Eu²⁺, Nd³⁺ crystals depending on Eu²⁺ concentration. (B) Decay curves depending on Eu²⁺ concentration. (C) Magnified views of the graph in (B). (D) Decay curves in log scale depending on Eu²⁺ concentration. (E) Relative initial intensity measured at 5s (relative values where the value of control sample #1 is 1.0) depending on Eu²⁺ concentration.

Table 1. Various nominal activator(Eu²⁺) compositions of the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals and the calculated decay times of the phosphorescence from the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals doped with various Eu²⁺ concentrations.

Decay times were calculated based on the three exponential components ($\mathrm{I}=\mathrm{a}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}1}}+\mathrm{b}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}2}}+\mathrm{c}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}3}}$) by a curve fitting technique.

Sample	#1	#2	#3	#4
Mol of Eu	CaAl2O4:Eu0.006,Nd	CaAl2O4:Eu0.008,Nd	CaAl2O4:Eu0.012,Nd	CaAl2O4:Eu0.014,Nd
t1[s]	202.4	164.1	220	132.6
t2[s]	30.74	25.15	31.26	25.04
t3[s]	0.5419	0.6566	0.9798	0.3373
a	44.17	42.53	28.79	29.15
b	145	131.6	107.3	79.45
c	0.9492	0.8655	0.6647	0.9011

Based on this result, we also evaluated the effect of different activator and co-activator compositions on the phosphorescence intensity. It is known that Nd³⁺ doping produces extremely brighter and longer blue phosphorescence than Dy³⁺ doping at room temperature in blue-emitting CaAl₂O₄: Eu²⁺[20, 21]. Its introduction is analogous to that of Dy³⁺ as an auxiliary activator in green-emitting SrAl₂O₄:Eu²⁺ to produce remarkably intense phosphorescence[7, 23–27]. Previous studies have reported that Nd³⁺ doping results in stronger phosphorescence at room temperature than Dy³⁺ doping in CaAl₂O₄:Eu²⁺, indicating that the local environment may affect the trap depth made by the co-dopant[20]. In our previous study, we reported that the optimum concentration of the activator Eu²⁺ was ~0.935 mol (per mol of SrAl₂O₄) and the optimum Dy³⁺/Eu²⁺ ratio was ~2.4[28]. In a similar way, we tested different Nd³⁺/Eu²⁺ ratios by varying the concentration of Nd³⁺ at a constant Eu²⁺ concentration (0.006 mol per mol CaAl₂O₄:Eu²⁺, Nd³⁺) to find the optimal concentration of Nd³⁺. The phosphor samples were prepared by firing mixtures of CaCO₃, Al₂O₃, Eu₂O₃, SiO₂, and small quantities of H₃BO₃ as a flux in a reducing atmosphere at 1300°C for 3–5 h and measuring the phosphorescence. The afterglow curves measured at 448 nm are shown in Fig 2. The Nd³⁺/Eu²⁺ ratio was varied in the range from 1 to 2.4 and the results are shown in Fig 2 and Table 2. The results indicate that the intensity decreases as the Nd³⁺/Eu²⁺ ratio increases, and the Nd³⁺/Eu²⁺ ratio of 1 results in the brightest and longest emission. This optimum concentration of Nd³⁺ (~0.006 mol per mol CaAl₂O₄:Eu²⁺, Nd³⁺) is much lower than the optimum concentration of Dy³⁺ (~2.244 mol per mol CaAl₂O₄:Eu²⁺, Nd³⁺) in SrAl₂O₄:Eu²⁺, Dy³⁺ reported in a previous study, which could be explained by the difference in the solubility of the two ions in the structure[28]. Since the ion radius of Dy³⁺ is smaller than that of Nd³⁺, Dy³⁺ is more soluble within the system than Nd³⁺[7]. Therefore, a higher concentration of Dy³⁺ can be incorporated into the structure, forming a relatively higher number of trapping levels and resulting in brighter phosphorescence in the SrAl₂O₄:Eu²⁺ system compared to the case of Nd³⁺ doping in CaAl₂O₄:Eu²⁺. Doping with a higher concentration of Nd³⁺ (reaching levels over the solubility limit) may result in the production of the by-product NdAlO₃, disrupting the phosphorescence process. The role of Nd³⁺ doping in CaAl₂O₄:Eu²⁺ can be explained by hole traps in the structure, similar to what happens with Dy³⁺ in SrAl₂O₄:Eu²⁺[23, 29]. Since Nd³⁺ and Dy³⁺ have relatively low 4f–5d transition energies and high charge-transfer energies, they can act as hole traps[23, 29]. These holes migrate to the excited Eu²⁺ centers where they are captured, followed by recombination. Phosphorescence is caused by this trapping of photo-generated holes and/or electrons, which, following a delayed radiative return after recombination of the charge carriers, causes luminescence. Therefore, phosphorescence is considered as thermo-luminescence with de-trapping at room temperature, and local distortions around the co-dopant ions seem to affect the trap depth. The trapping level of the CaAl₂O₄:Eu²⁺, Nd³⁺ phosphor is known to be located a little shallower than that of the SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor, resulting in relatively shorter phosphorescence, which agrees with our observations. However, the trapping level of the CaAl₂O₄:Eu²⁺, Nd³⁺ phosphor is probably not so shallow to show a fast decay that does not last for long, but deep enough to show long phosphorescence at room temperature. The de-trapping mechanism in CaAl₂O₄:Eu²⁺, Nd³⁺ is described in Fig 3. In this mechanism, Nd³⁺ acts as a hole trap and the holes move to the excited state of Eu²⁺. After capturing, recombination occurs, followed by phosphorescence. Therefore, local distortions around co-dopant ions seem to affect the trap depth, and hence, optimization of the activator and co-activator composition is important to produce high phosphorescence intensity.

Fig 2.

(A) Emission spectrum of CaAl₂O₄:Eu²⁺, Nd³⁺ crystals depending on [Nd³⁺]/[Eu²⁺] ratio. (B) Decay curves depending on [Nd³⁺]/[Eu²⁺] ratio. (C) Magnified views of the graph in (B). (D) Decay curves in log scale depending on [Nd³⁺]/[Eu²⁺] concentration. (E) Relative initial intensity measured at 5s (relative values where the value of control sample #1 is 1.0) depending on [Nd³⁺]/[Eu²⁺] concentration.

Table 2. Various nominal activator(Eu²⁺) and co-activator(Nd³⁺) compositions of the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals and the calculated decay times of the phosphorescence from the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals doped with various [Nd³⁺]/[Eu³⁺] ratios.

Decay times were calculated based on the three exponential components($\mathrm{I}=\mathrm{a}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}1}}+\mathrm{b}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}2}}+\mathrm{c}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}3}}$) by a curve fitting technique.

Sample	#1	#2	#3
Mol of Nd	CaAl2O4:Eu0.006,Nd0.006	CaAl2O4:Eu0.006,Nd0.008	CaAl2O4:Eu0.006,Nd0.012
t1[s]	171.3	159	193.5
t2[s]	25.16	22	29.18
t3[s]	0.2395	0.2581	0.2675
a	41.91	36.24	16.93
b	172.9	147.4	77.98
c	0.9816	0.9809	0.9806

Fig 3. Energy level diagram for CaAl2O4: Eu²⁺, Nd³⁺ Phosphor.

Doping with impurities

In blue-emitting CaAl₂O₄:Eu²⁺, Nd³⁺, the Eu²⁺ ions usually act as the luminescence centers and the transitions between the 4f⁷ ground state and the crystal field components of the 4f⁶ 5d excited state are responsible for the broad emission spectrum of Eu²⁺, as previously explained[20]. Such f–d transitions are known to be very sensitive to distortions of the crystal field in the luminescent host of alkaline earth silicates[22]. Therefore, if this stable host structure changes and crystallographic distortions occur by substitution or impurities, the crystal field environment of the rare earth ions in the host structure is influenced, affecting the trap depth and, finally, the characteristics of the phosphorescence. In order to create such crystallographic distortions and boost the luminescence in CaAl₂O₄:Eu²⁺, Nd³⁺, we substituted Ca²⁺ with alkali metal or alkaline earth metal ions of various sizes, or substituted the Al³⁺ cations with Si⁴⁺. These substitutions may lead to less forbidden transitions, thereby enhancing the phosphorescence.

Doping with impurities—Alkaline earth metal doping

As an initial doping test, we substituted Ca²⁺ in the calcium aluminate phosphor with alkaline earth metal ions of different sizes, in order to create crystallographic distortions in the host structure and boost the phosphorescence. High purity chemical reagents CaCO₃, Al₂O₃, Eu₂O₃, SiO₂, MCO₃ (M = Sr²⁺, Mg²⁺, and Ba²⁺) and small quantities of H₃BO₃ were used as starting materials, and the dried powder mixtures were fired in the furnace at 1300°C for 3–5 h. The phosphors were irradiated with 365-nm light for 5 min at room temperature and the afterglow spectra, decay curves, and fitting results from different phosphors and doping samples are shown in Fig 4 and Table 3. Since the ionic radii of alkaline earth metals decrease from Ba²⁺ to Mg²⁺ in the body-centered cubic crystal structure (Ba²⁺: 149 pm, Sr²⁺: 132 pm, Ca²⁺:114 pm, Mg²⁺: 86 pm), we expected different orders of break in the symmetry of the host crystal structure depending on the size difference (relative to Ca²⁺), resulting in expansion or shrinkage of the structure. The wavelength position, band shape, and bandwidth of the afterglow did not change with the addition of any alkaline earth doping, as shown in Fig 4, indicating that the emitting centers are still the Eu²⁺ ions. The decay times of the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals grown from different starting compositions have similar values; however, the intensity of the phosphorescence after illumination is different (Table 3). As expected, alkali metal doping significantly increases the luminescence up to 190% of the initial value, as compared to that observed for the non-doped crystal, probably due to the distorted crystal structure leading to less forbidden transitions. The largest alkaline metal, Ba²⁺, exhibits the largest increase in initial luminescence at 5 s, probably due to the largest size difference with respect to Ca²⁺, while the smallest alkaline metal, Mg²⁺, displays the smallest increase in initial luminescence at 5 s due to the smallest size difference with Ca²⁺.

Fig 4.

(A) Emission spectrum of CaAl₂O₄:Eu²⁺, Nd³⁺ crystals depending on alkali earth metal ion doping. (B) Decay curves depending on alkali earth metal ion doping. (C) Magnified views of the graph in (B). (D) Decay curves in log scale depending on alkali earth metal ion doping. (E) Relative initial intensity measured at 5s (relative values where the value of control sample is 1.0) depending on alkali earth metal ion doping.

Table 3. Nominal compositions of the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals doped with different Alkaline Earth metal ions and the calculated decay times of the phosphorescence from the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals doped with various alkaline earth metals.

Decay times were calculated based on the three exponential components ($\mathrm{I}=\mathrm{a}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}1}}+\mathrm{b}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}2}}+\mathrm{c}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}3}}$) by a curve fitting technique.

Sample	#1	#2	#3	#4
Alkaline Earth Metal	CaAl2O4:Eu,Nd	CaAl2O4:Eu,Nd,Mg	CaAl2O4:Eu,Nd,Sr	CaAl2O4:Eu,Nd,Ba
t1[s]	285.7	235.8	209.1	217.8
t2[s]	25.28	34.06	29.2	29.54
t3[s]	0.7052	0.405	0.3382	0.3097
a	37.46	64.91	71.9	48.11
b	150	202.6	194.6	191.1
c	0.8194	0.9499	0.9309	0.915

In order to optimize further the conditions, we next tested different concentrations of alkaline metal doping. We chose Sr²⁺ doping, rather than Ba²⁺ doping, since Sr²⁺ doping exhibits a slower decay after 10 s than Ba²⁺ doping. Various concentrations of SrCO₃ from 0 to 0.03 mol (per mol of CaAl₂O₄:Eu²⁺, Nd³⁺) were tested and the results are shown in Fig 5 and Table 4. Again, the shape and bandwidth of the UV-excited luminescence did not change at different concentrations of SrCO₃, indicating again the Eu²⁺ centers. We found that all of the phosphors doped with SrCO₃ displayed enhanced phosphorescence up to 206% of the initial value, as compared to that observed for the non-doped crystal. The strongest initial phosphorescence was observed with 0.015 mol of SrCO₃ (per mol of CaAl₂O₄:Eu²⁺, Nd³⁺). Concentrations below 0.015 mol of SrCO₃ may not be enough to enhance the electronic transitions of Eu²⁺, while concentrations above 0.015 mol of SrCO₃ may disrupt the overall crystal structure, decreasing the phosphorescence.

Fig 5.

(A) Emission spectrum of CaAl₂O₄:Eu²⁺, Nd³⁺ crystals curves depending on Sr²⁺ concentration. (B) Decay curves depending on Sr²⁺ concentration. (C) Magnified views of the graph in (B). (D) Decay curves in log scale depending on Sr²⁺ concentration. (E) Relative initial intensity measured at 5s (relative values where the value of control sample #1 is 1.0) depending on Sr²⁺ concentration.

Table 4. Nominal compositions of the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals doped with different Sr²⁺ concentrations and the calculated decay times of the phosphorescence from the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals doped with various Sr²⁺ concentrations.

Decay times were calculated based on the three exponential components ($\mathrm{I}=\mathrm{a}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}1}}+\mathrm{b}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}2}}+\mathrm{c}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}3}}$) by a curve fitting technique.

Sample	#1	#2	#3	#4	#5	#6
Mol of Sr	CaAl2O4:Eu,Nd	CaAl2O4:Eu,Nd,Sr0.005	CaAl2O4:Eu,Nd,Sr0.010	CaAl2O4:Eu,Nd,Sr0.015	CaAl2O4:Eu,Nd,Sr0.020	CaAl2O4:Eu,Nd,Sr0.030
t1[s]	285.7	159.5	182	175.4	217.8	183.4
t2[s]	25.28	22.82	26.25	26.42	29.54	28.02
t3[s]	0.2077	0.3476	0.2959	0.2927	0.3097	0.2954
a	37.46	52.94	51.35	50.35	48.11	48.43
b	150	161.5	161.4	173.4	191.1	154.5
c	0.7052	0.9284	0.9085	0.9097	0.915	0.9087

Doping with impurities—Alkali metal doping

To further break the centro-symmetry of the structure, we tried to substitute Ca²⁺ with alkali metals. Doping with alkali metals is expected to result in two effects: a decrease in cation vacancies due to charge differences, and changes in the crystal structure symmetry due to size differences. These two effects may cause the corresponding shrinkage or expansion of the host structure and changes in the formation of hole traps, thereby resulting in the change of the afterglow characteristics. The powder materials SrCO₃, Al₂O₃, Nd₂O₃, Eu₂O₃, and M₂CO₃ (M = Li, Na, and K) were weighed out and mixed according to the mole ratios of the elements in the final product, and boric acid was added as a flux to prepare the polycrystalline CaAl₂O₄: Eu²⁺, Nd³⁺. The compounds were pressed into pellets, followed by sintering into ceramics at 1300°C for 3–5 h in a N₂ / H₂ reducing atmosphere. The final products were irradiated with 365-nm UV light for 5 min, and the phosphorescence and afterglow curves were measured at 448 nm, as shown in Fig 6 and Table 5. The band position, shape, and width of the UV-excited luminescence were found to be identical, indicating the same Eu²⁺ centers in the different compounds doped with different alkali metals. However, the phosphorescence intensities varied significantly with the alkali metal doping, although the decay times of the CaAl₂O₄:Eu²⁺, Nd³⁺ doped with different alkali metal ions were almost similar. Only Li⁺ doping showed an increase in the initial phosphorescence intensity measured 5 s after removing the UV-light, and Na⁺ and K⁺ doping showed similar or lower intensity compared to the non-doped compounds, suggesting changes in the afterglow intensity by the different sized-alkali metal doping. The ionic radii of the alkali metals decrease smoothly from K⁺ to Li⁺ (Li⁺: 90 pm, Na⁺: 116 pm, K⁺: 152 pm, Ca²⁺: 114 pm); Na⁺ and K⁺ are larger than Ca²⁺, while Li⁺ is smaller than Ca²⁺. Therefore, the Na⁺ and K⁺ ions may not be able to enter the cation vacancies since they are larger than Ca²⁺, thus decreasing the number of cation vacancies and hole traps. The results suggest that Na⁺ or K⁺ co-doping likely quenches the afterglow luminescence intensity efficiently due to a decrease in the number of cation vacancies.

Fig 6.

(A) Emission spectrum of CaAl₂O₄:Eu²⁺, Nd³⁺ crystals depending on alkali metal ion doping. (B) Decay curves depending on alkali metal ion doping. (C) Magnified views of the graph in (B). (D) Decay curves in log scale depending on alkali metal ion doping. (E) Relative initial intensity measured at 5s (relative values where the value of control sample is 1.0) depending on alkali metal ion doping.

Table 5. Nominal compositions of the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals doped with different alkali metals and the calculated decay times of the phosphorescence from the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals doped with various alkali metals.

Decay times were calculated based on the three exponential components($\mathrm{I}=\mathrm{a}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}1}}+\mathrm{b}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}2}}+\mathrm{c}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}3}}$) by a curve fitting technique.

Sample	#1	#2	#3	#4
Alkali Metal	CaAl2O4:Eu,Nd	CaAl2O4:Eu,Nd,Li0.002	CaAl2O4:Eu,Nd,Na0.002	CaAl2O4:Eu,Nd,K0.002
t1[s]	294.8	246.6	191.6	283.1
t2[s]	52.81	34.46	33.45	47.04
t3[s]	0.07818	0.08444	0.628	0.1656
a	28.04	58.17	38.71	31.18
b	115.9	247.1	140.3	135.2
c	0.1524	0.004634	0.3993	0.1622

Next, in order to take full advantage of Li⁺ doping, we also tested various concentrations of Li⁺ doping from 0 mol to 0.016 mol (per mol of CaAl₂O₄:Eu²⁺, Nd³⁺). Doping with different concentrations of Li⁺ shows a similar band position, shape, and width, but different initial phosphorescence intensity (Fig 7 and Table 6). We found that all of the Li+ doping with different concentration of the Li⁺(0.005~0.016 mol) enhance the phosphorescence from 190% up to 239% of the initial value, as compared to that observed for the non-doped crystal. The optimal concentration of Li⁺ was 0.010 mol (per mol of CaAl₂O₄:Eu²⁺, Nd³⁺), which was similar to the previously reported optimal concentration of Li⁺ in the SrAl₂O₄:Eu²⁺, Dy³⁺. This concentration is likely enough to enter into the cation vacancies, enhancing the electronic transition of Eu²⁺ but not too high to disrupt the overall crystal structure.

Fig 7.

(A) Emission spectrum of CaAl₂O₄:Eu²⁺, Nd³⁺ crystals depending on Li⁺ concentration. (B) Decay curves depending on Li⁺ concentration. (C) Magnified views of the graph in (B). (D) Decay curves in log scale depending on Li⁺ concentration. (E) Relative initial intensity measured at 5s (relative values where the value of control sample #1 is 1.0) depending on Li+ concentration.

Table 6. Nominal compositions of the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals doped with different Li⁺ concentrations and the calculated decay times of the phosphorescence from the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals doped with various Li⁺ concentrations.

Decay times were calculated based on the three exponential components($\mathrm{I}=\mathrm{a}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}1}}+\mathrm{b}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}2}}+\mathrm{c}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}3}}$) by a curve fitting technique.

Sample	#1	#2	#3	#4
Mol of Li	CaAl2O4:Eu,Nd	CaAl2O4:Eu,Nd,Li0.005	CaAl2O4:Eu,Nd,Li0.010	CaAl2O4:Eu,Nd,Li0.016
t1[s]	287.2	284.1	280.4	331.3
t2[s]	46.2	45.18	44.95	49.54
t3[s]	0.03054	0.8594	0.4899	0.682
a	24.62	89.85	114.5	82.79
b	120.5	263.3	317.6	262.8
c	0.4253	0.4799	0.4799	0.7127

Doping with impurities—Si⁴⁺ doping

Next, we carried out the experiment of doping with SiO₂ in order to substitute Al³⁺ with Si⁴⁺ in the CaAl₂O₄:Eu²⁺, Nd³⁺ crystalline structure. Doping with SiO₂ is expected to cause not only the creation of cation vacancies, but also the shrinkage of the crystal structure since the size of Si⁴⁺ (~40 pm) in tetrahedral mode is smaller than the size of Al³⁺ (53 pm). If optimal, the cation vacancies, which act as hole traps, and breaking the symmetry of the crystal structure by shrinkage or expansion of the crystal structure contribute to boost the phosphorescence intensity and increase the lifetime of the phosphorescence. To figure out the effect of Si⁴⁺ doping on CaAl₂O₄:Eu²⁺, Nd³⁺, various concentrations of SiO₂ were tested, ranging from 0 mol to 0.065 mol. The compounds were prepared in the same way as the previous experiments, and the decay curves of the afterglow and the fitting results are shown in Fig 8 and Table 7. The band position, shape, and width of the UV-excited spectra appear similar, implying the same luminescent centers. However, the intensities of the afterglow appear different at the different concentrations of Si doping. Interestingly, the initial afterglow intensities were lower than that observed from the non-doped crystal at concentrations of Si⁴⁺ below 0.05 mol, whereas the initial afterglow intensities became brighter at concentrations of Si⁴⁺ above 0.06 mol. This observation could be explained by two different effects from Si⁴⁺ doping in the crystal structures: a shrinking effect caused by the smaller size of Si⁴⁺ and an expansion effect caused by the creation of cation vacancies. If these two effects cancel out each other, there is no or a minimal boost effect on the phosphorescence. It may be the case when the concentration of Si⁴⁺ is lower than 0.05 mol, and additional disruption of the overall crystal structure likely decreases the phosphorescence intensity. However, if these two effects are synergic and the hole traps with the optimal depth are created by cation vacancies, the phosphorescence is enhanced. This may be the case when the concentration of Si⁴⁺ is higher than 0.06 mol. Finally, we found that the optimal concentration of Si⁴⁺ is 0.06 mol, enhancing the phosphorescence intensity up to 144% that of the original value for the non-doped crystal.

Fig 8.

(A) Emission spectrum of CaAl₂O₄:Eu²⁺, Nd³⁺ crystals depending on Si⁴⁺ concentration. (B) Decay curves depending on Si⁴⁺ concentration. (C) Magnified views of the graph in (B). (D) Decay curves in log scale depending on Si⁴⁺ concentration. (E) Relative initial intensity measured at 5s (relative values where the value of control sample #1 is 1.0) depending on Si⁴⁺ concentration.

Table 7. Nominal compositions of the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals doped with different Si⁴⁺ concentrations and the calculated decay times of the phosphorescence from the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals doped with various Si⁴⁺ concentrations.

Decay times were calculated based on the three exponential components ($\mathrm{I}=\mathrm{a}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}1}}+\mathrm{b}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}2}}+\mathrm{c}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}3}}$) by a curve fitting technique.

Sample	#1	#2	#3	#4	#5	#6
Mol of Si	CaAl2O4:Eu,Nd	CaAl2O4:Eu,Nd,Si0.020	CaAl2O4:Eu,Nd,Si0.030	CaAl2O4:Eu,Nd,Si0.050	CaAl2O4:Eu,Nd,Si0.060	CaAl2O4:Eu,Nd,Si0.065
t1[s]	132.6	113.9	207.7	149.6	171.1	137
t2[s]	25.04	29.61	27.97	21.15	28.74	24.52
t3[s]	0.336	0.566	0.426	0.385	0.562	0.6541
a	29.15	15.4	14.33	27.08	43.46	48.8
b	79.46	37.49	61.81	83.47	119	111.5
c	0.9006	0.7582	0.7413	0.8444	0.9549	0.8646

Optimization of the flux

Further enhancements can be achieved by optimizing the flux conditions. In this study, H₃BO₃ was used as the flux, and we tested various concentrations of H₃BO₃ to find its optimal concentration. At high concentrations of H₃BO₃, it was hard to remove the hardened final product from the crucibles after firing and, even when it was removed from the crucibles, the final product was too hard to be ground in the mortar. Therefore, we tested concentrations of H₃BO₃ from 0.15 mol to 0.25 mol per mol of CaAl₂O₄:Eu²⁺, Nd³⁺. From the measurements (Fig 9 and Table 8), we found that the phosphorescence slightly increased with the concentration of H₃BO₃. The optimal concentration of H₃BO₃ was found to be 0.25 mol, since an excess of H₃BO₃ may form trigonal planar BO₃ by-product units, probably disrupting the phosphorescence mechanism.

Fig 9.

(A) Emission spectrum of CaAl₂O₄:Eu²⁺, Nd³⁺ crystals curves depending on H₃BO₃ concentration. (B) Decay curves depending on H₃BO₃ concentration. (C) Magnified views of the graph in (B). (D) Decay curves in log scale depending on H₃BO₃ concentration. (E) Relative initial intensity measured at 5s (relative values where the value of control sample #1 is 1.0) depending on H₃BO₃ concentration.

Table 8. Nominal compositions of the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals synthesized with different H₃BO₃ concentrations and the calculated decay times of the phosphorescence from the CaAl₂O₄:Eu²⁺, Nd³⁺ crystals doped with various Si⁴⁺ concentrations. Decay times were calculated based on the three exponential components ($\mathrm{I}=\mathrm{a}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}1}}+\mathrm{b}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}2}}+\mathrm{c}*{\mathrm{e}}^{-\frac{\mathrm{t}}{\mathrm{t}3}}$) by a curve fitting technique.

Sample	#1	#2	#3
Mol of H3BO3	0.15	0.20	0.25
t1[s]	156.3	174.6	203.6
t2[s]	24.63	28.23	27.73
t3[s]	0.3777	0.2626	0.3132
a	28.17	28.14	37.43
b	114.5	94.67	100.6
c	0.9703	0.9807	0.9036

Discussion

Here, we have described the systematic characterization of the phosphorescence properties of CaAl₂O₄:Eu²⁺, Nd³⁺ synthesized with various compositions to develop bright and persistent blue-emitting phosphors. In an initial test, the activator and co-activator compositions were optimized and then, we tried to substitute Ca²⁺ with alkali metal or alkaline earth metal ions of various sizes, or substituted Al³⁺ with Si⁴⁺ to create crystallographic distortions and boost the luminescence of CaAl₂O₄:Eu²⁺, Nd³⁺. In general, the band position, shape, and width did not vary, while the persistence times and intensities varied greatly with the different compositions, indicating the same luminescent Eu²⁺ centers are present in the different compositions we tried. Therefore, we quantitatively characterized the afterglow intensity to find the optimized conditions for bright and persistent blue-emitting phosphors. In the composition studies on the activator Eu²⁺ and the co-activator Nd³⁺, ~0.006 mol Eu²⁺ (per mol CaAl₂O₄:Eu²⁺, Nd³⁺) and a Nd³⁺/Eu²⁺ ratio of 1 resulted in the brightest and longest emission. These are much lower concentrations than the optimum concentration of Eu²⁺ (~0.935 mol Eu²⁺ per mol SrAl₂O₄:Eu²⁺, Dy³⁺) and Dy³⁺ (~2.244 mol per mol SrAl₂O₄:Eu²⁺, Dy³⁺) in the green-emitting SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor reported in our previous study, probably owing to the low solubility of Eu²⁺ and Nd³⁺ in the CaAl₂O₄ crystal due to their relatively large sizes[28]. The different compositions of Eu²⁺ and Nd³⁺ in CaAl₂O₄:Eu²⁺, Nd³⁺ resulted in big differences in the afterglow intensity and hence, the optimization of the activator and co-activator composition was found to be important towards a high phosphorescence intensity. In the alkaline earth metal doping test, the alkaline metal doping achieved significant enhancements of the luminescence up to 190% of the initial value, as compared to that observed for the non-doped crystal, probably due to the distorted crystal structure with alkaline earth metal doping leading to less forbidden transitions. Among them, the largest alkaline metal, Ba²⁺, showed the largest increase in initial luminescence, while the smallest alkaline metal, Mg²⁺, showed the smallest increase in initial luminescence. When we varied the concentration of SrCO₃, the phosphorescence was enhanced up to 206% of the initial value with 0.015 mol of SrCO₃ (per mol of CaAl₂O₄:Eu²⁺, Nd³⁺), as compared to that observed for the non-doped crystal. In the alkali metal doping test, only Li⁺ doping showed an increase in the initial phosphorescence intensity, while Na⁺ and K⁺ doping showed a similar or lower intensity compared to the non-doped compound. When we varied the concentration of Li⁺ to find the optimal value, the phosphorescence was enhanced up to 239% of the initial value with 0.010 mol per mol of CaAl₂O₄:Eu²⁺, Nd³⁺, as compared to that observed for the non-doped crystal. From the Si doping test, we found that the phosphorescence intensity could be enhanced up to 144% of the original value at the optimal concentration of Si⁴⁺ (0.06 mol). Lastly, the flux was also found to affect the phosphorescence. The phosphorescence could be slightly increased as the concentration of H₃BO₃ increased; however, amounts larger than 0.25 mol of H₃BO₃ make the compound too hard and difficult to remove from the crucibles after firing and grinding in the mortar.

Finally, we found that our combined optimized condition which are doping with ~0.006 mol Eu²⁺, ~0.006 mol Nd³⁺, 0.015 mol of SrCO₃, 0.010 mol Li⁺, 0.06 mol Si⁴⁺ and 0.25 mol of H₃BO₃ (per 1mol SrAl₂O₄:Eu²⁺, Dy³⁺) boosts the phosphorescence intensity to 257% of the initial value which is doping with ~0.006 mol Eu²⁺, ~0.006 mol Nd³⁺(Fig 10). This investigation is expected to provide a guideline for the synthesis of bright and long persistent blue-emitting phosphors, and facilitate the application of persistent phosphors with afterglow characteristics superior to those of conventional phosphors. Although the detailed mechanism of the doping effects on the persistent luminescence remains an open question, we note that the role of lattice defects in CaAl₂O₄:Eu²⁺, Nd³⁺ as traps are likely of great importance for the persistence of the luminescence. Further works utilizing different experimental spectroscopic and other techniques such as XRD and microscopy regarding phase and purity confirmation would be valuable to explore the details of the phosphorescence mechanism in CaAl₂O₄:Eu²⁺, Nd³⁺, the co-doping effects and the optimization of alkaline earth metals and alkali metals to further enhance the phosphorescence efficiency. This blue-emitting material is expected to be used as a novel phosphor with numerous applications in not only white LEDs, but also in the areas of energy saving and safety improvement.

Fig 10.

(A) Decay curves in log scale of the best optimized CaAl₂O₄:Eu²⁺, Nd³⁺ crystals(#2) compared with the control CaAl₂O₄:Eu²⁺, Nd³⁺ crystals which is doped with optimized Eu²⁺ and Nd³⁺(#1). (B) Relative initial intensity measured at 5s (relative values where the value of control sample #1 is 1.0)

Methods

The details of our experimental methods have been described previously[28]. Briefly, all CaAl₂O₄:Eu²⁺, Nd³⁺ powder samples were synthesized by a high-temperature solid-state reaction. High-purity SrCO₃, Eu₂O₃ (Rhône-Poulenc, 99.99%), Nd₂O₃, Al₂O₃, MCO₃ (M = Ca, Sr, and Ba; Merck, >99.0%), and SiO₂ (Aerosil OX 50, Degussa) were mixed and H₃BO₃ was added as a flux. After grinding the mixtures in an agate mortar, they were fired in molybdenum crucibles for 3–5 h at ~1300°C in a furnace under a weak reductive atmosphere of flowing N₂/H₂ (5%) gas. After cooling down the synthesized samples to room temperature, they were ground again in an agate mortar. The final samples were irradiated with 365 nm UV-light for 5 min. After turning off the UV lamp, the emission spectra were recorded with a Hitachi 850 fluorescence spectrophotometer in the wavelength range from 300 to 950 nm.

Data Availability

All relevant data are within the paper.

Funding Statement

The author(s) received no specific funding for this work.