Recent Developments in Lanthanide-Doped Alkaline Earth Aluminate Phosphors with Enhanced and Long-Persistent Luminescence.

Lanthanide-activated alkaline earth aluminate phosphors are excellent luminescent materials that are designed to overcome the limitations of conventional sulfide-based phosphors. The increasing research attention on these phosphors over the past decade has led to a drastic improvement in their phosphorescence efficiencies and resulted in a wide variety of phosphorescence colors, which can facilitate applications in various areas. This review article discusses the development of lanthanide-activated alkaline earth aluminate phosphors with a focus on the various synthesis methods, persistent luminescence mechanisms, activator and coactivator effects, and the effects of compositions. Particular attention has been devoted to alkaline earth aluminate phosphors that are extensively used, such as strontium-, calcium-, and barium-based aluminates. The role of lanthanide ions as activators and coactivators in phosphorescence emissions was also emphasized. Finally, we address recent techniques involving nanomaterial engineering that have also produced lanthanide-activated alkaline earth aluminate phosphors with long-persistent luminescence.

1. Introduction

Phosphorescence involves the emission of light for significant periods of time, even after removal of the exciting radiation. Unlike fluorophores, phosphors can store the absorbed light energy and release it as long-persistent luminescence in the form of a delayed weak radiation via forbidden energy state transitions. Such persistent, luminescent, and phosphorescent materials with an adequate lifetime have attracted considerable attention for a wide range of applications, such as devices based on organic light-emitting diodes (OLEDs) and in solar cells for energy conservation and emergency lighting.

Natural phosphorescence was first observed by Cellini in diamond in 1568, and several natural minerals have also been reported to generate similar emissions of light under illumination with sunlight, such as naturally doped willemite, scheelite, and calcite crystals [1]. The first artificial phosphor was synthesized via calcination of sulfur-rich barium sulfate in 1604 [2]. Sulfide-based phosphors, such as rare-earth alkali sulfides (CaS, SrS) and zinc sulfides doped with copper and cobalt (ZnS:Cu, ZnS:Co), were subsequently synthesized in the first half of the 20th century [3]. Among these, copper-doped zinc sulfide (ZnS:Cu) phosphors have been extensively employed as long-lasting phosphorescent materials in various areas, such as flat panel displays, cathode ray tubes, fluorescent lamps, and traffic signs [4]. Its afterglow intensity is known to be enhanced by doping with transition metals (such as cobalt) and radioactive elements (such as promethium); however, the mechanical and physical properties of the host are degraded when a large amount of a dopant is incorporated, which eventually leads to the rapid degradation of its chemical stability [5]. In addition, the low emission intensity and short intrinsic time of decay (1 h) of ZnS:Cu limit its applications [5].

Rare-earth-doped alkaline earth aluminates have been developed as potential phosphors to overcome such limitations. When alkaline earth aluminates (MAl2O4) are doped with rare-earth ions, the resulting structures can exhibit persistent luminescence for significantly long durations under sunlight illumination, thereby leading to them being considered as excellent luminescent materials [6,7,8,9,10]. Rare-earth elements are a group of 17 elements consisting of lanthanides, yttrium (Y), and scandium (Sc). Europium-doped strontium aluminates (SrAl2O4:Eu2+) were synthesized and studied as the first rare-earth-doped strontium aluminates in the late 1960s [11]. This development can be considered a breakthrough in the field of phosphorescence in terms of the applications of phosphor, primarily because of the high luminescence efficiency of such phosphors, which was ~10 times that of zinc sulfide; such phosphors also exhibited high chemical and physical stability [12]. Moreover, these phosphors are environmentally friendly, thanks to their low chemical toxicities and the lack of radioactive elements [13]. Particularly, the development of codoped SrAl2O4:Eu2+,Dy3+ (Dy: dysprosium) phosphors by Matsuzawa et al. has received considerable attention for the replacement of traditional ZnS-based phosphors, owing to their improved afterglow intensities, lifetimes, and chemical stabilities compared to those of the previously synthesized phosphors [5]. The intense emission of rare-earth-based phosphor has led to extensive applications in light-emitting diode (LED) devices, thin-film electroluminescent (TFEL) devices, optoelectronic or cathodoluminescent devices, safety marks, radiation dosimetry, X-ray imaging, bioimaging, and photodynamic therapy [14,15,16,17,18,19]. In particular, their applications as LED devices can help in replacing argon-mercury discharge fluorescent lamps, which are extensively used for general lighting purposes. They can also be applied as alternative mercury-free excitation sources for avoiding the use of hazardous ingredients and environmentally unsafe materials [20]. Moreover, they have been recently demonstrated as solar cell materials and high upconversion luminescent materials [21,22].

The phosphorescence from such lanthanide-activated alkaline earth aluminate phosphors can be tuned over the emission wavelength and lifetime, based on the 4f–5d transitions of lanthanides. The 4f–5d optical transition is electric-dipole allowed and, therefore, generally features a high radiative emission probability and short lifetime (of the order of tens of nanoseconds) [23,24,25]. This transition generates significantly broader absorption and emission spectra because of the high sensitivity of the 5d orbital to the surrounding environment. The luminescence properties arising from the 4f–5d transition of lanthanides can be tuned by varying the host material and lanthanide emitters. In particular, the luminescence properties arising from the 4f–5d transition of Eu2+ have been investigated in more than 300 compounds, and various emission colors from near-ultraviolet to deep red have been revealed [26].

Therefore, in this review, lanthanide-doped alkaline earth aluminate phosphors are discussed in the context of their potential as excellent luminescent materials. Lanthanide-doped strontium aluminate-, calcium aluminate-, and barium-aluminate-based phosphors are focused on, and their synthesis techniques, phosphorescence mechanisms, and effects of dopants and codopants are reviewed. Recent advances in the synthesis of lanthanide-doped alkaline earth aluminate phosphors are also highlighted. Finally, we discuss the prospects and challenges of the future development of lanthanide-doped alkaline earth aluminate phosphors.

2. Strontium Aluminate Phosphors

Strontium aluminates are the most extensively used alkaline earth aluminates and are convenient host crystals for rare-earth dopants [27]. They have attracted recent research interest that has resulted in their extensive applications in several fields owing to their excellent phosphor properties such as high brightness, long-persistent luminescence, and good chemical stability compared to those of other phosphor materials (Table 1). There are various types of strontium aluminate hosts, such as SrAl2O4, SrAl2B2O7, SrAl4O7, SrAl12O19, Sr3Al2O6, and Sr4Al14O25, all of which have different structures. For example, SrAl2O4 has a tridymite structure, whereas SrAl12O19 has a magnetoplumbite structure [28,29]. Among these, SrAl2O4 is a promising host material for ensuring persistent luminescence and acts as a convenient host crystal for rare-earth and transition metal dopants [30].

Table 1

Comparison of reported studies for the synthesis of lanthanide-doped strontium aluminate phosphors.

Host Material	Activator	Co-Activator	Synthesis Method	Color or λ	Remarks	Ref
SrAl2O4	Eu2+	-	Solid-state reaction(1300 °C)	Green(λem = 515 nm(295 K)λem = 445 nm(20 K))	It was found that the luminescent center is the same, but excitation processes are different at different temperatures.	[31]
			Solid-state reaction(1250 °C)	Green(λem = 512 nm)	The position of the Eu 4f states showed the charge transfer transition.	[28]
			Combustion method(600 °C)	Green(λex = 360 nm, λem = 513 nm)	The ratio of Eu2+ to Eu3+ is changed depending on the total concentration of Eu dopants, determining the luminescence color of the phosphors.	[32]
			Combustion method using urea at 500 °C and calcinated at 1000 °C	Green(λem = 520 nm)	The luminescence mechanism and temperature dependence of bands intensities are discussed on the crystal field theory and the vibronic approach.	[7]
	Dy3+	-	Combustion method(600 °C)	λex = 356 nm,λem = 480 nm, 573 nm, 670 nm	The piezo-electricity was suggested to be responsible for producing mechanoluminescence in prepared phosphor.	[27]
	Tb3+		Precursor route via the thermal decomposition of tartarate compounds	λem = 542 nm	They demonstrated that the precursor method via the thermal decomposition of multimetallic tartarate compounds is a quick, simple and inexpensive way for the preparation of alkaline-earth aluminate powder.	[33]
	Eu2+ or Ce3+	-	Combustion method(600 °C)	Eu2+:λex = 230, 350 nm, λem = 498 nm,Ce3+:λex = 266, 331 nm, λem = 371 nm	Experimental results matched with the predictions of Dorenbos’ model.	[30]
	Eu2+ or Nd3+	-	Solid-state reaction(1000 °C)	-	The structures of the alkaline earth aluminates were systematically studied using a combination of synchrotron X-ray and neutron powder diffraction.	[13]
	Eu2+	Dy3+	Solid-state reaction(1300 °C)	Green(λex = 365 nm, λem = 520 nm)	They observed that Dy3+ ion creates the highly dense trapping level by acting as the hole-trap.	[5]
			Floating zone technique	Green (λem = 520 nm)	The intensities and the persistent times of the phosphorescences are found to depend on the growth atmosphere.	[34]
			Laser-heated pedestal growth method	Green (λem = 520 nm)	It was found that multiple trapping centers are involved in the phosphorescence dynamic processes, which are responsible for the long persistence.	[35]
			Solid-state reaction(900–1350 °C)	λem = 518 nm	The depth of Dy3+ trap levels is in the order of BaAl2O4 host > CaAl2O4 host > SrAl2O4 host.	[36]
			Sol–gel method(900–1250 °C)	Green-blue (λem = 511 nm)	It was found that the single-phase SrAl2O4 was formed at 900 °C, which is 300 °C lower than the required temperature for the conventional solid-state reaction.	[37]
			Combustion method(600 °C)	Yellow-green(λem = 516 nm)	They proposed that phosphor samples obtain a persistent luminescence with the aid of the energy transfer at the trap level.	[38]
			Combustion method (500 °C)	λem = 528 nm	They found that the monoclinic crystal structures of both CaAl2O4 and SrAl2O4 are more appropriate in creating the traps, which is directly related to the long afterglow phenomena.	[39]
			Laser synthesis	Green (λem = 520 nm)	This laser melting method is a promising route for the synthesis of ceramic phosphors.	[40]
			Solid-state reaction(~1300 °C)	Green(λem = 520 nm)	A systematic investigation of the composition of phosphors, such as the concentrations of Eu2+,Dy3+, alkali metal, alkaline earth metal, Si ions.	[12]
			Flame spray pyrolysis technique	Green(λem = 525 nm)	The flame spray pyrolysis technique was demonstrated to manufacture the rounded and spherical particles of SrAl2O4:Eu2+/Dy3+ phosphor without any problem.	[41]
		Nd3+	Combustion method (550 °C)	Green-blue(λex = 280 nm, λem = 612 nm)	Nd3+ trap levels can be thought of as the lanthanide element that causes long phosphorescence at room temperature.	[42]
		Na+	Solid-state reaction followed by ball-milling	Green(λem = 520 nm)	This report presents the factors affecting the luminescence properties of the Eu2+-, R3+-doped SrAl2O4.	[43]
		Dy3+ or Nd3+	Combustion method followed by annealing at 1150 °C	Green(Dy3+: λem = 515 nm,Nd3+: λem = 480 nm)	Eu2+ photoluminescence is observed to be shifted in a monoclinic/orthorhombic structure.	[44]
		Dy3+,Tb3+	Combustion method(600 °C)	Green(λem = 513 nm)	Compared with SrAl2O4:Eu2+,Dy3+ phosphor, the initial luminescence brightness improved, and the long afterglow time was prolonged in SrAl2O4:Eu2+, Tb3+ phosphor.	[45]
		La3+–Lu3+, Y3+; excluding Pm3+ and Eu3+	Solid-state reaction(1250–1300 °C)	Green(λem = 520 nm)	The co-doping by Dy3+ intensifies the luminescence by an order of magnitude, whereas the easily reducible rare earths, such as Sm3+ and Yb3+, suppressed both the afterglow and the thermoluminescence.	[46]
Sr4Al14O25	Eu2+	Dy3+, Er3+	Solid-state reaction(1300 °C)	Green-blue(λem = 481, 492 nm, and 529 nm)	Appropriate Er3+ doping significantly enhanced the afterglow performance of the phosphors, but excessive Er3+ doping caused concentration quenching.	[47]
Sr4Al2O7	Eu3+, Eu2+	-	Solid-state reaction(1500 °C)	Red(λex = 450 nm, λem = 607 nm)	Sr4Al2O7 has higher emission intensity than Sr3Al2O6 due to the higher optimum doping concentration of Sr4Al2O7 phosphor.	[31]
	Eu2+	Ca2+	Halide-assisted solid-state reaction (1450 °C)	Red (λem = 610 nm)	Partial substitution of Sr2+ by Ca2+ in Sr4Al2O7:Eu phosphors is found to be an efficient way to increase the proportion of longer wavelength emission and luminescence intensity.	[48]
SrAl12O19	Eu2+	-	Combustion method (500 °C)	Red(λex = 341 nm,λem = 397 nm)	Europium ions were found to be present both in divalent as well as trivalent oxidation states in the sample, and Eu2+ was observed as the dominant luminescent site.	[49]
SrAl2O4,Sr4Al14O25	Eu2+	Dy3+	Solid-state reaction followed by ball-milling	Green (SrAl2O4),Blue (Sr4Al14O25)	The significant loss of luminescence was observed below 2 μm average crystallite size, and performance could be partially restored by reductive annealing above 1000 °C.	[50]
Sr3Al2O6, SrAl2O4, Sr4Al14O25	Eu2+	-	Solid-state reaction(1350 °C)	Sr3Al2O6:Eu2+: λem = 510 nmSr4Al14O25:Eu2+: λem = 483 nm	The influences of Al/Sr ratio, sintering temperature, the doping concentration of europium ions on structural transformation and luminescent properties of the phosphors were studied.	[51]
SrAl2O4, Sr4Al4O10, Sr3Al2O6	Ce3+	-	Sol–gel synthesis(700–1200 °C)	SrAl2O4:Ce: λex = 575~700 nmSr3Al2O6:Ce: λex = 585~675 nmSr4Al4O10:Ce: λex = 615 nm	The optical reflectance spectra clearly showed the influence of the strontium aluminate matrix on the optical properties of the synthesized phosphors.	[52]
SrAl2O4, SrAl4O7, SrAl12O19, Sr4Al14O25	Eu2+	-	Solid-state reaction(SrAl12O19:1300 °C, Sr4Al14O25:1400 °C, SrAl2O4:1350 °C)Citric acid method (SrAl4O7: 1050 °C)	SrAl12O19: λem = 397 nmSrAl4O7: λem = 470 nmSr4Al14O25: λem = 490 nm	The Eu2+ emission spectra in the other aluminates showed the trend that the Eu2+ emission shifts to longer wavelengths with an increasing Sr/Al ratio.	[53]

2.1. Lanthanide Ion-Doped Strontium Aluminate Phosphors

Eu2+ has been frequently used as an activator dopant in SrAl2O4 because it exhibits anomalously long phosphorescence. Because the Sr2+ and Eu2+ ions have similar ionic radii (1.21 and 1.20 Å, respectively), Eu2+ ions are very likely to be located in the Sr2+ positions; this was also confirmed by electron paramagnetic resonance (EPR) measurements [40,54]. Therefore, when Eu3+ ions are incorporated in the Sr2+ sites of SrAl2O4, they are observed to be easily reduced to Eu2+ [54,55]. In fact, two different sites are available for Sr ions to occupy, and they are slightly different with respect to the individual Sr–O distances (Sr+ and Sr2+ sites are coordinated at 6 and 7, respectively) [53]. The incorporation of Eu2+ ions in these different sites leads to a variation in the luminescent properties from both sites because of their different symmetries and orientations. The incorporated dopant ions are thought to introduce localized states of the bandgap in the host matrix, which facilitate the luminescent properties that arise from the various dopant-incorporated sites in the SrAl2O4 host material [56]. The doping sites in the host material are known to be determined by the host lattice components and the ionic radii of the dopants. For example, Eu2+, Eu3+, and Dy3+ are likely to be incorporated in the Sr2+ sites in the SrAl2O4 matrix because of their similar ionic radii+ (Eu2+: 1.2Å, Eu3+: 0.95 Å, Dy3+: 0.91 Å, Sr2+: 1.18 Å); the dissimilar ionic radii of Al3+ and O2-, compared to those of the dopants, ensure that the incorporation does not occur in their sites (Al3+: 0.53 Å, O2-: 1.4 Å). This was confirmed by EPR measurements [56].

When the SrAl2O4 host material is doped with Eu2+, the Eu2+ ion plays the role of a luminescence center via its luminescent 4f65d1 → 4f7 transition. Therefore, the 5d–4f transitions of Eu2+ in aluminate and silicate host materials generate similar broad emission spectra with a maximum in the blue-green region; the emission band is absent in systems without Eu2+ doping [57]. Moreover, the maximum peak positions are known to vary with the type of host material and are likely dependent on their surrounding configurations, such as symmetry, bond length, coordination, covalence, site size, and crystal field strength [57]. This probably occurs because of the displacement of the 5d energy level of Eu2+ in different crystal fields [56]. Therefore, numerous attempts have been made to tune the spectrum wavelength over a wide range by changing the composition and local crystal structure of the phosphors (Figure 1). For example, Eu2+ dopant ions in SrAl2O4 and Sr4Al14O25 are known to generate different emission wavelengths owing to their different crystalline structures (orthorhombic for Sr4Al14O25 and monoclinic for SrAl2O4) [44,58,59]. Kim et al. synthesized SrAl2O4:Eu2+,Dy3+ and Sr4Al14O25:Eu2+,Dy3+ as green and blue phosphors, respectively, by taking advantage of the different emission wavelengths [14].

Figure 1

Photoluminescence spectra for lanthanide doped strontium aluminate phosphors (A) Photoluminescence excitation (left) and emission (right) spectra for SrA2O4:Eu2+,Dy3+ and Sr4Al14O25:Eu2+,Dy3+. Reprinted from [61] with permission from Elsevier. (B) Photoluminescence excitation (i) and emission (ii) spectra for SrA2O4:Eu2+ (left) and SrA2O4:Ce3+ (right). Adapted from [30] under Creative Commons Attribution (CC BY) license.

Moreover, a new class of Eu2+-doped Sr4Al2O7 phosphors was recently reported to exhibit a longer wavelength relative to that of SrAl2O4:Eu2+ [31]. Zhang et al. also reported that the emission maximum shifted to a longer wavelength when the Al/Sr ratio increased in Eu2+-doped strontium aluminate phosphors [51]. Such spectral shifts in different host materials have been thoroughly explored with computational tools at the atomic and molecular levels using quantum mechanical methods, such as the density functional theory (DFT) and Hartree–Fock (HF) method. Theoretical investigations have indicated that the 4f–5d transition energy of lanthanides becomes redshifted after doping into the host lattice because it can affect the difference in energies of the lowest 4fn and the first 4fn−1d levels in the lanthanide dopants [26]. Three important factors are thought to determine the spectroscopic redshift of lanthanide ions in host materials: centroid shift, crystal field splitting, and ligand polarization [23]. First, the centroid shift of the 5d orbital can be explained using the nephelauxetic effect. Given that the covalency between the luminescent center and its neighboring anions is proportional to the nephelauxetic effect, the centroid shift is expected to be in the F− < Cl− < Br− < I− < O2− < S2− order. Next, the type of the coordination polyhedron of anions around the luminescent center is known to strongly affect the highly susceptible nature of the 5d orbitals; this is referred to as crystal field splitting. Therefore, this effect is considered to play a crucial role in determining the spectral redshift of lanthanides in the host materials. Finally, ligand polarization was proposed as an important factor for determining the spectroscopic redshift [60]. The emission wavelengths of SrO:Eu2+ and Sr2SiO4:Eu2+ phosphors, which cannot be explained using the centroid shift, can be elucidated using ligand polarization.

2.2. Synthesis of Strontium Aluminate Phosphors

Various methods have been employed for the synthesis of lanthanide-doped strontium aluminate phosphors, such as sol–gel, solid-state reaction, combustion, microwave sintering, precursor, and coprecipitation. The selection of the appropriate synthesis method is crucial because it can significantly affect the quality of the luminescent material.

Among these methods, solid-state reactions have been extensively used for phosphor synthesis. This method is based on the chemical reactions between precursors in a powder form at high temperatures (1300–1600 °C). During this process, Eu3+ is reduced to Eu2+ in a reducing atmosphere; various reductants, such as H2 + N2, HI, and NH4I, are employed to facilitate this. The reductive atmosphere is crucial for avoiding sample decomposition or oxidization. Moreover, fluxing agents such as H3BO3 or LiF are often employed during this process to facilitate grain formation and crystal growth [62]. This method is popular in industrial settings because it is a conventional and robust method for the preparation of lanthanide-activated phosphors. For example, the solid-state reaction method was employed by Kim et al. and He et al. to synthesize SrAl2O4:Eu2+,Dy3+ and Eu2+-doped Sr4Al2O7 phosphors, respectively [12,48]. This method does not produce toxic or unwanted wastes and is, therefore, considered environmentally friendly. However, it is relatively difficult to accurately control and uniformly mix the individual components via chemical reactions in solid phases, in contrast to those in liquid phases. Additionally, the high-temperature sintering employed in this process for a relatively long reaction time often results in poor homogeneity in the product. Therefore, to avoid the formation of inhomogeneous grain boundaries, protocols involving pretreatment of the starting precursors at low temperatures (500–600 °C) followed by sintering at high temperatures (>1000 °C) have been employed [63,64].

The sol-gel method has been considered an attractive and straightforward alternative to the solid-state reaction method. In this process, solid particles suspended in a liquid (“sol”) are transformed into a three-dimensional network throughout the liquid (“gel”) via polycondensation reactions of molecular precursors. This process is conducted at a relatively low reaction temperature (~950 °C) to facilitate the uniform mixing of the starting materials and the formation of homogeneous products compared to those obtained from the solid-state reaction. This method has been employed for the synthesis of various mixed-metal oxides, nanomaterials, and organic–inorganic hybrids over the last few decades. Both nonaqueous and aqueous sol–gel methods have been employed. The aqueous sol–gel process is considered better than the nonaqueous process because the former facilitates the formation of a homogeneous solid-state structure at the atomic level, which is based on the chemical interactions among the precursor species in the mixture [65]. Misevicius et al. employed this concept in using the aqueous sol–gel approach for the synthesis of various Ce-doped strontium aluminates using glycolate intermediates, such as SrAl2O4, Sr3Al2O6, and Sr4Al4O10 [52].

Although the sol–gel technique can be successfully employed at a relatively low temperature (950 °C), both the solid-state reaction and sol–gel methods have unavoidable limitations, such as the extreme temperatures for long durations. To overcome them, the combustion method has been used as a promising technique for the synthesis of aluminate phosphors. This method is based on a self-sustaining exothermic redox reaction between the materials present in the starting mixture, which includes metal nitrates and urea as the oxidant and fuel, respectively. The reaction between the nitrates and fuel results in the formation of crystalline phases using the sufficient heat even at low temperatures. High levels of chemical homogeneity are also obtained because of their dissolution in the aqueous solution; this is followed by a uniform distribution of luminescent centers. In the Eu2+-doped phosphors, Eu3+ ions are reduced to Eu2+ by the gas released in this process [66]. This method has been extensively used to prepare various oxide materials, because it is relatively easy to avoid washing, filtration, and drying. This method is also preferred for the preparation of oxide materials at low temperatures. It is safe and efficient with respect to energy conservation because this process can be completed in only a few minutes (~5 min) at low temperatures. This reasoning was employed by Singh et al. to prepare the SrAl12O19:Eu2+ phosphor via the combustion method, which was found to conserve time, energy, and costs [49]. The combustion method has also been used to prepare dysprosium-doped strontium aluminate phosphors (SrAl2O4:Dy3+) and SrAl2O4:Eu2+,Nd3+ phosphors [27,42].

Finally, the precursor method involves the thermal decomposition of a single molecular precursor at high temperatures, which leads to the formation of nanophosphors. In this method, selection of the appropriate precursor is crucial for the synthesis of pure mixed oxides. Therefore, multimetallic complex compounds are typically preferred in this approach because they allow an intimate molecular-level contact of the metal ions. The selection of the appropriate complexation agent is also important for facilitating the production of complex compounds at low temperatures that can decompose easily. The molar ratios of the chemical elements in the final product can be easily controlled by changing the precursor concentration. This method is considered economically viable compared to other phosphor synthesis methods. The precursor method has also been used to prepare Tb3+-doped alkaline earth aluminates, such as SrAl2O4:Tb3+; scanning electron microscopy (SEM) analysis on these phosphors revealed that a homogeneous microstructure with a fine particle size was obtained [33].

In most of the aforementioned phosphor synthesis methods (solid-state reaction, sol–gel processing, combustion), crystalline materials with an average particle size of several tens of micrometers are produced. Therefore, to prepare nanometer-sized phosphors, post-treatment of the bulk phosphors that involves grinding of the large phosphor particles is necessary. Therefore, the particle size of the phosphor can be controlled. However, this process has several disadvantages, such as the unexpected oxidization of a few Eu2+ ions into inactive Eu3+ ions in the host lattice during the particle size reduction. In addition, the luminescent performance of the phosphors is degraded because of changes in the local coordination environment, such as crystallinity and lattice microstructure, around the dopants in the strontium aluminate hosts. For instance, Havasi et al. demonstrated the use of the ball-milling method for the production of submicrometer-sized particles of rare-earth-metal-doped strontium aluminate phosphors [50]. A comparative study of the mechanical stress resistance of various phosphors was used to observe the significant loss of long-persistent luminescence. However, this method is acceptable for use in industrial settings, although the elucidation of mechanisms involved and the feasibility of thermal restoration of the performance remain unclear. In addition to phosphor milling, various nanoengineering methods have been developed to synthesize ultrafine nanophosphors. For example, the use of surfactants or chelating reagents in hydro (solvo) thermal or microwave-assisted reactions was found to be effective for controlling the growth of the phosphor size [23]. In addition, laser ablation, template-directed synthesis, and microemulsion routes have been attempted to control the size and morphology of the phosphors [67].

2.3. Codoping of Strontium Aluminate Phosphors with Various Trivalent Lanthanide Ions

The luminescent properties of lanthanide-doped strontium aluminate phosphors can be further improved by codoping with other trivalent lanthanide ions as coactivators (such as Ln3+). The codoping of SrAl2O4:Eu2+ with Dy3+ resulted in a phosphor with an improved afterglow lifetime, intensity, and chemical stability compared to those of the SrAl2O4:Eu2+ phosphors [68]. In particular, the afterglow of SrAl2O4:Eu2+,Dy3+ was noted to last for over 10 h after exposure to illumination, and showcased a >10-fold increase in the initial intensity, making it an optimal material with persistent luminescence [43]. Interestingly, the position and shape of the luminescence emission band do not change, and the characteristic luminescence of Dy3+ is rarely observed in this coactivator-doped compound [56]. The direct excitation of Dy3+ or the energy transfer from Eu2+ to Dy3+ is noted to be negligible during the afterglow process. This implies that the luminescence center in these compounds does not change upon the incorporation of Dy3+, and it remains centered at Eu2+. Although the Dy3+ profiles are often observed only in the radioluminescence spectrum, they are likely to be hidden because of their weak intensities under the bright emission profiles of Eu2+ in most cases [41,69]. Other lanthanide ions such as Nd3+ and Tm3+ have also been employed as coactivators and were noted to exhibit similarly strong emission bands from the Eu2+ luminescence center without their characteristic photoluminescence [44,70].

The effect of coactivator concentration on the afterglow has also been investigated. Kim et al. reported that the SrAl2O4:Eu2+,Dy3+ phosphor exhibited the strongest persistent luminescence at a Dy3+/Eu2+ ratio of ~2.4 [12]; the afterglow luminance intensity was found to decrease as the Dy3+/Eu2+ ratio surpassed ~2.4, possibly because of the formation of the DyAlO3 by-product from residual and insoluble Dy3+ ions. The afterglow intensity from Eu2+ has been observed to increase as the dopant and codopant concentration increased [32]; the afterglow intensity was noted to decrease beyond its maximum value because of the concentration quenching effect, similar to that in the study discussed previously [12]. Such concentration effects involving the dopant and codopant can be explained by their distances.

Although codoping with lanthanide ions is known to significantly improve the luminescence, the exact role of Dy3+ in host materials remains unclear. The general understanding of codoping with lanthanide ions involves their contribution to the trapping centers of the material. For example, Dy3+ ions doped in the Sr2+ sites have been proposed to act as electron traps [38,70,71,72]. This is because the Dy3+ codopant ions are likely to replace the Sr2+ ions because of their similar radii and result in a +1 charge incompatibility. In addition, the concentration ratio of Eu2+ and Eu3+ after the chemical reduction process can be modified via the codopant ions because they can stabilize the Eu valences in the phosphors [73]. Similarly, Dy3+ ions are expected to modify the environment of Eu2+ ions in the SrAl2O4 host materials. Other possibilities include the trapping of a hole by Dy3+ or the attraction of other defects for charge compensation [56].

2.4. Mechanisms of Long-Persistent Luminescence from Lanthanide-Doped Strontium Aluminate Phosphors

Several models have been suggested to elucidate the persistent luminescence of codoped SrAl2O4:Eu2+,Dy3+ (Figure 2). The differences in these models arise from the assumptions regarding the charge carriers (holes or electrons), pathways of the charge carriers (valence band, conduction band, trapping centers), nature of trapping centers (intrinsic defects or codopants), and excitation mechanism of luminescence. Although the elucidation of persistent luminescence mechanisms in codoped SrAl2O4:Eu2+,Dy3+ remains incomplete, the general background in all these models involves the generation of migrating charge carriers during excitation and subsequent localization in trapping centers. The important models that try to explain the mechanism behind the long afterglow in the codoped SrAl2O4:Eu2+,Dy3+ phosphor are discussed in this section.

Figure 2

The (A) Matsuzawa, (B) Aitasalo, (C) Dorenbos, (D) Clabau models for the photoluminescence of SrAl2O4:Eu2+,Dy3+.

The first of these models was proposed by Matsuzawa et al. [5]. Briefly, the model involves the generation of a hole during Eu2+ excitation and subsequent Eu+ formation, subsequent release and migration of the hole to Eu+ through the valence band at a high temperature, and generation of Eu2+ in the excited state; a photon is also subsequently released from the excited Eu2+. In this model, Dy acts as a hole-trapping center by localizing the holes released from Eu2+, and it facilitates the conversion of Dy3+ to Dy4+. Therefore, the hole release and migration from the trapping centers to Eu+ is crucial to determining the afterglow. However, the main problem in this model involves the energetically unfavorable formation of Eu+.

To overcome this limitation, Aitasalo et al. proposed a new model for codoped SrAl2O4:Eu2+,Dy3+ [43,74,75]. In this model, the possibility of Eu+ formation is excluded, and the formation of electron traps and migration of electrons is considered instead of hole traps. Trivalent rare-earth codopants play an important role in trapping electrons at the defect level in this model. As shown in Figure 2B, electrons from the Eu2+ excited state are thermally promoted to the conduction band and are eventually trapped on defect levels, such as oxygen vacancies, trivalent rare-earth ions, cation vacancies, and interstitial ions. The charge carriers subsequently migrate back to the luminescence center, Eu2+, which is followed by the characteristic luminescence of the transition of Eu2+ to the ground state. Studies involving X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses have verified this model by confirming the accumulation of Eu3+ during excitation. The Eu2+/Eu3+ oxidation has also been observed in XANES measurements of SrAl2O4:Eu2+,Dy3+, which support this model [46]. Qiu et al. suggested that such trapping and detrapping processes from the defect levels can be repetitive [38].

Dorenbos et al. proposed a similar model [72], in which the dopant and codopant energy levels in the bandgap of the SrAl2O4 host material were estimated. In particular, the energy levels of Eu3+ and Dy3+ were presumed to be positioned immediately below the bottom of the conduction band, and ~0.9 eV below the bottom of the conduction band, respectively. Because the difference between these energy levels and that of the conduction band is small, thermal ionization is likely to occur at room temperature. The electron that migrates through the conduction band is eventually trapped by Dy3+, which is followed by recharging to Dy2+. Subsequently, the electron can be thermally released from Dy2+ and migrates to the excited Eu2+ center, which eventually leads to the photon emission. The main difference between this model and the Aitasalo model is the type of electron trap discussed. The model presented by Dorenbos et al. only considers trivalent rare-earth ions, whereas that by Aitasalo et al. suggests various kinds of defects, such as oxygen vacancies and trivalent rare-earth ions [43,74,75].

Clabau et al. proposed a new model that involved the formation of Eu3+ and electron migration, in a manner similar to that discussed in the abovementioned models [54]. Based on results from EPR experiments, the ionization of Eu2+ to Eu3+ was assumed to occur after excitation by UV irradiation. The main difference between this and the previous models involves the direct migration of electrons between the luminescence and trapping centers and not via the conduction band. This can occur if the energy levels of Eu3+ and trapping centers are located close to each other.

Broadly speaking, all the aforementioned models describe the generation of charge carriers during excitation, and their subsequent localization in trapping centers postmigration. The charge carrier is subsequently released from the trapping centers and eventually recombined with the excited luminescent center. The model that best represents the persistent luminescence in SrAl2O4:Eu2+,Dy3+ is likely to involve the creation of Eu3+ and the radiative Eu2+ transition to the ground state. Moreover, there may be several trapping centers present in the phosphor, although their exact nature remains unclear.

The use of multiple codopants has also been attempted because of the effectiveness of codoping for the enhancement of the luminescence of Eu2+. Song et al. and Li et al. employed Tb3+ and Er3+ as additional codopants in Sr4Al14O25:Eu2+,Dy3, respectively [45,47]. In addition, Havasi et al. demonstrated the photoluminescence properties of Sr4Al14O25:Eu2+,Dy3,Ho3+, which was synthesized using Dy3+ and Ho3+ as additional codopants [50]. During sample preparation in this study, Dy3+ in Sr4Al14O25:Eu2+/Dy3+ was presumed to be partially substituted with Ho3+, which remarkably increased the photoluminescence intensity and shifted the main emission peaks. This was explained via the effect of Dy3+/Ho3+ on the trapping/de-trapping and energy transfer processes in this phosphor [76].

3. Calcium Aluminate Phosphors

Calcium aluminate is also considered as a useful host matrix material for phosphors (Table 2). Similar to strontium aluminate, calcium aluminate has been known to exhibit a bright emission over a wide visible range and a high energy efficiency, quenching temperature, and chemical stability. In addition, its high toughness, strength, and high-temperature resistance facilitate its use as cement materials [77]. In particular, calcium aluminates have been applied as dental cements and bone grafts because of their bioactive, biocompatible, physical, and mechanical properties.

Table 2

Comparison of reported studies for the synthesis of lanthanide-doped calcium aluminate phosphors.

Host Material	Activator	Co-Activator	Synthesis Method	Color or λ	Remarks	Ref
CaAl2O4	Eu2+	-	Solid-state reaction(1250–1300 °C)	Blue(λem = 440 nm)	The new mechanism was proposed, which involves the excited state absorption of two 530 nm photons via deep traps followed by trapping of electrons in shallow traps.	[75]
		-	Solid-state reaction(1300 °C)	Blue(λem = 442 nm)	Good morphology and the best luminous intensity could be gained when H3BO3 mass ratio was 0.5 wt%.	[78]
	Tb3+		Precursor route via the thermal decomposition of tartarate compounds	λem = 542 nm	They demonstrated that the precursor method via the thermal decomposition of multimetallic tartarate compounds is a quick, simple and inexpensive way for the preparation of alkaline-earth aluminate powder.	[33]
	Pr2+	-	Sol–gel method	λem = 390 nm, 520 nm, 790 nm	The interlinked small granular structured particles finally formed bigger particles.	[9]
	Eu2+ or Nd3+	-	Solid-state reaction(1000 °C)	-	A systematic study of the structures of the alkaline earth aluminates using a combination of synchrotron X-ray and neutron powder diffraction.	[13]
	La3+ or Tb3+	-	Sol–gel method	Blue-greenλem = 395 nm, 535 nm	Emission peak position is not altered by doping with La3+, Tb3+, but variation in the intensity is observed.	[79]
	Eu2+ or Ce3+	-	Combustion method(600 °C)	Eu2+:λex = 275, 329 nm, λem = 440 nmCe3+:λex = 247, 300 nm, λem = 370 nm	Experimental results matched with the predictions of Dorenbos’ model.	[30]
	Eu2+	Nd3+	Floating zone technique	Blue(λem = 450 nm)	The intensities and the persistent times of the phosphorescences are found to depend on the growth atmosphere.	[34]
			Laser-heated pedestal growth method	Blue(λem = 445 nm)	It was found that multiple trapping centers are involved in the phosphorescence dynamic processes, which is responsible for the long persistence.	[35]
			Combustion method	Blue (λem = 440 nm)	Eu2+, Nd3+ co-doped calcium aluminate showed bright phosphorescence with a long duration.	[80]
			Solid-state reaction(1300 °C)	Blue(λem = 442 nm)	The composition of the activator Eu2+ and the co-activator Nd3+, the doping conditions with alkaline earth metals, alkali metals, and Si were optimized.	[81]
			Combustion method (550 °C)	Blue(λex = 355 nm, λem = 492 nm)	Nd3+ trap levels can be thought of as the lanthanide element that causes long phosphorescence at room temperature.	[42]
		Dy3+	Solid-state reaction(900–1350 °C)	λem = 445 nm	The depth of Dy3+ trap levels is in the order of BaAl2O4 host > CaAl2O4 host > SrAl2O4 host.	[36]
			Combustion method (500 °C)	λem = 449 nm	They found that the monoclinic crystal structures of both CaAl2O4 and SrAl2O4 are more appropriate in creating the traps, which is directly related to the long afterglow phenomena.	[39]
		Na+	Solid-state reaction followed by ball-milling	λem = 440 nm,	This report presents the factors affecting the luminescence properties of the Eu2+-, R3+-doped SrAl2O4.	[43]
		La3+	Combustion method (600 °C)	blue-purple(λem = 440 nm)	They proposed that phosphor samples obtain a persistent luminescence with the aid of the energy transfer at the trap level.	[38]
		Dy3+, Nd3+, La3+	Solid-state reaction(1380 °C)	Blue(λem = 440 nm)	Both initial brightness and persistent afterglow time of CaAl2O4: Eu2+, Nd3+ is better than those of CaAl2O4: Eu2+,Dy3+, and CaAl2O4: Eu2+, La3+.	[82]
		La3+–Lu3+, Y3+; except Pm3+, Eu3+	Solid-state reaction(1250–1300 °C)	Green(λem = 440 nm)	The co-doping by Dy3+ intensifies the luminescence by an order of magnitude, whereas the easily reducible rare earths, such as Sm3+ and Yb3+, suppressed both the afterglow and the thermoluminescence.	[46]

3.1. Calcium Aluminate Phosphors with Diverse Colors

Calcium aluminates belong to the spinel group of minerals, similar to the other alkaline earth aluminates. The typical chemical representation of calcium aluminate is CaAl2O4, and it exists in the monoclinic or orthorhombic forms [83]. As discussed earlier, the host crystal structure and activator are the important factors for determining the main emission peaks of aluminate-based phosphors. For example, CaAl2O4:Eu2+ is known to exhibit blue light emission, which corresponds to a shorter emission wavelength in the SrAl2O4:Eu2+ system. Among the various lanthanide-doped calcium aluminate phosphors, CaAl2O4:Eu2+ is extensively used as a phosphor material (Figure 3). As an emission center, Eu2+ is known to emit blue light via the 4f65d → 4f7 transition, whose peak is located at ~442 nm [78]. Its absorption is observed at near-UV light, which is similar to that via LED chips. Although there are fewer studies on CaAl2O4 phosphors than those on SrAl2O4, the former has attracted considerable recent interest as a luminescent host owing to its high color purity and stability; it also meets the high-efficiency-based requirements for novel blue-emitting phosphors [84].

Figure 3

Photoluminescence spectra for lanthanide doped calcium aluminate phosphors (A) Photoluminescence excitation (i) and emission (ii) spectra for CaAl2O4:Eu2+. Adapted from Ref. [30] under Creative Commons Attribution (CC BY) license. (B) Emission spectra of (i) CaAl2O4:Eu2+, (ii) CaAl2O4:Eu2+,Dy3+, (iii) CaAl2O4:Eu2+,Nd3+, (iv) CaAl2O4:Eu2+,La3+. Adapted with permission from [82]. Copyright Elsevier, 2003.

Several activators have been experimented as dopants for CaAl2O4. For example, Tb3+, which has been doped into CaAl2O4 in several studies, can substitute a Ca2+ ion as Eu2+, which is followed by the formation of a charge defect [85]; CaAl2O4:Tb3+ was found to exhibit an emission wavelength of 545 nm, which corresponded to the f-f transitions of Tb3+. La3+-doped CaAl2O4 synthesized via the sol–gel method exhibited an emission peak at 395 nm in the blue region [79]; its emission band intensity was observed to be stronger than that of CaAl2O4:Tb3+. The particle sizes of CaAl2O4:La3+ and CaAl2O4:Tb3+ were obtained as 27 nm and 31 nm, respectively, and both had no effect on the phase composition of CaAl2O4 [79]. Additionally, several studies have attempted the Ce3+ ion doping of CaAl2O4 and have resulted in a much shorter emission wavelength (330–350 nm). This emission band corresponds to the 5d–4f transitions of Ce3+ ions. The broad emission band was found to be remarkably intense because of the transition being parity-allowed [30]. Nonlanthanide ions, such as Mn, have also been employed as activators in CaAl2O4. Mn-doped aluminate phosphors synthesized via the combustion process were found to exhibit red emission from the Mn4+ ions [86]; Mn ions were noted to exist in the CaAl2O4 host material in both Mn2+ and Mn4+ states; they also occupied distorted lattice sites in the host matrix.

3.2. Codoping of Calcium Aluminate Phosphors with Various Trivalent Lanthanide Ions

Similar to strontium aluminate phosphors, codoping of CaAl2O4 phosphor materials with various trivalent lanthanide ions has been attempted to improve their luminescence properties. Lin et al. suggested that the incorporation of Dy3+, Nd3+, and La3+ can possibly enhance the brightness and persistent afterglow time [82]. Regardless of the type of codopant ions used, the excitation and emission of these three phosphors resulted in spectra that were similar in shape to those of the CaAl2O4:Eu2+ phosphor, which corresponded to the 4f7–4f65d inter-configuration transitions of Eu2+ ions. Among these phosphors, the afterglow from CaAl2O4:Eu2+,Nd3+ was found to be the brightest and with the longest duration.

The CaAl2O4:Eu2+,Nd3+ phosphor has, therefore, been extensively investigated. Zhao et al. investigated the UV-excited luminescence of CaAl2O4:Eu2+,Nd3+ and observed a broad band in the blue region (λmax = 440 nm) that arose from the 5d–4f transitions of Eu2+ [80]. The bright afterglow luminescence was observed for a long duration. This long-persistent luminescence was presumed to result from the trapping–transporting–detrapping of the holes, a process in which Nd3+ ions behaved as hole traps between the ground and the excited states of the Eu2+ ion. Kim et al. reported the optimized composition of the Eu2+ activator and Nd3+ coactivator for CaAl2O4 [81]. The introduction of Nd3+ into the CaAl2O4:Eu2+ system was found to significantly boost its phosphorescence intensity and lifetime, similar to that of Dy3+ doping in the SrAl2O4:Eu2+ system. Composition-based studies on the activator (Eu2+) and coactivator (Nd3+) have revealed that the afterglow intensity and lifetime were strongly affected by the concentrations of Eu2+ and Nd3+ in CaAl2O4:Eu2+,Nd3+; this implies that the optimization of the activator and coactivator concentrations appears to be important for obtaining a high intensity of phosphorescence. The afterglow of phosphors with various concentrations of the activator (Eu2+) and coactivator (Nd3+) were measured, and ~0.006 mol of Eu2+ per mol of CaAl2O4:Eu2+,Nd3+, and an Nd3+/Eu2+ ratio of 1 were found to result in a product with the brightest phosphorescence emission for the longest duration. This optimized concentration was noted to be much lower than that of Eu2+ (~0.935 mol per mol of SrAl2O4:Eu2+,Dy3+) and Dy3+ (~2.244 mol per mol of SrAl2O4:Eu2+,Dy3+) in the green-emitting SrAl2O4:Eu2+,Dy3+ phosphor, probably because of their relatively large sizes.

3.3. Synthesis of Calcium Aluminate Phosphors

The CaAl2O4:Eu2+ phosphors are typically prepared in a manner similar to that for the SrAl2O4:Eu2+ phosphors. The high-temperature solid-state reaction is intensively used for the preparation of CaAl2O4:Eu2+; however, the high calcination temperature and the formation of heterogeneous particles with relatively large microscale sizes limit their applications. Therefore, the various properties of the fluxing agent in the solid-state process have been modified to control the particle size and reduce the sintering temperature. This is attempted because the fluxing agent is presumed to facilitate the incorporation of lanthanide ions in the matrix lattice. For example, the effect of the concentration of H3BO3 as a fluxing agent on the structure, morphology, and luminescent properties of Ca1−xAl2O4:xEu2+ have been explored by Zeng et al. [78] A comparison of different amounts of H3BO3 for phosphor synthesis revealed that an H3BO3 mass ratio of 0.5 wt.% resulted in a product with an adequate morphology without agglomeration and the best luminous intensity among the various samples. Kim et al. also presented a comparative study of varying amounts of H3BO3 [81], which suggested that 0.25 mol per mol of CaAl2O4:Eu2+,Nd3+ resulted in the brightest phosphorescence among the various samples; however, a higher concentration of H3BO3 was found to produce a hardened final product, which can create difficulties in the subsequent mortar grounding process.

Several alternative methods involving liquid phases, such as sol–gel, combustion, coprecipitation, and microwaves, have been employed for the preparation of CaAl2O4:Eu2+, in a manner similar to that for strontium aluminate phosphors. In the liquid phase, each component can be uniformly mixed and accurately controlled. For example, the sol–gel process facilitates the homogeneous mixing of the starting materials and synthesis at a relatively low reaction temperature, which results in the formation of homogeneous products with a fine grain size [37,82]. The combustion method is another efficient technique for the preparation of CaAl2O4:Eu2+ at a relatively low temperature and is known for being facile, safe, quick, cost-effective, and energy conserving. Zhao et al. reported that the CaAl2O4 phase was formed at a combustion initiation temperature of 400 °C via the combustion method; this method resulted in the formation of persistent luminescent CaAl2O4:Eu2+-based phosphors with bright phosphorescence and a long duration [80].

Several methods have been recently attempted for the preparation of Eu2+-doped calcium aluminate phosphors. Jia et al. employed a LASER-heated pedestal growth method to synthesize the CaAl2O4:Eu2+,Nd3+ phosphor, which resulted in bright and long-persistent phosphorescence being observed from the final product [35]. Katsumata et al. attempted a loading zone technique to grow a single-crystal CaAl2O4:Eu2+,Nd3+ phosphor, and the resulting product exhibited luminescent properties similar to those of the previously reported SrAl2O4:Eu2+,Dy3+ phosphor [34].

4. Barium Aluminates

Barium aluminate phosphors are persistent and exhibit high luminescent intensity, long afterglow time, and chemical stability (Table 3). Moreover, their synthesis does not require a reducing atmosphere for dopant reduction [87].

Table 3

Comparison of reported studies for the synthesis of lanthanide-doped barium aluminate phosphors.

Host Material	Activator	Co-Activator	Synthesis Method	Color or λ	Remarks	Ref
BaAl2O4	Eu2+	-	Solid-state reaction(1400 °C)	λex = 340 nm, λem = 498 nm	The Eu3+ reduction in BaAl2O4:Eu2+ prepared in the air could be explained with the charge compensation model.	[8]
	Ce3+	-	Solid-state reaction(900–1350 °C)	λex = 357 nm, 335 nmλem = 450 nm, 402 nm	Site-selective thermoluminescence spectra showed that traps were close to the corresponding Ce3+ ion.	[88]
	Cr3+	-	Combustion method (500 °C)	Red (λem = 705 nm)	The site symmetry of Cr3+ ion in this phosphor is responsible for a distorted octahedron.	[89]
			Hydrothermal route followed by a thermal treatment	-	The dopant Cr3+ cations increased lattice strain and disturbed the crystallites to grow by acting as defects in the barium aluminate structure.	[90]
	Eu2+ or Nd3+	-	Solid-state reaction(1000 °C)	-	A systematic study of the structures of the alkaline earth aluminates using a combination of synchrotron X-ray and neutron powder diffraction.	[13]
	Eu2+ or Ce3+	-	Combustion method(600 °C)	Eu2+:λex = 270, 328, 397 nm, λem = 485 nm,Ce3+:λex = 246, 292, 308 nm, λem = 386 nm	Experimental results matched well with the predictions of Dorenbos’ model.	[30]
	Eu2+	Dy3+	Solid-state reaction (900–1350 °C)	λem = 496 nm	The depth of Dy3+ trap levels is in the order of BaAl2O4 host > CaAl2O4 host > SrAl2O4 host.	[36]
			Solid-state reaction(700–1500 °C)	Green-blue(λem = 500 nm)	The dopant (Eu2+) and co-dopant (Dy3+) concentrations affect the crystallinity and luminescence properties of the materials.	[87]
			Combustion method (500 °C)	λem = 450 nm	The hexagonal structure of BaAl2O4 can only produce shallow traps, resulting in a short afterglow.	[39]
			Combustion method (400–600 °C)or Solid-state reaction (1500 °C)	λem = 505 nm	They found that the method of preparation has a significant effect on the defect structure of the materials.	[87]
			Combustion synthesis method assisted by microwave irradiation	Blue-green(λem = 496 nm)	The surface of the BaAl2O4:Eu2+,Dy3+ powder samples showed lots of voids and pores.	[91]
			Solid-state reaction(1300 °C)	Green(λex = 355 nm,λem = 499 nm)	The photoluminescence efficiency increased with increasing Eu2+ concentration until 3 mol% then decreased at higher concentrations due to the concentration quenching effect.	[92]
			Combustion method (500 °C)	Blue–green(λex = 340 nm, λem = 505 nm)	The powders exhibited high initial brightness luminescence with subdued long afterglow characteristics.	[93]
		Nd3+	Combustion method(600 °C)	Green-blue(λem = 500 nm)	They proposed that phosphor samples obtain a lifetime of persistent luminescence with the aid of the energy transfer at the trap level.	[38]
			Combustion method (550 °C)	Blue(λex = 355 nm, λem = 495 nm)	Nd3+ trap levels can be thought of as the lanthanide element that causes long phosphorescence at room temperature.	[42]
		Cr3+	Solid-state reaction(1300 °C)	-	Fibre shaped morphology of the grown material was formed with sharp surface morphology like single crystals.	[94]
		Dy3+, Nd3+, Gd3+, Sm3+, Ce3+, Er3+, Pr3+ and Tb3+	Combustion method (600 °C)	Blue-green(λem = 500 nm)	The highest intensity was observed from Er3+ co-doping, whereas the longest afterglow was observed from Nd3+ followed by Dy3+ co-doping.	[95]

Barium aluminate (BaAl2O4) has a stuffed tridymite structure that is derived from the SiO2 β-tridymite structure observed in other alkaline earth aluminates [96]. It has a hexagonal phase, which is different from those of strontium aluminate and calcium aluminate. Barium aluminates exhibit a stable monoclinic phase at low temperatures which are transformed from the hexagonal phase after cooling. Barium aluminate (BaAl2O4) also has a high melting point (1815 °C), and exhibits adequate chemical stability and decent dielectric, pyroelectric, and hydraulic-hardening properties [97]. In its structure, two different sites are available for Ba2+: one with a C3 symmetry and a relatively longer Ba–O distance (2.86–2.87 Å), and the other with a C1 symmetry and a relatively shorter Ba–O distance (2.69 Å) [94]. Ba2+ has a larger ionic radius (1.34 A°) than that of most other rare-earth ions; this facilitates the straightforward substitution of vacant Ba2+ sites with lanthanide ions upon doping [88].

4.1. Synthesis of Barium Aluminate Phosphors

Various techniques have been employed to prepare barium aluminate phosphors in a manner similar to those for strontium and calcium aluminate phosphors, such as solid-state reactions, combustion methods, and microwave heating techniques.

The conventional solid-state reaction method has been typically used to successfully prepare barium aluminate phosphors [98]. In contrast to the synthesis of strontium and calcium aluminates, barium aluminate can be synthesized in an oxidizing atmosphere. Peng et al. were the first to report the reduction ofEu3+ to Eu2+ in an oxidizing atmosphere (air) via a high-temperature solid-state reaction for doping into an AlO4 tetrahedron in the BaAl2O4 crystal [8]. The tetrahedral AlO4 anion groups can form a hard three-dimensional network, which can induce the Eu3+ reduction even when the barium aluminate phosphors are prepared in air. Therefore, diverse atmospheric conditions such as reducing, weak reducing, and oxidizing atmospheres have been employed to synthesize barium aluminate phosphors. This method is straightforward and does not require expensive or sophisticated equipment; it is also convenient for large-scale industrial production. In addition, it can produce a structurally pure final product with desirable properties, depending on the final sintering temperatures.

Combustion synthesis is another extensively used method for the preparation of barium aluminate phosphors. Annah et al. prepared trivalent lanthanide codoped BaAl2O4:Eu2+ phosphors via the combustion method at an initiating temperature of 600 °C and annealing at 1000 °C [95]. The annealing process was found to have no effect on the general properties of the phosphors. Rodrigues et al. also employed this method at similarly low temperatures between 400 and 600 °C; Mothudi et al. synthesized BaAl2O4:Eu2+,Dy3+ at a combustion initiation temperature of 500 °C with urea as an organic fuel for combustion [39,99]. The combustion method can be performed at a much lower temperature compared to that in the solid-state reaction, and it is considered to be a simple, time-saving, and cost-effective technique. As previously discussed, it can produce smaller crystals than those obtained from a solid-state reaction. The size of particles produced via the combustion method was found to decrease from 98 nm to 85 nm as the reaction temperature increased from 400 to 600 °C [96]. Another difference between these two methods involves the number of traps produced; one trap is formed in the combustion method, whereas three traps are formed in the solid-state method, which hints at the formation of various defect structures in the materials, depending on the synthesis methods.

BaAl2O4 phosphors have also been synthesized by microwave heating. Zhang et al. successfully synthesized BaAl2O4 using BaCO3 and Al(OH)3 powders as raw materials via microwave sintering [100]. These phosphors were characterized by thermogravimetry–differential scanning calorimetry (TG–DSC), X-ray diffraction (XRD), and optical microscopy; this method was found to be feasible for the preparation of persistent luminescence materials of barium aluminates. Although this technique is not employed as frequently as the two previously discussed methods, it has several advantages, such as the low temperatures and short durations in sintering, a simple and easy setup, cost-effective energy source, and high rate of synthesis.

4.2. Barium Aluminate Phosphors with Various Colors

Eu2+ is the most popular rare-earth element for doping in BaAl2O4, similar to that in the alkali metal-based aluminate phosphors [98] (Figure 4). BaAl2O4 phosphors have been considered for application in plasma display panels (PDPs) and mechanoluminescence (ML) dosimetry owing to their enhanced luminescence intensity, long-lasting duration, and suitable emitting colors via Eu2+ doping. The peak of the broad excitation spectra is observed at 340 nm, and the emission spectra are present in the blue–green region under vacuum/ultraviolet (VUV) light excitation, which corresponds to the 5d–4f transition of Eu2+. The spectral peak does not appear uniform, which implies the occurrence of multiple events at the luminescent centers. Peng et al. reported that Eu2+ ions can occupy 2 different lattice sites after doping in BaAl2O4: the Eu2+ ion in the first site exhibited a major emission peak at 495 nm, and the Eu2+ ion in the other exhibited a weak emission peak at 530 nm [8]. The main emission peak is noted to be positioned between the emission peaks of SrAl2O4:Eu2+ (528 nm) and CaAl2O4:Eu2+ (449 nm), implying that the crystal structure of the host plays a crucial role in determining the main emission peaks of aluminate-based phosphors. In addition, Stefani et al. observed that the relative intensity of the two emission peaks in BaAl2O4:Eu2+ can be modified by varying the dopant and codopant concentrations [87]. The intensity of the emission peak at a shorter wavelength was observed to increase as the dopant and codopant concentrations increased, which suggested that Eu2+ preferentially occupied the Ba2+ site responsible for longer wavelength emission and subsequently occupied another site corresponding to the shorter wavelength emission. Feilong et al. reported 1 mol.% as the optimal Eu2+ concentration for the enhancement of luminescent intensity of BaAl2O4 [91]. Roh et al. also studied the effect of Eu2+ concentration on the photoluminescence of these phosphors [92]. The photoluminescence efficiency was noted to increase as the Eu2+ concentration increased up to 3 mol.%. and concentrations greater than 3 mol.% quenched the photoluminescence of BaAl2O4:Eu2+.

Figure 4

Photoluminescence spectra for lanthanide doped barium aluminate phosphors (A) Photoluminescence excitation (i) and emission (ii) spectra for BaAl2O4:Eu2+. Adapted from [30] under Creative Commons Attribution (CC BY) license. (B) Photoluminescence emission spectra of BaAl2O4:Eu2+, Re3+ (Re = Dy3+, Er3+, Sm3+, Gd3+, Ce3+, Pr3+ and Nd3+). Adapted with permissions from [95]. Copyright Elsevier, 2012.

In addition to Eu2+ ions, Cr3+ ions have also been employed as dopants in BaAl2O4. Singh et al. prepared red-emitting BaAl2O4:Cr3+ phosphors via the urea combustion method [89]. The excitation spectra of this synthesized phosphor featured two broad bands with high intensities at 421 and 552 nm, which were ascribed to the Cr3+ ions in octahedral symmetry. The emission peak observed at 750 nm corresponded to the transition from Cr3+ ions. Vrankic et al. investigated the oxidation state of Cr dopant in a Cr-doped BaAl2O4 structure using XRD and synchrotron-based X-ray absorption spectroscopy (XAS) [90]. Two different oxidation states for chromium ions were found. Cr6+ was observed in a small amount in an impure phase (BaCrO4), whereas Cr3+ was noted to participate in the formation of the doped BaAl2O4:Cr3+ phase, in which it behaved as a defect.

4.3. Codoping of Barium Aluminate Phosphors with Various Trivalent Lanthanide Ions

The persistent luminescence of lanthanide-doped BaAl2O4 phosphors can also be enhanced by codoping with trivalent lanthanide ions, similar to that in the other alkaline earth aluminates. Among the various lanthanide elements that have been used as codopants, Eu2+ and Dy3+ have been particularly successful as codopants in the synthesis of polycrystalline barium aluminate phosphors (BaAl2O4:Eu2+,Dy3+) [36,87,93,99]. The photoluminescence efficiency of the Eu2+-doped BaAl2O4 phosphor was observed to increase after codoping with Dy3+; this phosphor exhibited afterglow properties with the longest duration among the various MAl2O4:Eu2+,Dy3+ phosphors (M = Sr, Ca, Ba).

Liu et al. reported that the codoping of BaAl2O4:Eu2+ with Dy3+ did not modify the positions of either the emission band or the excitation band; however the luminescence intensity and afterglow duration of the phosphor increased [36]. The generally accepted mechanism for the photoluminescence of BaAl2O4:Eu2+,Dy3+ is similar to that ofSrAl2O4:Eu2+,Dy3+ and includes the following steps: (1) electron migration induced by UV radiation from the 4f65d1 levels in Eu2+ to the conduction band, (2) electron trapping from the conduction band to defects such as oxygen vacancies or codopant ions, (3) reverse electron migration from traps to the 4f65d1 levels in Eu2+, and (4) radiative relaxation of the returned electron to the ground state of the luminescent center, Eu2+. In this process, the type of codopant can affect the formation of electron traps, and therefore, the Dy3+ trap levels are presumed to be responsible for the long afterglow phosphorescence [36,93]. The effect of Dy3+ concentration on the afterglow of BaAl2O4:Eu2+,Dy3+ has also been investigated [87,99]. Stefani et al. found that the concentrations of both Eu2+ and Dy3+ play an important role in the determination of the crystallinity and luminescence properties of BaAl2O4:Eu2+,Dy3+ phosphors [87]. Rodrigues et al. investigated the concentration of Eu2+/Dy3+ (in mol.% with respect to the amount of Ba) from 0.1/0.1 to 1.0/3.0, and found that the luminescence intensity increased as the Eu2+ and Dy3+ concentrations increased [99]. The high concentration of Eu2+ and the resulting enhancement of luminescence can be explained by an increase in the number of emitting centers. Increased Dy3+ concentration is also presumed to reduce the distance between the electron traps and the emitting center, which increases the efficiency of energy transfer.

Cr3+ ions have also been employed as codopants in BaAl2O4:Eu2+. Ryu et al. synthesized BaAl2O4:Eu2+,Cr3+ using various concentrations of Cr3+ [94]. Crystalline fibers were obtained via the different concentrations of Cr3+ (0.01, 0.05, and 0.1 mol%); fibers with larger dimensions were obtained at higher concentrations of doped Cr.

Ho3+ ions have also been codoped in BaAl2O4:Eu2+ to facilitate the enhancement of photoluminescence. Liu et al. reported that the BaAl2O4:Eu2+,Ho3+ samples exhibited excitation and emission spectra with shapes and positions similar to those from BaAl2O4:Eu2+,Dy3+ but with slightly lower emission intensities [101].

Annah et al. systematically investigated the use of several trivalent rare-earth ions (Dy3+, Nd3+, Gd3+, Sm3+, Ce3+, Er3+, Pr3+, and Tb3+) as codopants in BaAl2O4:Eu2+ [95]. All tested samples were found to exhibit blue–green emission at ~500nm, which corresponds to the 4f6d1–4f7 transitions of Eu2+; no changes were observed in the phase structure after codoping. The emission intensity of the Er3+-codoped phosphor was noted to be the highest, and the afterglow of the Nd3+-codoped phosphor was observed to be the longest.

5. Outlook

Lanthanide-activated alkaline earth aluminates phosphors are excellent luminescent materials and can have extensive applications. In this review, lanthanide-doped strontium aluminate-, calcium aluminate-, and barium-aluminate-based phosphors, which are among the popular alkaline earth aluminates, were discussed with an emphasis on their synthesis methods, phosphorescence mechanisms, and the effects of dopants and codopants on phosphor properties. This systematic review featuring the development of various lanthanide-activated alkaline earth aluminate phosphors is expected to stimulate further research on lanthanide-based phosphors for applications in a wide range of areas. Various attempts have been made to optimize the synthesis methods and compositions of lanthanide-activated alkaline earth aluminates phosphors to improve their long-persistent luminescence properties; however, recent strategies involving nanomaterial engineering, which have been successfully implemented for different types of nanoparticles, can also be adopted to further enhance the luminescence properties of phosphors (Figure 5).

Figure 5

Various efforts to enhance the photoluminescence intensity of lanthanide-doped alkaline earth aluminate phosphors (A) Enhanced photoluminescence intensity by alkali metal (left), alkaline earth metal (middle), and Si (right) ions doping. Adapted from [12] under Creative Commons Attribution (CC BY) license. (B) (left) Emission spectra of the green (SrAl2O4:Eu2+,Dy3+), blue (Sr4Al14O25:Eu2+,Dy3+), and violet (CaAl2O4:Eu2+,Nd3+) phosphors used for the phosphor mixing method with the aim of energy transfer between them. (middle) Photoluminescence decay curves in log scale for the green and blue phosphors mixed sample, showing enhanced luminescence intensity of green phosphor by mixing with the blue phosphor. (right) Photoluminescence decay curves in log scale for the blue and violet phosphors mixed sample, showing enhanced luminescence intensity of blue phosphor by mixing with the violet phosphor. Adapted from [14] under Creative Commons Attribution (CC BY) license.

Energy transfer between the different lanthanide ions in dual-emitting lanthanide-ion-codoped phosphors has been utilized to facilitate ratiometric temperature sensing and enhancement of photoluminescence of the phosphors. Energy transfer between two lanthanide codoped ions has been observed in several dual-color emitting phosphors, such as Ba2Y(BO3)2Cl:Bi3+,Eu3+, BaLu6(Si2O7)2(Si3O10):Ce3+,Tb3+, and LaOBr:Ce3+,Tb3+ [47,102]. Two emission peaks were observed herein because of the presence of two luminescence centers in these dual-emitting phosphors; their ratio was found to change with the extent of energy transfer. Applying this phenomenon, Zhang et al. demonstrated the ratiometric temperature sensing of LaOBr:Ce3+,Tb3+ over a wide temperature-sensing range (293–443 K) with a sensitivity of 0.42% K−1 [103]. This approach has also been recently demonstrated in lanthanide-codoped aluminate phosphors with various colors, such as SrAl2O4:Eu2+,Dy3+, Sr4Al14O25:Eu2+,Dy3+, and CaAl2O4:Eu2+,Nd3+ as green, blue, and violet phosphors, respectively [14]. Novel protocols have been developed based on the effect of energy transfer between spectrally different phosphors, and they were noted to significantly improve the afterglow intensities and lifetimes of green and blue phosphors. Multistep energy transfer between the three phosphors with different colors was also achieved, and a much higher afterglow intensity was generated: ∼2 times that via single-step energy transfer. Therefore, it is vital to consider the synthesis of various lanthanide-doped alkaline earth aluminate phosphors as a new strategy for the development of bright and long-persistent phosphors.

Distortion of crystal fields has also been induced via the introduction of impurities to facilitate the tuning of the luminescence properties of phosphor crystals [104]. Various alkali metals have been employed as dopants to increase the afterglow intensity and lifetime. Dhananjaya et al. observed that the photoluminescence intensity of the Gd2O3:Eu3+ phosphor remarkably increased after the incorporation of Li+, Na+, and K+ into this phosphor [105]. Kim et al. also reported that the doping of SrAl2O4:Eu2+,Dy3+, and CaAl2O4:Eu2+,Nd3+ phosphor with alkali metals (Li+, Na+, K+) and alkaline earth metals (Mg2+, Ca2+, Ba2+) can significantly boost the phosphorescence intensity and increase the afterglow lifetime [12,81]. These apparently imply that Si4+ doping is also effective for enhancing the phosphorescence intensity. Upon the incorporation of Si4+ into the SrAl2O4 crystal, the local symmetry of the crystal structure was presumed to be broken owing to the smaller size of Si4+ (~40 pm) compared to that of Al3+ (53 pm). Therefore, an increase in luminescence was observed for both phosphors at the optimal concentration of Si4+. The effect of incorporation of additional impurities on photoluminescence is presumed to be a result of the local distortion of the crystal field surrounding the luminescence center, which is known to considerably affect the f–d transitions. Such impurity effects are expected to play an important role in the future development of effective codopants.

Ideas for the straightforward fabrication of nanoparticles with desirable shapes and sizes have also been suggested. Liu et al. attempted to control the size, shape, and surface properties of rare-earth-doped nanomaterials at the atomic scale using oleate anions (OA−) and molecules (OAH) [106]. This level of control was observed to facilitate the fabrication of various sub-50 nm-sized monodispersed nanoparticles. Similarly, Sui et al. reported the use of oleate salts as ligands that can shorten the reaction time (down to 5 min) during the synthesis of the ultrasmall (~13 nm) hexagonal phase of the NaYF3 nanocrystals; this was facilitated via induction of the orderly arrangement of Y3+ and lowering of the energy barrier for the phase transition to occur [107].

Overall, recent approaches based on nanomaterial engineering can be expected to be readily expanded to lanthanide-doped alkaline earth aluminate phosphor systems for the enhancement of their photoluminescence properties. However, results from previous studies on the design of phosphors for obtaining desirable sizes of the phosphor particles need to be carefully considered. Most of the studies discussed in this review involve bulk structures; however, the properties of nanoparticles, such as luminescence and physical and chemical properties, can be different at the nanoscale. The coordination complex of a doped lanthanide ion can be easily distorted in nanoparticles compared to that in the bulk lattice because nanophosphors typically have large surface areas and high densities of interfacial boundaries. Recent approaches based on theoretical modeling are expected to play an essential role in the estimation of the photoluminescence characteristics and mechanisms in newly designed nanophosphors. The development of new lanthanide-doped phosphors should include systematic characterization to investigate the sizes and scales involved.

6. Conclusions

Considerable attempts have been made to develop bright and long-persistent lanthanide-doped alkaline earth aluminate phosphors for replacing the conventionally and extensively used ZnS phosphors; as a result, the luminescence properties of such phosphors have been significantly improved. Various synthesis methods have been employed to prepare lanthanide-doped alkaline earth aluminate phosphors, and their pros and cons have been found to arise from the different reaction temperatures, reaction phases (solid or liquid), and particle size of the product. The luminescence properties of these aluminate phosphors are mainly determined by the compositions of the doped lanthanide ions and the host matrix. The localized environment surrounding the lanthanide activator, which is a luminescence center, appears to play a crucial role in persistent luminescence. It is important to determine the effect of the synthesis steps and compositions of phosphors on the persistent luminescence because subtle changes in the phosphor synthesis conditions can lead to significant variations in their luminescence properties. The development of lanthanide-doped alkaline earth aluminate phosphors has great potential; such phosphors can also be expected to find application in a wide range of areas.