A Facile Method Using a Flux to Improve Quantum Efficiency of Submicron Particle Sized Phosphors for Solid-State Lighting Applications

Abstract

This work successfully verified that the addition of a flux (NH₄F, NH₄Cl, and H₃BO₃) during synthesis has an impact on the crystallite size and quantum efficiency of submicron-sized particles of CaMgSi₂O₆:Eu²⁺ phosphors. The addition of NH₄F or NH₄Cl increased the crystallite size in the submicron-sized particles, yielding an increase in emission intensity and quantum efficiency. On the other hand, the use of the H₃BO₃ flux crystallized a secondary phase, SiO₂, and changed the lattice parameters, which degraded the luminescent properties. In addition, an excessive amount of NH₄Cl was examined, resulting in nucleation of a secondary phase, CaSiO₃, which changed the lattice parameters with no improvement in luminescent properties. These results demonstrate that the addition of a flux could be a method to improve the quantum efficiency of submicron-sized particles composed of nanocrystallites; however, a judicious choice of the flux composition and amount has to be carefully considered.

1. Introduction

Powder phosphors produced by the conventional solid-state reaction method have been widely researched for application in near UV-emitting LEDs (nUV-LEDs) [1,2]. This method produces micron-sized powders that have higher quantum efficiencies than smaller-sized powders [3,4], whereas chemical synthesis methods produce submicron-sized powders composed of nanocrystallites. Particle size and crystallite size should be differentiated to understand the quantum efficiency of phosphors. A powder particle can be a single crystal or consist of crystallites, the crystallite size is typically measured by X-ray diffraction or transmission electron microscopy, whereas the particle size is typically measured by dynamic light scattering or scanning electron microscopy. In the remote phosphor configuration, the phosphor particles are on a substrate that is suspended above a nUV-LEDs, as opposed to the conventional configuration where the phosphors are embedded in a polymer around a blue-emitting LED. In addition, in the remote configuration, the packing density of the large particles is low, which generates substantial light scattering [5]. To overcome this issue, phosphors with a small, narrow particle size distribution are required. If the particle radii are <~400 nm, these particles will negligibly scatter visible and near UV radiation, because particle size is smaller than the wavelength of the radiation. However, phosphor particles in the submicron-size regime with nano-sized crystallites have poor quantum efficiency compared to micrometer-sized, single crystal phosphor particles [2,6].

One method that is used to improve the crystallinity and quantum efficiency of micrometer-sized particles is to use a flux [7,8,9]. A flux material is an inert high-temperature solvent used to accelerate crystallite growth. Generally, 0.5 wt.%~10 wt.% of a flux is used [8,10], forming a thin layer of molten material around crystals during the annealing process and facilitating a high diffusivity path through the flux [10]. Crystals grow in the molten salt solvents, thus normally called flux growth. The process is a well-known method for crystal growth in materials [1,11,12]. A flux is typically used in preparing phosphor powders through a solid-state reaction method, producing regular-shaped particles and enlarged crystallites, which cause the emission intensity to be enhanced [7,8,9,13,14,15,16,17]. However, this process is rarely researched for nanocrystalline, submicrometer-sized phosphors prepared by wet chemical processes [18,19,20,21].

Table 1. Reported results of the addition of flux on phosphor preparation. T_m = melting temperature, T_b = boiling temperature, Φ = quantum efficiency.

Flux/Tm/Tb (°C)	Phosphor Composition	Synthesis Method	Annealing Temperature (°C)	Results	Ref.
CaF2/1418/2533	(Ca0.99Ce0.01)3Sc2Si3O12	Solid state reaction	1100–1450	Reduced impurities, decreased formation temperature, no reported crystallite size and Φ, emission intensity increased 2×, narrow particles distribution, removed flux by sublimation after reaction	[13]
BaF2/1368/2260	Y2.965Ce0.035Al5O12	Spray pyrolysis	1300–1600	Enlarged, regular morphology, and non-aggregated particles, no reported crystallite size and Φ, emission intensity increased 1.4×	[18]
	Y2.95Ce0.05Al5O12	Solid state reaction	1000–1500	Able to reduce annealing temperature BaAl2O4, byproduct from BaF2 Spherical shape and smooth surface Φ external) increased 1.3× over commercial sample	[7]
	Ba0.85Eu0.15Si3Al3O4N5	Solid state reaction	1550	Enlarged crystallite size (no specific number) and particles size, narrow particles distribution, emission intensity increased slightly, no reported Φ	[14]
	Ca0.99Ce0.01Sc2O4	Solid state reaction	1550 and 1450	Φ external) increased 1.1×, no reported crystallite size, enlarged and regular particles	[8]
LiF/845/1673	Ba0.9Eu0.1Mg0.98Mn0.02Al10O17	Molten salt synthesis	1100–1400	Particles size enlarged, Li+ into the host lattice analyzed by lattice parameter, no report crystallite size from XRD, no reported Φ, emission intensity increased 2×	[15]
NaF/993/1695	Lu2.925Ce0.075Al4.79Si0.21O11.79N0.21	Solid state reaction	1500	Emission intensity increased 1.3×, regular morphology of particles, no report crystallite size and Φ	[16]
NaF/993/1695 LiF/845/1675 H3BO3/171/300 NH4F/100/decomposes	Y1.55Eu0.45Ti2O7	Solid state reaction	1350	Crystallite size enlarged (no specific number), emission intensity increased 11× (NaF), 9× (LiF), 5× (H3BO3), 2.5× (NH4F), 39% of Φ (NaF), no reported Φ without flux, enlarged particles size	[9]
NH4Cl/338/decomposes	Ba1.488Sr0.5Eu0.012SiO4	Spray pyrolysis	900–1400	Enlarged particles, enlarged crystallite size (no specific number), no reported Φ, emission intensity increased 1.3×, optimum annealing temperature decreased	[19]
K2CO3/891/decomposes	Ca0.68Eu0.12Mg0.2SiO3	Co-precipitation	1200	Charge compensation, crystallite size increased 1.1×, Φ increased 2.5×, no phase composition change, no reported particles size	[20]
Li2CO3/734/1310	(Sr0.92Eu0.08)8Al12O24S2	Solid state reaction	900	Improved purity, but still impurities remained. No report crystallite size and Φ	[17]
SrCl2/874/1250	Sr1.56Eu0.04Ba0.4SiO4	Combustion	800–950	Crystallite size increased (no specific number), emission intensity increased 2.7×, no reported Φ, similar particles size	[21]

shows the effect of various flux compositions on phosphor properties for application in nUV-LED lighting [7,8,9,13,14,15,16,17,18,19,20,21]. In each case, the phosphor properties (e.g., quantum efficiency, particle size and surface smoothness) were enhanced with the addition of a flux. In fact, after adding a flux, aluminates and oxides showed enlarged particles and smooth particle surfaces [7,8,15,18], silicates and oxy-nitrides have larger crystallite sizes and regular particle morphology [14,16,19,20]. Most importantly, these phosphors all have enhanced emission intensity or quantum efficiencies with the addition of a flux.

A flux is typically mixed with reactants [7,8,14,15,16,19] or as-synthesized product [20] before an annealing step; thus, is present as the molten phase during the post-synthesis annealing process. Since the flux material should be evaporated after annealing to avoid formation of impurities or second phases in the final product, the melting and boiling temperatures should be considered in relationship to the annealing conditions (temperature, atmosphere), when selecting a flux material. The criteria for selecting a flux are: (1) a low melting temperature, so that it is a liquid during the annealing process [9]; (2) a boiling temperature lower than the annealing temperature, so that the flux can be evaporated to avoid impurity or a second phase formation [13]. Fluxes having a higher boiling temperature than the annealing temperature had been used in most of the previous literature (Table 1); and (3) the difference in ionic radii of the flux and the phosphor elements must be more than 30% to avoid doping of the flux elements in the phosphors [22]. Chiang et al. [7] reported a formation of a second phase of BaAl₂O₄ in Y_2.95Ce_0.05Al₅O₁₂ with a BaF flux. This suggests that some fluxes can remain after the reaction, producing by-products.

Dai et al. [9] discussed the effect of various fluxes (NH₄Cl, NH₄F, H₃BO₃, LiF, and NaF) on the emission intensity of Y_1.55Eu(III)_0.45Ti₂O₇ phosphors with an orange-red emission under near UV light for display devices such as high-resolution and field emission displays, as well as high-power white light-emitting diodes. It was found that uniform micrometer-sized (~4 µm) particles formed with NaF and LiF fluxes, and the maximum emission intensity was achieved with NaF flux, while a narrow size distribution of the particles was not achieved with NH₄Cl, NH₄F, or H₃BO₃ fluxes. Additionally, Zhang et al. [8] examined the influence of different concentrations of BaF₂ flux on the formation of Ca_0.99Ce_0.01Sc₂O₄ with green emission prepared by a solid-state reaction. After introducing BaF₂, a higher particle growth rate, larger particle sizes, and more narrow particle size distribution were verified, which resulted in improved emission intensity. The emission intensity increased with the increase of the concentration of BaF₂ and the maximum emission intensity corresponded to 0.5 wt.% of BaF₂. The emission intensity decreased when the concentration of BaF₂ was higher than 0.5 wt.%, which was attributed to particle agglomeration. Wang et al. [20] examined the effect of Li₂CO₃ and K₂CO₃ fluxes on the formation of Ca_0.68Mg_0.2Eu_0.12SiO₃ prepared by a co-precipitation method. The crystallite size increased from ~93 nm to 99 nm (6% of Li₂CO₃) or to 100 nm (5% of K₂CO₃), and the quantum efficiencies were improved (from 12% to 27% with Li₂CO₃ flux and to 31% with K₂CO₃ flux).

In this work, blue-emitting Ca_0.94Eu_0.06MgSi₂O₆ powders were synthesized through a co-precipitation method [23,24]. The powders were annealed with three different fluxes (NH₄F, NH₄Cl, or H₃BO₃). In our previous report [24], the Ca_0.94Eu_0.06MgSi₂O₆ powders formed submicrometer-sized particles having blue color with x = 0.14 and y = 0.05 on the CIE diagram, similar to those defined by the National Television System Committee color (0.14, 0.08), but the quantum efficiency was found to be low (Φ, ~5%). A low Φ is a drawback of nanocrystalline-sized phosphors; therefore, the main goal of this study was to improve the Φ using several different types of flux materials. NH₄F, NH₄Cl, and H₃BO₃ were selected as flux materials from the reported flux candidates [7,8,9,13,14,15,16,17,18,19,20] due to their low melting and boiling temperatures (

Table 2. Melting and boiling temperature of the fluxes.

Flux	Melting Point (°C)	Boiling Point (°C)
NH4F	100	Decomposition *
NH4Cl	338	Decomposition **
H3BO3	171	300 ***

* NH₄F (s) → NH₃ (g) + HF(g) > 100 °C, ** NH₄Cl (s) → NH₃ (g) + HCl (g) > 338 °C, *** H₃BO₃ (s) → B₂O₃ (s) + H₂O > 235 °C.

), which are expected to decompose or evaporate during annealing process. Additionally, the large (>30%) ionic radii difference between the flux and the phosphor components indicates that flux contamination of the phosphor is unlikely to occur.

2. Experimental Procedure

2.1. Reagents

All chemicals were used without further purification and included tetraethyl orthosilicate (TEOS, 99.9%, Sigma Aldrich, St. Louis, MO, USA), Mg(NO₃)₂·6H₂O (98.3%, Fisher Scientific, Hampton, NH, USA), Ca(NO₃)₂·4H₂O (99.0%, Macron Fine Chemicals, Center Valley, PA, USA), Eu₂O₃ (99.99%, Alfa Aesar, Haverhill, MA, USA), nitric acid (69.3%, Fisher Scientific, Hampton, NH, USA), citric acid (C₆H₈O₇·H₂O, ACS reagent grade, Macron Fine Chemicals, Center Valley, PA, USA), ethylene glycol (C₂H₆OH, certified, Fisher Scientific, Hampton, NJ, USA), polyethylene glycol (PEG, C₂H₄O·nH₂O, molecular weight = 20,000 g/mol, Sigma Aldrich, St. Louis, MO, USA), ammonium hydroxide (28~30%, BDH Aristar Plus, Center Valley, PA, USA), NH₄F (96%, Alfa Aesar, Haverhill, MA, USA), NH₄Cl (ACS reagent grade, Macron Fine Chemicals, Center Valley, PA, USA), and H₃BO₃ (99.5%, Sigma Aldrich, St. Louis, MO, USA).

2.2. Preparation of Ca_0.94Eu_0.06MgSi₂O₆ with and without a Flux

The co-precipitation method was used to synthesize the powders by following previously reported procedures [23,24]. The concentration of 6 at.% Eu²⁺ activator was chosen as it is reported to have the highest photoluminescence (PL) emission intensity [24]. Tetraethyl orthosilicate (2.23 mL) was added to ethanol (20 mL) with several drops of nitric acid and deionized water while stirring for 30 min. Meanwhile, Eu₂O₃ (0.0015 mol) was dissolved in nitric acid (0.4 mL) solution to form aqueous Eu(NO₃)₃ solution. Mg(NO₃)₂·6H₂O (0.005 mol) and Ca(NO₃)₂·4H₂O (0.0047 mol) were dissolved in deionized water with stirring. After the two nitrate solutions became transparent, all three solutions (tetraethyl orthosilicate, Eu(NO₃)₂, and Mg(NO₃)₂ with Ca(NO₃)₂) were mixed together and then stirred for 1 h. Subsequently, ammonium hydroxide was added dropwise into the solution to reach a pH of 10 and initiate precipitation. White precipitates were formed, and the suspension was stirred for 8 h. Next, NH₄F, NH₄Cl, or H₃BO₃ was added to the solution at amounts of 2 wt.%, 6 wt.%, or 10 wt.% of Ca_0.94Eu_0.06MgSi₂O₆. The conversion of wt.% to mol.% for each flux is shown in

Table 3. Conversion of wt.% to mol.% for each flux in the solid phosphor powders.

wt.% Flux	NH4F (mol.%)	NH4Cl (mol.%)	H3BO3 (mol.%)
2	12	8	7
6	28	22	19
10	40	31	28

. The solutions were then centrifuged and dried at 80 °C for 12 h. Finally, a post-synthesis annealing step was performed at 1100 °C for 2 h in air and then at 1100 °C for 4 h in a 5% H₂/95% N₂ atmosphere to transform Eu³⁺ to Eu²⁺.

2.3. Characterization

The powders were analyzed by X-ray diffraction (XRD) on a D2 Phaser (Bruker, Karlsruhe, Germany) using CuKα radiation and a step size of 0.014° over a 2θ range of 20 to 80°. The crystallite sizes, lattice parameters, and ratio of the phases presented were calculated by Rietveld refinement using the TOPAS 4.2 software (Bruker). The sizes of the powders were examined by dynamic light scattering (DLS) on a Nanotrac Wave II system (Microtrac Inc., York, PA, USA) [25,26,27]. Particles were distributed in an aqueous solution by sonication in a water bath for DLS experiments. A field emission scanning electron microscope (FESEM, XL30, Philips, Amsterdam, Netherlands) at 10 keV was used to image the powders to confirm sizes and determine morphology. Samples were coated with iridium at 85 µA for 10 s before imaging. Energy dispersive spectroscopic analysis (EDS) was performed with a scanning electron microscope (Apreo SEM, FEI, Hillsboro, OR, USA) to analyze the concentration of elements. Absolute quantum efficiency (Φ) measurements were performed using an integrating sphere system, with sodium salicylate (Φ = 44%) as a reference standard. PL emission and excitation spectra were acquired with a fluorescence spectrophotometer (Hitachi F-7000, Hitachi High-Technologies Corporation, Tokyo, Japan) using λ = 350 nm excitation wavelength (pulse = 0.025 s). This excitation wavelength was selected as it produced the highest PL emission intensity for Ca_0.94Eu_0.06MgSi₂O₆ [24].

3. Results and Discussion

3.1. Crystal Structure and Lattice Parameters

Figure 1 shows the crystal structure of CaMgSi₂O₆. Also known as diopside, it has a monoclinic crystal structure with space group C2/c. The lattice parameters are a = 0.9743 nm, b = 0.8879 nm, c = 0.5230 nm, and β = 105.53° [24]. The coordination numbers for Ca²⁺, Mg²⁺, and Si⁴⁺ of 8, 6, and 4, respectively. The ionic radii of the ions are listed in

Table 4. The ionic radii (nm) of ions of in CaMgSi₂O₆ and fluxes, NH₄F, NH₄Cl, H₃BO₃.

Ions	4-Coordinated	6-Coordinated	8-Coordinated
Ca2+	-	-	0.112
Mg2+	-	0.072	-
Si4+	0.026	-	-
O2−	0.138	0.140	0.142
B3+	0.011	0.027	-
F−	0.131	0.133	-
Cl−	-	0.181	-

. The radii differences between ions in CaMgSi₂O₆ and the flux are Mg²⁺-B³⁺ = 90%, Si⁴⁺-B³⁺ = 81%, O²⁻-F⁻ = 5% (4-coordinated, 6-coordinated), O²⁻-Cl⁻ = 26% (6-coordinated). The F⁻ from NH₄F and Cl⁻ from NH₄Cl are likely to be composed to HF (g) and HCl (g) during the annealing process (Table 2) although the radii difference between O²⁻ and F⁻/or Cl⁻ is less than 30%. H⁺ may occupy interstitial sites in the lattice because of its small size. Eu³⁺ (0.107 nm for 8-coordinated) is expected to occupy the Ca²⁺ sites due to the radii size similarity (5% difference) before reduction annealing and transforms to Eu²⁺ after reduction.

XRD patterns of Ca_0.94Eu_0.06MgSi₂O₆ with and without NH₄F and NH₄Cl fluxes are shown in Figure 2. The peak widths narrowed with an increase in the concentration of NH₄F (Figure 2a), indicating an increase in crystallite size. Several small peaks from CaEu₄(SiO₄)₃O were recorded for 10 wt.% NH₄F. CaEu₄(SiO₄)₃O was previously identified in Ca_0.94Eu_0.06MgSi₂O₆ under annealing temperature of 1247 °C [29], which is a higher temperature than used in the present study, with no flux. As shown previously, a flux can decrease the temperature for the crystallization of phosphor materials [7,13], so that excessive NH₄F may lead to the new formation of CaEu₄(SiO₄)₃O at the lower temperature by reducing the corresponding formation temperature. For the diffraction patterns from the powders prepared using NH₄Cl flux, shown in Figure 2b, the peak width also slightly narrowed with the increase of concentration of NH₄Cl. However, this narrowing is less pronounced in comparison with the NH₄F, indicating that the crystallite sizes with NH₄F flux were larger. The crystallite sizes for different concentrations of NH₄F and NH₄Cl fluxes are shown in Figure 2c. For the NH₄F flux, the crystallite size increased from roughly 13 nm with no flux to about 31 nm with 10 wt.% NH₄F. The maximum crystallite size for NH₄Cl flux was about 19 nm with 2 wt.% NH₄Cl. Different diffusion rates of the reactants through a flux may explain why the material produced with NH₄F flux shows enlarged crystallite sizes compared to that made with NH₄Cl flux. Ions move through liquid flux during the annealing process with a diffusion coefficient expressed by D = (1/f)·kT, where f is the frictional coefficient, k is the Boltzmann’s constant, and T is the absolute temperature. The frictional coefficient can be expressed by f = πμr, where μ is viscosity and r is the radius of the ion. Therefore, a relationship D ∝ 1/r is found. F⁻ (0.119 nm) is smaller than Cl⁻ (0.167 nm), therefore NH₄F may act as a more effective flux compared to NH₄Cl from the point of view of enhanced diffusivity, assuming equivalent viscosities.

With the H₃BO₃ flux, SiO₂ impurities were detected, as shown in the XRD patterns in Figure 3a. Even though B³⁺ and Si⁴⁺ have quite different ionic radii (0.026 nm and 0.011 nm) [30], a substitution of B³⁺on Si⁴⁺ sites on the tetrahedral site has been previously reported [31,32] although there is a charge difference between B³⁺ and Si⁴⁺. Marler et al. [31] reported synthetic tourmaline (olenite) to replace partial silicon ions with excessive boron ions; and the difference in charge between Si⁴⁺ and B³⁺ was compensated by protons leading to the unusually high water content. Xia et al. [32] studied La₅(Si_2−xB_1−x)(O_13−xN_x):Ce³⁺ by replacing partially B³⁺ - O²⁻ by the Si⁴⁺ - N³⁻; and the charge difference from the replacement of B³⁺ by Si⁴⁺ was compensated by the substitution of O²⁻ by N³⁻. In the current study, the charge difference from the partial replacement of Si⁴⁺ by B³⁺ could be compensated to produce oxygen vacancies due to no other ion replacements.

With an assumption that B ions partially replace the Si ions, the corresponding chemical reaction is:

CaMgSi₂O₆ + xH₃BO₃ → CaMg(Si_1−x/2B_x/2)₂O₆ + xSiO₂ + xH₂O + x/2H₂.

(1)

As the amount of H₃BO₃ increased (2–10 wt.%), the corresponding amount of SiO₂ increased (53–84 mol.%, Figure 3b). The crystallite size of the phosphor increased from ~13 nm to ~27 nm, while for SiO₂ it increased from 20 nm to 80 nm (Figure 3c).

The calculated lattice parameters of Ca_0.94Eu_0.06MgSi₂O₆ with and without each flux are shown in Figure 4. The lattice parameters of powders prepared with NH₄F and NH₄Cl (Figure 4a,b) were identical to the phosphors prepared without flux, indicating that these two fluxes did not affect the crystal structure of the resultant materials. In contrast, with H₃BO₃ flux, the b and c parameters did not show significant change, while a decreased from 0.9743 nm to 0.9670 nm. This is presumably due to the substitution of B³⁺ (0.011 nm) with Si⁴⁺ (0.036 nm) sites, causing the lattice parameter to decrease. Vegard’s law [33], which is an empirical rule based on a linear relationship found between lattice parameters and the size of the constituent elements. The formation of ½O vacancies from the charge difference between Si⁴⁺ and B³⁺ can affect the lattice parameters. Therefore, the relationship between the initial lattice parameter and the one with B³⁺ is $a=a_0-x\left(r_{Si}-r_B\right)-\frac{1}{2}x{r}_O$, where a is the lattice parameter (0.9670 nm) with 10 wt.% B³⁺ addition, a₀ is the initial lattice parameter (0.9743 nm), $x$ is the partial substitution of B on Si, $r_{Si}$, $r_B$, $r_O$ are the radii of Si⁴⁺, B³⁺, and O²⁻, respectively. The $x$ value obtained is 0.09, which is smaller than the molar fraction of H₃BO₃ added (0.28 converted from 10 wt.% H₃BO₃). The excess B³⁺ was not detected in the XRD patterns (detection limit of 3 ~ 5 wt.%). Therefore, due to the low boiling temperatures, it likely does not remain in the resultant powder. The change of lattice parameter a, while parameters b and c remain unchanged, can be explained by the ion arrangement. Along the [100] and [001] directions (see Figure 1b,d), there are two Si⁴⁺ along the a-axis, four along the b-axis, and three along c-axis. The fractions of one Si⁴⁺ along the a-, b-, and c-axes are 0.50, 0.25, and 0.33, respectively. The fraction of Si⁴⁺ along the a-axis is the largest, so that the replacement of Si⁴⁺ is affected more along this axis, resulting in a decrease of a with an addition of 10 wt.% H₃BO₃. Another possible reason for a change in lattice parameter of Ca_0.94Eu_0.06MgSi₂O₆ is H⁺ dissolved in interstitial sites, but this would cause an increase of lattice parameters, which was not observed.

3.2. Scanning Electron Microscopy and Dynamic Lighting Scattering Analysis

SEM images of Ca_0.94Eu_0.06MgSi₂O₆ are shown in Figure 5a for no flux, Figure 5b–d for NH₄F flux addition, Figure 5e–g for NH₄Cl flux addition, and Figure 5h–j for H₃BO₃ flux addition. For NH₄F or NH₄Cl fluxes, the particle sizes and morphologies were similar to those without a flux. However, the particles with the H₃BO₃ flux were aggregated, and the particle sizes were irregular. From EDS analysis, the ratio of Ca:Mg:Si:O was 1:1:7:13 in the large particles (yellow point in Figure 5h), where the amounts of Si and O are two to three times more than those of CaMgSi₂O₆ (Ca:Mg:Si:O = 1:1:2:6). In the small particles (red point in Figure 5h), the ratio of Ca:Mg:Si:O was 1:1:3:5, which is closer to that of CaMgSi₂O₆. Although the results from EDS are considered more accurate for polished surfaces than the powders, the ratio of Ca, Mg, Si, O demonstrated differences between small and large particles. Also, given that the EDS analysis penetrates 1–2 µm below the surface [34], CaMgSi₂O₆ could be detected under the SiO₂ particles, which could be the reason EDS analysis showed a small amount of Ca and Mg, when presumably large SiO₂ particles were analyzed. Therefore, large particles can be considered as SiO₂ and the aggregated, small particles as CaMgSi₂O₆.

From the DLS analysis (Figure 6), the average particle sizes were 83 nm for powders without flux. With the NH₄F flux, the average particles sizes were 80 nm (2 wt.%), 100 nm (6 wt.%), and 133 nm (10 wt.%), respectively. For the NH₄Cl flux, the corresponding particle sizes were found to be 84 nm (2 wt.%), 82 nm (6 wt.%), and 118 nm (10 wt.%). Overall, this indicates that particles were still submicron-sized with the addition of flux. However, some studies reported uniformly shaped and enlarged particles with up to 14 wt.% of flux addition [8,14,15,19]. For the H₃BO₃ flux, the average particle sizes were 307 nm (2 wt.%), 318 nm (6 wt.%), and 375 nm (10 wt.%), which resulted from an increase in the amount of the SiO₂ particles.

3.3. Photoluminescence Spectra and Quantum Efficiency

The PL excitation (PLE) was monitored at 458 nm, and the PL emission spectra were obtained under λ_ex = 350 nm as an excitation wavelength. Although the Eu ions were introduced from a strong nitric acid solution during the synthetic process, this acid may cause traces of transition metal ions impurities (Cr, Cu, Pb, Ni, Zn, Au, Ti). However, these are negligible due to the very low concentration in the whole solution (<0.01 ppm). If all transition metals are present in the phosphor powders, the concentration would be ~0.1 ppb, which would not influence the luminescence properties. Figure 7a–c shows the PLE spectra (dashed lines) with a broadband absorption in the near UV region from 200 nm to 400 nm, with a maximum at 350 nm, which is attributed to the allowed transition of Eu²⁺. The PL emission (solid lines) shows a spectrum with a maximum at 458 nm, which corresponds to the parity allowed 4f⁶5d¹ → 4f⁷ transition of Eu²⁺. In Figure 7a, the absorption and emission intensities, together with the quantum efficiency (Φ), increased with the increase of the amount of NH₄F. A maximum Φ of 17% was found at 11 wt.% of NH₄F. For the NH₄Cl flux (Figure 7b), the quantum efficiency increased from 5% to 11% when 2 wt.% of NH₄Cl was added. For the amounts of NH₄Cl > 2 wt.%, Φ decreased. As shown in Figure 7c, Φ with H₃BO₃ was very low, ~1%, because of the significant amount of SiO₂ in the sample, as was confirmed by the XRD (Figure 3b). Figure 7d is a plot of Φ as a function of the amount of flux for all three fluxes used in the current study. A cumulative analysis of Figure 2c and Figure 7d demonstrates that the change in the crystallite sizes of the materials produced with NH₄F and NH₄Cl fluxes, directly relates to the change in their corresponding quantum efficiencies [35]. For 0–10 wt.% of NH₄F, the average crystallite size increased from 13 nm to 31 nm, and the corresponding quantum efficiencies improved from 5% to 17%. Similarly, for 0 wt.%, 2 wt.%, 6 wt.%, and 10 wt.%, of the NH₄Cl flux, the average crystallite sizes were 13 nm, 19 nm, 15 nm, 18 nm, respectively, and the corresponding quantum efficiencies showed similar trend. In constant, with 0–10 wt.% of H₃BO₃, the crystallite size increased from ~13 nm to ~27 nm, but the quantum efficiency was low due to the presence of a large fraction of the secondary phase, SiO₂.

3.4. Effect of High Concentration of NH₄Cl

Increased amount of NH₄Cl was examined to determine if the high concentration of this particular flux resulted in the formation of second phase, a change in crystal structure, or further improvement of Φ while phosphors with NH₄F and H₃BO₃ already consist of secondary phases below 10 wt.% of the flux. Figure 8a shows the XRD patterns of CaMgSi₂O₆ produced with 14, 17, and 20 wt.% of NH₄Cl flux. There were no secondary phases with 14 and 17 wt.%, but CaSiO₃ was detected for 20 wt.% of NH₄Cl. The lattice parameters a and c were not affected, but b decreased from 0.8867 nm (for 14 wt.%) to 0.8852 nm (for 20 wt.%) (Figure 8b). Given that the radii difference between Cl⁻ and O²⁻ is 26%, excessive Cl⁻ may substitute for O²⁻ and produce a charge imbalance. This could cause cation defects and/or the creation of secondary phases such as CaSiO₃. MgSiO₃ could also potentially be produced due to the substitution O²⁻ by Cl⁻, but it was not detected by XRD. Vegard’s law can also be applied to the b–axis change, with an assumption that there are vacancy defects of ½ Ca²⁺, ½ Mg²⁺ or ¼ Si⁴⁺ due to the charge difference between O²⁻ and Cl⁻. To obtain the maximum difference between b and b₀, Ca²⁺ vacancies were assumed. The lattice parameter change is $b=b_0-\frac{1}{2}x{r}_{Ca}-x\left(r_O-r_{Cl}\right)$, where b is the lattice parameter (0.8852 nm) with 20 wt.% Cl⁻ additions, b₀ is the initial lattice parameter with no flux (0.8900 nm), x is the partial substitution of Cl⁻ on O²⁻, and $r_{Ca}$, $r_{Si},$ $r_O$, $r_{Cl}$ are radii of Ca²⁺, Si⁴⁺, O²⁻, and Cl⁻, respectively. The obtained x value is 0.32, which is smaller than the molar fraction of Cl⁻ in CaMgSi₂O₆ (0.50, converted from 20 wt.% NH₄Cl). The remaining Cl⁻ is likely from the excessive addition of NH₄Cl. The crystal structure along the [001] (Figure 1d) and [010] (Figure 1c) directions shows one Ca²⁺ and one Mg²⁺ along the a- and c-axes and two Ca²⁺ and two Mg²⁺ along the b-axis, resulting in the fraction of Mg²⁺ and Ca²⁺ along the b-axis being twice higher than that along the a- and c-axes. Therefore, the b-axis was altered more than the a- and c-axes.

The PL emission spectra and Φs with additional NH₄Cl flux are shown in Figure 8c. The Φs were not significantly changed with an increase in the amount of NH₄Cl. A comparison between crystallite size and Φs with the amount of NH₄Cl flux is shown in Figure 8d. The maximum Φ corresponded to 2 wt.%; over 2 wt.% Φ initially decreased and eventually leveled off. However, the crystallite sizes increased with an increase in the concentration of the flux. This implies that the flux, although increasing the crystallite size, also changed lattice parameters resulting in no enhancement of Φ due to the inappropriate amount of flux addition.

4. Conclusions

The crystallite size and quantum efficiency of blue-emitting Ca_0.94Eu_0.06MgSi₂O₆ submicrometer-sized phosphors prepared by the co-precipitation method were altered with the addition of a flux, NH₄F, NH₄Cl, or H₃BO₃. The particle sizes both with and without fluxes (NH₄F or NH₄Cl) were submicrometer-sized (~100 nm). A direct correlation between crystallite sizes of the materials produced with NH₄F or NH₄Cl fluxes and their corresponding quantum efficiencies was verified. For the NH₄F flux, the crystallite size increased from 13 nm (no flux) to 31 nm (10 wt.% flux). The corresponding quantum efficiencies improved from 5% (no flux) to 17% (10 wt.% flux), correlating with the increase in crystallite size. For the NH₄Cl flux, the crystallite sizes increased to 18 nm with 10 wt.% from 13 nm with no flux, with a corresponding increase in quantum efficiency from 5% to 11%. This demonstrates that NH₄Cl is not an effective flux, likely due to its lower liquid diffusion coefficient compared to NH₄F. Additionally, it was shown that further increasing the amount of the NH₄Cl flux (14–20 wt.%) only slightly improved the corresponding quantum efficiency. The H₃BO₃ flux produced a substantial amount of SiO₂ as a secondary phase, which negatively affected the quantum efficiency of the resultant material. From lattice parameter measurements, it is shown that the phosphor was contaminated with boron ions, and silicon was leached out of the lattice.

In summary, it is shown that the poor quantum efficiency of submicron-sized phosphors can be improved by using NH₄F flux for CaMgSi₂O₆:Eu²⁺, but the flux composition and amount must be carefully assessed to evaluate the presence of secondary phases.