1. Introduction

Phosphor-converted white-light-emitting-diodes (pc-wLEDs) with superiorities of friendly environment, high light efficiency and long-life have received extensive attention in displays and solid-state lighting for fluorescent tubes, traffic and medical lighting [1–5]. Generally, the prominent commercial wLEDs are packaged with Y₃Al₅O₁₂: Ce³⁺ yellow phosphors excited by GaN blue chips. Nevertheless, obvious deficiencies of low color rendering index (CRI) distributed to the absence of red emitting and overflow of blue emitting limit its application for high-quality white lighting [6–8]. Near-ultraviolet (NUV) chip combined with red, green and blue (RGB) phosphors is considered as an efficient solution to acquire warm white light with high CRI and full-visible-spectrum emission, which also avoids the “blue light hazard” compared with directly exciting multicolor phosphors by blue chips [9,10]. Besides, dentistry curing light with wide-band blue emitting is a cry for curing photoinitiator, such as Camphor Quinone excited by 400∼500 nm [11].

Hence, it is important to exploit efficient and stable RGB phosphors for NUV wLEDs, especially for blue emitting phosphor doped with the rare earth ions (Eu²⁺) [12,13]. In general, there are two typical luminescent properties of 4f⁶5d – 4f⁷ in Eu²⁺ ions: (i) broadband emission and (ii) adjustable wavelengths covering all visible areas, because of the exposed electron in 5d orbital which is highly influenced by the external crystal field [14–18]. At present, commercial Eu²⁺ activated aluminate-type blue phosphor (BaMgAl₁₀O₁₇:Eu²⁺ (BAM)) has the obvious disadvantages of light degradation and poor structural stability with weak absorption under 350∼410 nm excitation, which impedes its application in NUV-excited color converter; Qiao et al. successfully synthesized phosphate-type (K₂BaCa(PO4)₂: Eu²⁺) phosphor by the method of solid-state reaction (SSR), which displayed 460 nm emitting under 350 nm excitation [19]. The same method was used by Ahn et al. to prepare silicate-type (Sr, Ba)₃MgSi₂O₈: Eu²⁺ phosphor with broad excitation from 250 nm to 450 nm and strong emission of 460 nm [20]. Compare to the phosphors with above types, the fluorapatite-type of M₅(BO₄)₃F (M is alkalis, alkaline earth, lanthanide ions or their mixture; B is P⁵⁺, Si⁴⁺ or B³⁺ ions) phosphors with great luminescence properties have following advantages: (i) the excellent chemical and thermal stability of host lattices, (ii) the strongest electronegativity of fluorine atoms for accepting electron and (iii) the great bioactivity and biocompatibility in potential application for fluorescence immunoassay [21–27]. Kim et al. realized adjustable blue emission by changing the ratio of co-doped alkaline metal cations of M [28]. Sadhana et al. used (Y, Ba), Si, Gd ions as bigger cations of M, smaller cation of B and luminescence center respectively to obtain double emission peaks at 361 nm and 467 nm [29]. Ma et al. researched the energy transfer between Tb³⁺ and Eu³⁺ to receive a multicolor from green to red under the excitation of 373 nm for Ba₂La₃(SiO₄)₃F fluorapatite-type phosphor [30]. However, the fluorescence properties (e.g., photoluminescent (PL) intensity and decay lifetime) of fluorescence center are greatly influenced by the uniformity of its distribution and crystallinity of host crystals. And the serious aggregation of activator will be emerged during SSR process in abovementioned reports, resulting in the quenching of luminescence [31].

In this paper, M₅(PO₄)₃F: Eu²⁺ (M = Ca, Sr) phosphors were newly synthesized via deposition-precipitation process (DP), which has enhanced the 6.7 times of luminescence intensity under 350 nm excitation when compared with traditional SSR method. Meanwhile, the DP-Ca₅(PO₄)₃F: 0.03Eu²⁺ sample revealed a broadband absorption from 225 nm to 425 nm and adapted to diverse excitation of NUV-LEDs. Uniform distribution of Eu²⁺ activators were powerfully proved by energy dispersive X-ray analysis system. Analyzing the coordination environment and the external crystal field around Eu²⁺, the peaks at 446 nm and 479 nm obtained by gaussian fitting of emission spectrum (at 447 nm) are concluded to the effect of the occupied Eu²⁺ in 4f and 6h symmetry sites, respectively. Novel types of all-inorganic perovskite quantum dots for green-emitting and red Lu₃Al₅O₁₂: Mn⁴⁺ phosphor are combined with Sr₅(PO₄)₃F: Eu²⁺ sample in a high power NUV-wLED device (1 W) to cover a wide color gamut of 123.4% NSTC, which also shows a high- quality white light with color rending index (CRI) of 86 and color temperature (CCT) of 7107 K. In addition, tunable chromaticity with high color purity (97.79%) and steady photoelectric converting of M₅(PO₄)₃F: Eu²⁺ samples indicating the excellent application prospect for wide color gamut pc-wLEDs excited by NUV.

2. Materials and methods

2.1 Materials and synthesis of the Ca₅(PO4)₃F precursor

Solid state reaction method (SSR) was utilized to synthesize Ca₅(PO4)₃F precursor (FAp) by using CaCO₃ (A.R., Aladdin, China), NH₄H₂PO₄ (A.R., Aladdin, China), and CaF₂ (A.R., Aladdin, China) as the raw materials. The stoichiometric mixed products were ground in an agate mortar and sintered at 900 °C in air atmosphere for 5 h.

2.2 Synthesis and methods of Ca_5-x(PO₄)₃F: xEu²⁺ (0 ≤ x ≤ 0.10) phosphors

A series of Ca_5-x(PO₄)₃F: xEu²⁺ (0 ≤ x ≤ 0.10) phosphors were obtained via different processes of DP (DP-Ca₅(PO₄)₃F: Eu²⁺) and SSR (SSR-Ca₅(PO₄)₃F: Eu²⁺), which were denoted as DP-FAp and SSR-FAp, respectively. The schematic diagram of fabrication processes and formation mechanisms about above processes were demonstrated in Fig. 1.

 

Fig. 1. Schematic diagram of fabrication processes and formation mechanisms via traditional solid-state reaction and deposition precipitation processes respectively.

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2.2.1 DP-Ca₅(PO4)₃F: Eu²⁺

0.8 − 8 mmol/L Eu³⁺ aqueous solutions were obtained by fully dissolving Eu₂O₃ in 50 ml HNO₃ solution (pH = 2). After adjusting to pH = 6 through dripping NH₃·H₂O, 4 mmol Ca₅(PO₄)₃F was added. Then adjusting to pH = 11 to obtain Ca₅(PO₄)₃F·Eu(OH)₃ precipitates. Subsequently, deionized water and absolute ethanol were used to wash the precipitates for several times and dried at 80 °C. Finally, the DP-Ca₅(PO₄)₃F: Eu²⁺ samples were prepared by sintering in the tubular furnace (1100 °C, 5 h) under a flow condition of 3% H₂/N₂.

Concurrently, a series of DP-Ca₅(PO₄)₃F: Eu²⁺ samples were sintered at different treated temperatures of 1100 °C, 1200 °C and 1300 °C respectively.

2.2.2 SSR-Ca₅(PO4)₃F: Eu²⁺

The reference samples were synthesized via traditional two-step SSR. Firstly, a series of different concentrations of Eu₂O₃ (0.02 − 0.2 mmol, 99.99%, Aladdin, China) were mixed with the same raw materials of FAp (4 mmol) and sintered at 900 °C for 5 h in air atmosphere. Then, SSR-FAp: Eu³⁺ were restored by treating at 1100 °C for 5 h under 3% H₂/N₂ atmosphere to obtain SSR-FAp: Eu²⁺.

2.3 Synthesis of Sr_5-xCa_x(PO₄)₃F: yEu²⁺ (0 ≤ x ≤ 5, 0 ≤ y ≤ 0.10) phosphors

Certain proportions of Ca²⁺ were replaced with SrCO₃ (A.R., Aladdin, China) to obtain a series of Sr_5-xCa_x(PO₄)₃F: Eu²⁺ (0 ≤ x ≤ 5) phosphors via the method of DP.

2.4 Materials characterization

The phase structures of samples were confirmed by X-ray diffraction (XRD-7000S, Shimadzu, Cu-Kα, 40 kV, 30 mA) patterns with step size of 5° min^-1 (20-70°). Microstructures and element distributions of the synthesized samples were characterized by field emission electron microscopy (SEM instrument, JEOL JSM-7800 F) equipped with an energy dispersive X-ray analysis system (EDS, X-Max50, Oxford) at 15 kV. A fluorescence spectrometer (F-7000, Hitachi) with a 150 W xenon lamp excitation source was used to measure the photoluminescence excitation (PLE) and PL spectra of samples. The PL decay lifetime curves were investigated by a spectrophotometer (FLS1000, Edinburgh) under excitation of VPL-375 laser source. The PL quantum yield (PLQY) were measured by a spectrophotometer (FLS980, Edinburgh) with the scatter range of 330–370 nm and the emission range of 371–600 nm.

3. Results and discussion

3.1 Phase and structure

XRD patterns of Ca₅(PO4)₃F: 0.03Eu²⁺ samples synthesized by DP method under the treated temperatures of 1100 °C, 1200 °C and 1300 °C are displayed in Fig. 2(a). And XRD pattern of SSR-FAp (1100 °C) is displayed for comparison. Obviously, the diffraction peaks of DP-FAp: 0.03Eu²⁺ sample are well indexed to the standard data of PDF#071-0881 and no other impurity peaks or position shift can be observed in all treated temperatures. However, there exist the weak diffraction peaks at 29.22° (marked as a green ), 37.42° and 53.87° (marked as purple ) in SSR-FAp prepared by 1100 °C, assigned to Ca₄(PO₄)₂O phase (PDF#070-1379) and CaO phase (PDF#082-1690), respectively. It is concluded that the Eu²⁺ ions will occupy Ca²⁺ sites ideally in Ca₅(PO₄)₃F crystal because of the close ionic radius of Eu²⁺ (0.109 nm) and Ca²⁺ (0.114 nm) and no lattice distortion of host structure can be observed from the XRD patterns, which demonstrates that FAp: 0.03Eu²⁺ phosphor with single phase can be successfully obtained via the DP process. On the contrary, the extraneous phases of Ca₄(PO₄)₂O and CaO are appeared by SSR method, owing to the inadequate response in this process. Compared to the PDF card 050-1744, extraneous phases in Sr₅(PO₄)₃F: 0.03Eu²⁺ are emerged, which are highlighted in pink diamonds as shown in Fig. 2(b). The diffraction peaks of extraneous phase are found at 29.57°, 33.22° and 57.11° respectively, which are excellently indexed to the (0 1 5), (1 1 0) and (1 2 5) crystal planes of the standard data of PDF#80-1614 (Sr₃(PO₄)₂). Worse still, diffraction intensity of extraneous phase is enhanced when increasing the synthesis temperature. According to the principle of three major XRD diffraction peaks, it is proved that the extraneous phases of Sr₅(PO₄)₃F: Eu²⁺ samples belong to Sr₃(PO₄)₂ and more likely to occur under high temperature. Compared with Sr₅(PO₄)₃F: Eu²⁺ samples, the purer single phase of Ca₅(PO4)₃F: Eu²⁺ samples can be accessibly synthesized during DP process.

 

Fig. 2. (a) XRD patterns of DP/SSR-Ca₅(PO₄)₃F: 0.03Eu²⁺ and (b) DP-Sr₅(PO₄)₃F: 0.03Eu²⁺ samples under different treated temperatures.

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Figure 3(a) exhibits the XRD patterns of Ca_5-xSr_x(PO₄)₃F: 0.03Eu²⁺ (x = 0, 2.5, 5) phosphors with different Ca²⁺ and Sr²⁺ concentrations prepared by DP method at 1100 °C. It is found that, after removing the tiny impurities, the diffraction peak positions and intensities of the main phases are consistent with the corresponding standard cards when x = 0 and 5, revealing that there is little influence on the host structures of Ca₅(PO₄)₃F: Eu²⁺ and Sr₅(PO₄)₃F: Eu²⁺ when a small amount of doped Eu²⁺ replace the sits of Ca²⁺ and Sr²⁺ ions respectively. When the concentration ratio of Ca²⁺: Sr²⁺ reaches to 1:1, the diffraction peak position of Ca_2.5Sr_2.5(PO₄)₃F: Eu²⁺ is different from the mixed phases of Ca₅(PO₄)₃F: Eu²⁺ and Sr₅(PO₄)₃F: Eu²⁺. Moreover, as shown in Fig. 3 (b), the magnified XRD patterns of Ca_5-xSr_x(PO₄)₃F: Eu²⁺ phosphors at 2θ = 30 − 36° represent that the diffraction peak at ∼32° shifts to a lower angle continuously as more Ca²⁺ were replaced by Sr²⁺. It is indicated that Ca²⁺ and Sr²⁺ ions are both the components of the host structure of Ca_2.5Sr_2.5(PO₄)₃F: Eu²⁺ phosphor. For Ca_2.5Sr_2.5(PO₄)₃F sample, it can be viewed as a solid solution with pure phase because there is no possible hybrid phase corresponding to it. The Bragg's law is applied as following functions to explain the shift of diffraction peak at ∼ 32° [32],

The θ moves toward a lower angle when the larger radius of Sr²⁺ ions constantly substitute Ca²⁺ ions and generate a bigger cell volume of (Ca, Sr)₅(PO₄)₃F (e.g., 523.1 ų for FAp and 595.8 ų for SFAp) due to the increase of lattice constants (a and c) in hexagonal lattice apatite structure.

 

Fig. 3. (a) XRD patterns of DP-Ca_5-xSr_x(PO₄)₃F: 0.03Eu²⁺ (x = 0, 2.5, 5) phosphors prepared at 1100 °C and (b) their magnified XRD patterns at 30.0 − 36.0°.

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The unit cell diagram of the crystal structures of FAp and SFAp are displayed in Fig. 4(a, c). Hexagonal crystal lattice is exhibited in host lattice of M₅(PO₄)₃F, which has a same space group of P 63/m (176) with cell parameters of a = b = 9.367Å, c = 6.884 Å, α = β = 90°, γ = 120° (FAp) and a = b = 9.717Å, c = 7.285 Å, α = β = 90°, γ = 120° (SFAp) [21]. Furthermore, as shown in Fig. 4 (b, d), two types of sites are donated as M1 and M2 in the crystal structures of M₅(PO₄)₃F where either Ca or Sr ions can be substituted by rare-earth ions. The ratio of M1 and M2 is 40:60 in undoped rare-earth ions host lattice of M₅(PO₄)₃F. 40% of the M1 ions are at the Wyckoff 4f site having C₃ point symmetry, which are coordinated with nine oxygen atoms. 60% of the M2 ions are surrounded by six oxygen atoms and one fluorine atom with C_s point symmetry at the Wyckoff 6h site. Owing to the close effective ionic radius of the Ca²⁺ (R = 1.06 Å for CN = 7 and R = 1.18 Å for CN = 9) / Sr²⁺ (1.21 Å and1.31 Å for CN = 7 and 9, respectively) and Eu³⁺ (radius for CN = 7 is 1.10 Å and CN = 9 is 1.20Å), the sites of Ca²⁺ / Sr²⁺ ions are easily replaced by Eu³⁺ ions in FAp / SFAp lattice during the M₅(PO₄)₃F·Eu(OH)₃ synthetic process of the DP [15,33]. Then, Eu³⁺ is reduced to Eu²⁺ (effective ionic radius is 1.25 Å and 1.30 Å for coordination number of 7 and 9, respectively) in reducing atmosphere. Compared with traditional SSR method, Eu²⁺ ions are expected to uniformly distributed in host lattice of M₅(PO₄)₃F: via the DP method in this work. The luminescence intensity is enhanced by avoiding the reunion of Eu²⁺ activator.

 

Fig. 4. Crystal structures diagram of FAp and SFAp (a, c) and different coordination surrounding of Ca and Sr ions (b, d).

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3.2 Microstructure and elemental mapping

To intuitively express the uniform distribution of luminescent center (Eu²⁺), the comparison of SEM images and EDS mapping analyzes between DP-FAp:0.03 Eu²⁺ and SSR-FAp:0.03 Eu²⁺ are represented in Fig. 5(a, b) respectively. The uniform diameter grain of DP-FAp:0.03 Eu²⁺ sample (7∼12 µm) is found compared with 3∼13 µm in SSR-FAp:0.03 Eu²⁺ sample. The compositional analysis shows that the Ca/P molar ratio are 1.79 and 1.73 in DP-FAp:0.03 Eu²⁺ and SSR-FAp:0.03 Eu²⁺ samples respectively, which are in accordance with the ideal ratio of 1.67. The distribution of Eu²⁺ ions can be determined by observing the Eu ion signal on the surface of a single phosphor body, instead of observing the whole EDS mapping of Eu. Interestingly, homogeneous distribution of Eu signal is observed on the DP-FAp:0.03 Eu²⁺ surface in Fig. 5(a). On the contrary, there are apparent aggregations of Eu signal on SSR-FAp:0.03 Eu²⁺ sample, marking with red circles in Fig. 5(b) and further demonstrating that the activator reunion of FAp: Eu²⁺ can be efficiently avoided by DP process.

 

Fig. 5. SEM images and their EDS mapping analyzes for FAp with different synthetic methods of DP (a) and SSR (b).

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3.3 Luminescence mechanism

The unsymmetrical PL spectrum of DP-FAp: 0.03Eu²⁺ phosphor excited at 350 nm is observed in Fig. 6(a), which exhibits an obviously enhanced blue emission intensity (at 447 nm) of 6.4 times compared with SSR-FAp:0.03 Eu²⁺ phosphor. As shown in Fig. 6(b), four various excitation peaks at 244 nm, 293 nm, 341 nm and 350 nm are found and present broad excitation bands from 225 nm to 425 nm in DP-FAp: 0.03Eu²⁺ and SSR-FAp: 0.03Eu²⁺ phosphors, which is attributed to the 4f⁶ → 4f⁷5d transition. Revealing that the efficient fluorescence enhancement of FAp: Eu²⁺ is favorably acquired through the deposition-precipitation process and the multi-mode excitation is greatly adapted with various commercial NUV-LED.

 

Fig. 6. The emission (a) and excitation (b) spectra (PL and PLE) of DP-FAp:0.03 Eu²⁺ and SSR-FAp:0.03 Eu²⁺ phosphors; Gaussian fitting of PL peak under 350 nm excitation (c) and multi-mode excitation energy levels of Eu²⁺ (d) for DP-FAp:0.03 Eu²⁺ sample.

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Nonlinear fitting for the emission peak of DP-FAp: 0.03Eu²⁺ is used to research the luminescence mechanism of Eu²⁺ in M₅(PO₄)₃F at Fig. 6(c), and two emission peaks at 446 and 469 nm with distinct intensities are primely disassembled. According to the description in the section of Fig. 4, there are two kinds of site symmetries (4f site with C₃ point symmetry for Ca1; 6 h site with C_s point symmetry for Ca2) in FAp. Naturally, two corresponding Eu²⁺ luminescent centers are deduced when Eu ions randomly occupy the sites of Ca1 and Ca2 ions into the host lattice. The following equation is employed to calculate the extent of 5d energy level splitting (D_q) influenced by the crystal environment around Eu²⁺ (Z for anion charge; e for electron charge; r for radius of the d wavefunction; R for bond length) [12,34],

The equation is derived by a point charge model, which can export an approximation of the energy level splitting (D_q) positively correlated with bond length (R) and anion charge (Z). The bond length and anion charge of Ca/Eu-O and Ca/Eu-F in 4f and 6h sites of FAP: Eu²⁺ are estimated in Table 1. Firstly, the R of Ca1/Eu1-O is 2.3940∼2.8064 Å with average bond length ($\bar{R}$) of 2.5501 Å, and the range of Ca2/Eu2-O/F is from 2.2944 to 2.6831 Å ($\bar{R}$=2.4301 Å). Moreover, the average anion charge ($\bar{Z}$) is 1 and 1.93 for Ca1 sites (4f sites; CN = 9; Z(Ca-O) = 1) and Ca2 sites (6h sites; CN = 7; Z(Ca-O) = 2; Z(Ca-F) = 1.5), respectively. As a result, the Eu1 ions in 4f sites have narrower 5d energy level splitting than Eu2 ions ($\bar{R}$ (4f) > $\bar{R}$ (6h); $\bar{Z}$ (4f) < $\bar{Z}$ (4f)). Generally, the shorter wavelength will be emitted due to the weaker level of crystal field splitting with longer bond length and smaller anion charge. It is indicated that the shorter emission wavelength at 446 nm in Fig. 6(c) belongs to the luminescence center of Eu1 sites (4f), according to the result of D_q(4f) < D_q(6h), and the longer 469 nm emission is emitted by the Eu²⁺ luminescence center (6h). The smaller FWHM for 4f site with value of 39 nm (from 427 nm to 466 nm) further proves a narrower splitting situation of 5d energy level when compared with 6h site (FWHM = 68nm; from 438 nm to 504 nm). Further, the Eu1 luminescence center has greater influence for the emission of FAP: Eu²⁺ than Eu2 via comparing the intensity of the two emission peaks in 446nm and 469nm. The possible reason may be that the doped Eu ions predominately occupy the Ca1 sites (4f) in FAp. Based on the intensity of separate narrow peaks (the stronger at 293 nm; the weaker at 244 nm) of PLE spectra in Fig. 6(b), it is reasonable to conclude that the overlap PL spectra of FAp: Eu²⁺ is attributed to the energy level transition from the energy band (the gravity at 34130 cm^-1 / 40984 cm^-1 and the lowest excited states 22422 cm^-1 / 21322 cm^-1 for 4f / 6h, respectively) to the ground state level. And the energy level schematic diagram for luminescence mechanism of Eu²⁺ in FAp crystal is exhibited in Fig. 6(d).

Table 1. The selected interatomic bond length and anion charge in DP-FAp: Eu²⁺ sample [33]

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3.4 Optical properties

The emission spectra of FAp: xEu²⁺ (0.02 ≤ x ≤ 0.10) with their intensity and wavelength of PL position under 350nm excitation are displayed in Fig. 7(a, b), indicating that the optimum Eu²⁺ doping concentration is x = 0.03 and a weeny red shift is found from 449 to 450 nm with the increase of doped Eu²⁺. There is an analogous variation tendency for SFAp: xEu²⁺ (0.02 ≤ x ≤ 0.05) samples as shown in Fig. 7(c, d). The strongest PL intensity of SFAp: xEu²⁺ is appeared when x = 0.03 and the PL band shifts to longer wavelength (436.1∼437.5 nm) determined by enhanced Eu²⁺ doping proportion. Generally, shorter distance and stronger interaction between activated ions lead to the concentration quenching, then non-radiative energy transfer between Eu²⁺ and the host crystal reduces the luminescence intensity when x > 0.03. In addition, the defects of FAp and SFAp crystal lattices will be produced due to the bigger ionic radius of substitute Eu²⁺ than that of Ca²⁺ and Sr²⁺, resulting in the slight red shift for emission band.

 

Fig. 7. (a, c) PL spectra of DP-FAp and DP-SFAp with different doping concentrations of Eu²⁺ excited by 350 nm. (b, d) The corresponding position and intensity of emission peaks.

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Figure 8 is exhibited to research the regulatory ability of Ca_5-xSr_x(PO₄)₃F: 0.03Eu²⁺ phosphors for emission intensity and wavelength when excited at 350 nm. It is distinctly concluded that the PL intensity is decreased when a small amount of Ca²⁺ are replaced by Sr²⁺, then a gradually enhancement occurs after x exceeds 0.5. The reason for this phenomenon is that when Sr ions are doped, especially with relatively low doping amount, a large lattice distortion will be formed. Compared with FAp and SFAp, the distortion is relatively large, which leads to a great influence on the fluorescence properties including the reduced intensity of PL. The strongest intensity is found when Ca²⁺ is completely replaced, which is 1.45 and 3.79 times than Ca₅(PO₄)₃F: 0.03Eu²⁺ and Ca_4.5(PO₄)_0.5F: 0.03Eu²⁺ samples respectively, owing to the influence for the sublattice surrounding field around Eu²⁺ luminescence center by Sr²⁺ ions. Not only that, with the increased ratio of Sr²⁺, the emission band has a slight red shift from 449 nm to 454 nm until the ratio of Ca²⁺:Sr²⁺ is 1:1. While continual increase of x can cause an obvious blue shift with maximum range at 437 nm (x = 5). Interestingly, the larger amount of substitutes can lead to a significant blue shift and the emission peak ultimately moves to 437 nm for SFAp: 0.03Eu²⁺ sample in illustration of Fig. 8, indicating a potential application of Ca_5-xSr_x(PO₄)₃F: 0.03Eu²⁺ phosphors for adjustable blue emission.

 

Fig. 8. The PL spectra with different Ca : Sr ratio for Ca_5-xSr_x(PO₄)₃F: 0.03Eu²⁺ phosphors under 350 nm excitation. The inset shows the corresponding variation tendency of luminescence intensity and wavelength.

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Double exponential fitting, considering the two kinds of Eu²⁺ sites (Eu1, Eu2) in host lattice, is applied to the PL decay curves (in Fig. 9(a)) of DP-FAp: xEu²⁺ (x = 0.02, 0.03, 0.1) and SSR-FAp: 0.03Eu²⁺ samples by using the following formula [22],

Note, compared with SSR-FAp: 0.03Eu²⁺ (τ_avg = 620.31 ns), that DP-FAp: 0.03Eu²⁺ has a similar τ_avg about 610.30 ns but represents a 6.7-fold increase of PL intensity. It is indicated the sample fabricated by DP process keep a higher PL quantum yield (PLQY) than that of SSR method when the averaged lifetime is similar, owing to the positive correlation between PL intensity and PL quantum yield in static luminescence quenching. The PLQY is 2.12% and 3.85% for SSR / DP-FAp: 0.03Eu²⁺ samples respectively, and the detailed fitting parameters of PL decay curves are displayed in Table 2.

 

Fig. 9. (a) PL decay lifetimes of DP-FAp: xEu²⁺ (x = 0.02, 0.03, 0.1) and SSR-FAp: 0.03Eu²⁺, (b, c) parameters of double exponential fitting and (d) weighted residuals of decay curves fitting (χ²) for corresponding samples.

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Table 2. Double exponential fitting for PL decay curves of FAp: xEu²⁺ samples with different preparation methods

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3.5 Chromaticity analysis and pc-wLEDs application with high CRI and wide color gamut

The CIE coordinates change track of FAp: xEu²⁺ (x = 0.02, 0.03, 0.04, 0.10) and Ca_5-xSr_x(PO₄)₃F: 0.03Eu²⁺ (0 ≤ x ≤ 5) are represented in Fig. 10 (a, b) respectively to prove the adjustable chromaticity of relevant samples. The CIE coordinates of FAp: xEu²⁺ hardly changed with a small amount of doped Eu²⁺, and a longer shift from (0.156, 0.081) to (0.158, 0.098) is marked by white circle in Fig. 10(a) when the doping content of Eu²⁺ is multiply risen to 0.1. Similarly, the slight change of coordinate from (0.156, 0.098) to (0.154, 0.099) is discovered with little enhanced ratio of Sr²⁺ in Fig. 10 (b), then visibly moves to (0.156, 0.024) when all of the Ca²⁺ ions are gradually substituted by Sr²⁺. Moreover, the color purity of relevant samples is exhibited in Table 3, which is calculated by the following formula [35,36],

 

Fig. 10. Chromaticity coordinates diagram of (a) FAp: xEu²⁺ and (b) Ca_5-x(PO₄)_xF: 0.03Eu²⁺ phosphors synthesized by DP process and excited under 350 nm. (c) Wide color gamut composed and CIE of Sr₅(PO₄)₃F: Eu²⁺ (red ball), all-inorganic perovskite quantum dots (yellow ball) and Lu₃Al₅O₁₂: Mn⁴⁺ (grey ball). Inset shows the emission spectrum of encapsulated pc-wLED device excited by a high power 365 nm chip (1 W). (d) The detailed PL spectra for corresponding RGB fluorescent materials of this device, respectively.

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Table 3. Detailed CIE coordinates distribution and color purity of relevant samples

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3.6 Application in dentistry curing light and stability analysis

As shown in the illustrations of Fig. 11, DP-FAp: 0.03 Eu²⁺ phosphor emits visible blue light under 365 nm UV lamp. Then, it was encapsulated with a NUV-LED (1 W, 365 nm) to obtain a lighting device with intense blue emission, denoted as DP-FAp: 0.03Eu²⁺@365 nm LED. In addition, the enlarged blue light output can be found at 451 nm with the increasement of driving currents from 10 to 150 mA in Fig. 11(a), demonstrating an excellent stability of DP-FAp: 0.03Eu²⁺ for the application of high-power blue emission LED device. Notably, the full width at half maxima (FWHM) of this device spreads over 46 nm (429∼475 nm), compared with the narrower spectrum of blue LED, which can realize a single light source to act on different types of light curing materials (e.g., 468 nm for Camphor Quinone noted as CQ) as shown in Fig. 11(b) and provides a candidate for curing light in dental repair.

 

Fig. 11. (a) The EL spectra of DP-FAp: 0.03Eu²⁺ encapsulated by 365 nm LED with different driving currents (the insets are DP-FAp: 0.03Eu²⁺ phosphor under UV lamp and corresponding packaged LED device in condition of power off and on, respectively.) and (b) the spectral overlap between absorption of dental photoinitiators and emission of DP-FAp: 0.03Eu²⁺@365 nm LED.

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4. Conclusions

In summary, a series of Eu²⁺ activated (Ca, Sr)₅(PO₄)₃F: phosphors with the symmetrical particle size of 7∼12 µm have been synthesized via deposition precipitation process, which efficaciously avoided the reunion of Eu²⁺ luminescence centers and significantly enhanced the emission intensity with 6.7 times over that of traditional solid state reaction method. Pure phase of FAp: Eu²⁺ under lower synthesized temperatures (1100 °C) was exhibited to restrain the generation of impurity phase Sr₃(PO₄)₂ for SFAp: Eu²⁺ crystals. The structure refinement and Gaussian fitting of unsymmetrical PL spectra certify that the 447 nm blue emission of DP-FAp: Eu²⁺ under 350 nm is distributed to two types of site symmetries in host crystal structure (dominated 4f site is for the stronger peak of 446 nm and subordinate 6h site is indexed to the weaker peak of 469 nm). The color-tunable blue emitting with high color purity (97.79%) can be effectively acquired by modulating the ratio of Ca : Sr to cover more areas of color gamut for pc-LEDs, which can reach to 123.4% NSTC by matching CsPbBr₃, Sr₅(PO₄)₃F: Eu²⁺ and Lu₃Al₅O₁₂: Mn⁴⁺ phosphors. And a high CRI of 86 and a chromaticity coordinate (0.292, 0.376) closed to the pure white lighting (0.33, 0.33) are estimated in the pc-wLED device via packaging the above RGB fluorescent materials. Besides, the DP-FAp: 0.03Eu²⁺@365 nm LED device exhibits immobile 451 nm output with FWHM of 46 nm under mutative driving current. It is revealed that (Ca, Sr)₅(PO₄)₃F: Eu²⁺ phosphors prepared via DP have a potential application of steady blue emitting for curing light and wide color gamut for pc-wLEDs.