Single-Phase Silicate Phosphors (Ba1.3Ca0.7−xSiO4:xDy3+) Doped with Dysprosium for White Solid-State Lighting

Abstract

Single-phase phosphors have potential advantages such as simple processability, competitive cost, and other optical and optoelectronic properties. Hence, in this study, the silicate phosphors (Ba_1.3Ca_0.7−xSiO₄:xDy³⁺) doped with Dy³⁺ ions (x = 0.01–0.05) were synthesized and characterized in detail. X-ray diffraction patterns showed that all the silicate phosphors have a τ-phase hexagonal unit cell independent of doping. However, d-spacing was reduced for the doped samples, indicating that the interplanar interactions were enhanced. Resultantly, the doped phosphors exhibited relatively larger domains with connectivity than the host, although there were sometimes microscale pores. Photoluminescence spectra stipulated that the optimized doping concentration is x = 0.03 for the silicate Ba_1.3Ca_0.7-xSiO₄:xDy³⁺ phosphors. Finally, the CIE coordinates (0.30, 0.33) confirm that the Dy³⁺-doped silicate phosphors are well applicable to the white light-emitting diodes for solid-state lighting and display devices.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

The light-emitting diode (LED)-based solid-state lighting (SSL) is relatively a green technology by replacing the conventional incandescent and fluorescence lamps requiring a large amount of energy consumption [1–4]. Therefore, since the Nichia Corporation invented a blue indium-gallium-nitride (InGaN) LED chip coated with a yellow phosphor (Y₃Al₅O₁₂:Ce³⁺, YAG: Ce³⁺) in 1996, the phosphor-converted white LED (pc-WLED) has received a surge of global interests due to their high luminous efficacy, stability/reliability, competitive cost, eco-friendliness, and low power consumption [5–7]. However, the low color-rendering index (CRI) and high correlated color temperature are common drawbacks to this kind of LED [8, 9]. Hence, to overcome this limitation, the ultraviolet (UV) LED chips coated with red-green-blue (RGB) trichromatic phosphors were introduced, resulting in high CRI with a partial sacrifice in their quantum efficiencies [10, 11]. However, there are still some technical challenges such as the reabsorption of blue light by the green and red phosphors and the complicated mixing process of these RGB phosphors with a specific ratio for a desirable color balance [12–16].

Meanwhile, the single-phase pc-WLED has been proposed as an alternative to the aforementioned UV-LED with yellow or RGB phosphors [17–28]. Here, the single host matrix (e.g., aluminate, borate, carbonate, fluoride, molybdate, niobate, nitride, phosphate, silicate, tantalite, titanate, and tungstate) was doped with rare earth element (REE) ions (e.g., scandium, yttrium, and other lanthanides) and/or transition metal ions [3, 29]. Here, if the process of doping in luminescent semiconductors is more simplified by using a single dopant instead of two or three ones, it will be clearly advantageous because both time and cost are saved. Therefore, it should be necessary to study the single-phase single-doped phosphor materials. For this purpose, the trivalent dysprosium ion (Dy³⁺) should be a good activator because it has two main emissions through the magnetic dipole transition at 482 nm (blue: ⁴F_9/2⟶⁶H_15/2) and the electric dipole transition at 577 nm (yellow: ⁴F_9/2⟶⁶H_13/2) [17–28]. Herein, the yellow-emitting process is hypersensitive to the local environment, indicating the luminous intensity is adjustable by modulating the crystal field. As a host material, the silicate phosphors are promising because it has chemical and thermal stabilities [30, 31]. Hence, to date, the versatile silicate phosphors have been examined for pc-WLEDs, which includes Ca₃Si₂O₇, Ca₂Al₂SiO₇, Sr₂MgSi₂O₇, Ca₂MgSi₂O₇, Ca₃MgSi₂O₈, K₄CaSi₃O₉, BaCa₂Si₃O₉, KBaScSi₃O₉, Ca₉La(PO₄)₅(SiO₄)F₂, and Ca₉La(PO₄)₅(SiO₄)Cl₂ [3, 29–34].

In this study, we report the concentration effects on the properties of the unique τ-phase silicate Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ phosphors, showing chemical and structural stabilities. For this purpose, we investigated the structural, morphological, and optical properties of Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ as a function of the activator (Dy³⁺) amounts. Here, the photoluminescence (PL) and CIE chromaticity diagram of the Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ samples were of special interests for the single-phase pc-WLED applications.

2. Experimental

2.1. Synthesis of Ba_1.3Ca_0.7−xSiO₄:xDy³⁺

The single-phase silicate (Ba_1.3Ca_0.7−xSiO₄:xDy³⁺) phosphors were prepared with varying the doping concentration of Dy³⁺ ions (x = 0.00, 0.01, 0.02, 0.03, 0.04, and 0.05). Calcium nitrate (Ca(NO₃)₂4H₂O (99.9%)), barium nitrate (Ba(NO₃)₂ (99.9%)), dysprosium(III) nitrate (Dy(NO₃)₃ (99.9%)), urea (CH₄N₂O (99.9%)), and tetraethyl orthosilicate (TEOS) ((Si(OC₂H₅)₄ (99.9%)) were purchased from Sigma-Aldrich and used as received. Stoichiometric amounts of nitrates, TEOS, and urea were dissolved in 10 mL of deionized water [35, 36]. The mixture was constantly stirred at the temperature of 80–90°C, transferred to an alumina crucible, and put into a preheated furnace at 550°C. Then, the solid products (Ba_1.3Ca_0.7−xSiO₄:xDy³⁺) were annealed at 1000°C for 2 h and subsequently crushed for obtaining the fine powder samples.

2.2. Characterization

The X-ray diffraction (XRD) pattern data were obtained using the Model Philips Bruker D8 advance. The microstructure of samples was characterized by using the scanning electron microscope (SEM: Model JEOL JSM-7800F). Elemental analysis was carried out using the energy-dispersive X-ray spectroscope (EDS) (Oxford Aztec). Optical bandgap was estimated by using the ultraviolet-visible (UV-Vis) spectrophotometer (Shimadzu evolution 100). The PL data could be obtained by using the Cary Eclipse fluorescence spectrophotometer (model LS-55) equipped with a xenon flash lamp.

3. Results and Discussion

3.1. Structural Analysis

Figure 1(a) shows the XRD patterns for the prepared silicate phosphors. The diffraction peaks were well indexed with the JCPDS (Joint Committee on Powder Diffraction Standards) card number 36–1449. The silicate phosphors (Ba_1.3Ca_0.7−xSiO₄:xDy³⁺) have the space group ($p\overline{3}m1$) and a hexagonal unit cell [37] with parameters a = 0.5749 nm, c = 1.466 nm, and volume (V) = 0.484 nm³. Figure 1(b) shows the analysis of the most intense (110) peaks. The host material (Ba_1.3Ca_0.7SiO₄) shows the d-spacing of 0.291 nm based on Bragg’s law $\lambda =2d\sin \theta$, where the wavelength of X-ray light ($\lambda$) is 0.154 nm at the diffraction angle $\theta$ = 15.35°. On the other hand, the Dy³⁺-doped silicate phosphors (Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ (x = 0.01–0.05)) display the average d-spacing of 0.2888 ± 0.001 nm at the angles of $\theta$ = 15.47° (x = 0.01), 15.49° (x = 0.02), 15.45° (x = 0.03), 15.55° (x = 0.04), and 15.40° (x = 0.05), respectively. This result indicates that when Dy³⁺ ions were incorporated into the host silicate, the interplanar interactions between (110) crystallographic planes were enhanced, resulting in the reduction of d-spacing, i.e., the shift of (110) peaks to higher $2\theta$ angles as shown in Figure 1(b). Importantly, it is notable that the ionic radii of Dy³⁺, Ca²⁺, and Ba²⁺ ions are 1.03 Å, 1.06 Å, and 1.34 Å, respectively [38–40], indicating that the dopant Dy³⁺ ions may substitute Ca²⁺ instead of Ba²⁺ ions based on the similarity of ionic sizes [41, 42]. However, uncompensated charges remain in the phosphors due to the aliovalency effect between Dy³⁺ and Ca²⁺ ions, which could be studied as a future work.

[figure(s) omitted; refer to PDF]

The crystallite size ($D_S$) of the phosphor samples can be calculated using Scherrer’s equation [43, 44]:$$\begin{array}{}\text{(1)}& D_S=\frac{0.92\lambda }{\beta \cos \theta },\end{array}$$where $\beta$ is the full width at half maximum (FWHM) at the diffraction angle $\theta$, $D_S$ value was estimated to be 29.4 nm for the host sample, whereas the average crystallite size ${\overline{D}}_S$ for the Dy³⁺-doped silicate phosphors (x = 0.01–0.05) was 32.8 ± 5.7 nm based on the data given in Table 1.

Table 1

Crystallite size ($D_S$) of the silicate phosphors, Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ (x = 0.00, 0.01, 0.02, 0.03, 0.04, and 0.05), as a function of Dy³⁺ doping concentration.

Ba1.3Ca0.7−xSiO4:xDy3+	$\beta$ (radians)	$\theta$ (°)	$D_S$ (nm)	d-spacing (nm)
x = 0.00	0.00366	15.35	29.4	0.2910
x = 0.01	0.00383	15.47	36.8	0.2889
x = 0.02	0.00358	15.49	36.8	0.2885
x = 0.03	0.00520	15.45	24.5	0.2892
x = 0.04	0.00366	15.55	36.8	0.2874
x = 0.05	0.00400	15.40	29.4	0.2902

The parameter $\beta$ is the FWHM at the diffraction angle $\theta$.

3.2. Morphological and Elemental Analyses

Figure 2 shows the SEM images, displaying various morphologies for the silicate powder samples. As shown in Figure 2(a), the host material shows some microscale aggregations of polycrystalline particles. However, when 1 mol% of Dy³⁺ ions was incorporated into the host lattice (i.e., Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ (x = 0.01)), the microstructural morphology was changed dramatically, exhibiting relatively uniform domains and their boundaries with good connectivity and aggregation (Figure 2(b)). Note that the Dy³⁺ doping process enhanced the interplanar interactions as confirmed through the XRD data in Figure 1(b). This phenomenon is validated again through the phosphor sample doped with 3–5 mol% (i.e., x = 0.03–0.05) of Dy³⁺. Figure 2(c) shows that the sample (Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ (x = 0.03)) has the increased aggregation of particles with partial connectivity, but contains significant amounts of micropores, presenting the nonuniform growth process of polycrystalline domains during the rapid gel-combustion reactions in the presence of dopant Dy³⁺ ions. In the same vein, when the doping concentration was increased up to 5 mol % of Dy³⁺ ions (Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ (x = 0.05)), the morphology displays an impressive microstructure with relatively large domains and microscale pores, implying the viscous flow in the lateral direction during crystallization and growth of the doped silicate phosphors (Figure 2(d)).

[figure(s) omitted; refer to PDF]

Furthermore, the EDS analysis provides the elemental composition of the silicate phosphor samples. As shown in Figure 3, the host material (Ba_1.3Ca_0.7SiO₄) shows O, Ba, Si, and Ca elements, whereas the Dy³⁺-doped phosphor samples (Ba_1.3Ca_0.7-xSiO₄:xDy³⁺) exhibit the presence of the activator (Dy) in addition to the host components, as given in Table 2.

[figure(s) omitted; refer to PDF]

Table 2

The weight and atomic percentage of Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ samples as a function of Dy³⁺ doping concentration (x = 0.00, 0.01, 0.03, and 0.05) based on the energy-dispersive X-ray spectroscopy (EDS) analysis.

Phosphor sample	Element	Weight (%)	Atomic (%)
Ba1.3Ca0.7−xSiO4:xDy3+ (x = 0.00)	Ba	29.0	6.06
	Ca	16.0	11.46
	Si	21.0	21.47
	O	34.0	61.01
Ba1.3Ca0.7−xSiO4:xDy3+ (x = 0.01)	Ba	38.7	9.48
	Ca	20.2	16.95
	Si	10.4	10.40
	O	28.9	60.75
	Dy	1.8	0.37
Ba1.3Ca0.7−xSiO4:xDy3+ (x = 0.03)	Ba	39.6	10.21
	Ca	20.8	18.37
	Si	12.4	15.63
	O	25.0	55.31
	Dy	2.20	0.48
Ba1.3Ca0.7−xSiO4:xDy3+ (x = 0.05)	Ba	39.6	10.51
	Ca	21.6	19.64
	Si	10.3	13.36
	O	24.4	55.57
	Dy	4.1	0.92

3.3. Optical Analysis

The optical properties of materials are determined by the electronic structure of elements as well as the microstructures (or orbital overlapping) of materials including the shape and size of particles. Figure 4(a) shows the diffuse reflectance spectra of the silicate phosphors, Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ (x = 0.00, 0.01, 0.02, 0.03, 0.04, and 0.05), in the range of 250–800 nm. As shown in Figure 4(a), when a sample was doped with Dy³⁺ ions, the reflectance was enhanced. However, depending on the doping concentration, the remission percentage was diversified. For example, when the doping concentration was only 1 mol% of Dy³⁺ ions, the enhanced reflectance was limited in the range of 320–800 nm. On the other hand, when doping was increased more than 2 mol% of Dy³⁺ ions, the reflectance (a back-scattering of light) increased in the whole range of 250–800 nm. Interestingly, when the doping amount was 2 or 5 mol% of Dy³⁺ ions, the reflectance percent was the largest among the samples, indicating that the microstructure including inhomogeneity affect significantly the optical property of materials. Importantly, the prominent absorption band in the range of 300–425 nm should be attributed to the electronic structure (i.e., the transition from ground state of excited one) of Dy³⁺ ions [45, 46].

[figure(s) omitted; refer to PDF]

The Kubelka–Munk (K-M) coefficient was employed to estimate the optical bandgap from the diffuse reflectance spectra [47]:$$\begin{array}{}\text{(2)}& FR=\frac{{1-R}^2}{2R},\end{array}$$where $FR$ is the K-M function and R is the diffused reflectance. Figure 4(b) shows the plot of “${FR\cdot h\nu }^2$ vs. photon energy,$h\nu$” for the direct bandgap semiconductor Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ (x = 0.00, 0.01, 0.02, 0.03, 0.04, and 0.05), in which $h$ is Planck’s constant and $\nu$ is the frequency of a light wave. As shown in Figure 4(b), the bandgap of Ba_1.3Ca_0.7SiO₄ (host) is estimated about 3.85 eV (corresponding to $\lambda$, 33f0 nm), whereas the average bandgap of the doped samples is 3.74 ± 0.09 eV (corresponding to $\lambda$, 340 nm). Hence, the doping of Dy³⁺ ions in the silicate host (Ba_1.3Ca_0.7SiO₄) not only brought forth the enhanced interplanar interactions (i.e., a reduced d-spacing) but also a red-shift in absorption (i.e., a reduced optical bandgap from 3.85 eV to 3.74 eV). Hence, the UV-vis reflectance and X-ray scattering analyses support each other about the effect of Dy³⁺ doping on the enhanced interplanar interactions of silicate phosphors.

3.4. Photoluminescence Analysis

Figure 5(a) shows the PL excitation spectra of Ba_1.3Ca_0.7-xSiO₄:xDy³⁺ as a function of Dy³⁺ concentration, whereas Figure 5(b) is the PL emission spectra showing the most intense peak at 482 nm. Here, in the case of the host material, the observed PL may arise from a radiative recombination between electrons and holes trapped in the defect states in the energy bandgap [36]. As shown in Figure 5(a) (specifically, the black solid line), the two weak broad emission bands were observed at 275 and 375 nm, respectively. As shown in Figures 5(a) and 5(b), the PL intensity increases dramatically with increasing the doping concentration of Dy³⁺ ions because the dopant species (Dy³⁺) is a dominant factor for the excitation band of the silicate phosphor materials. In Figure 5(a), all the bands except at 238 nm are assigned to electronic transitions from the ground state ⁶H_15/2 to the various excited states in the 4f⁹ orbital configuration of Dy³⁺ ions. Here, the weak PL peak at 238 nm should originate from the host absorption band, whereas that at 294 nm from the O²⁻ ⟶ Dy³⁺ transition [24, 48–50]. The other excitation peaks are attributed to the electronic transition ⁶H_15/2 ⟶ ⁴K_15/2 at 324 nm, ⁶H_15/2 ⟶ ⁶P_7/2 at 351 nm, ⁶H_15/2 ⟶ ⁶P_5/2 at 363 nm, and ⁶H_15/2 ⟶ ⁴I_13/2 at 382 nm, respectively [51–58]. Figure 5(c) shows the emission spectra of the host material (Ba_1.3Ca_0.7SiO₄) without any dopant, exhibiting a broad emission band at 482 nm from the self-trapped luminescent recombination.

[figure(s) omitted; refer to PDF]

Under the excitation of λ = 351 nm, the Dy³⁺-doped phosphors (Ba_1.3Ca_0.7−xSiO₄:xDy³⁺) exhibit three emission bands in the range of 450–800 nm with peaks at blue (482 nm), yellow (577 nm), and red (710 nm), which are induced by the ⁴F_9/2 ⟶ ⁶H_15/2, ⁴F_9/2 ⟶ ⁶H_13/2, and ⁴F_9/2 ⟶ ⁶H_11/2 transitions, respectively (Figure 5(b)). Importantly, the blue light emission at 482 nm is ascribed to the magnetic dipole transition, whereas the yellow-light emission at 576 nm is due to electric dipole transition which is known to be hypersensitive and thus strongly influenced by the surrounding environment around the Dy³⁺ ion [52, 53]. Furthermore, according to the Judd-Ofelt theory [54], the electric dipole transition (yellow emission) will be intensified when Dy³⁺ ions are located at low-symmetry sites with no inversion center, whereas the magnetic dipole transition (blue emission) will be strengthened when Dy³⁺ is located at high symmetry with an inversion center. However, the blue emission at 482 nm is usually stronger than the yellow emission, indicating that Dy³⁺ ions occupy a high-symmetry local site with inversion symmetry in this matrix [55]. The intensity of the radiative transition increases with the increase of activator molar concentration from 0 to 3 mol% and then decreases thereafter due to concentration quenching (Figure 5(d)). Notably, the hypersensitive transition affords a tunable white color emission by adjusting the ratio of yellow/blue light intensity. Furthermore, the silicate matrix itself should play an important role in this hypersensitive transition because it provides an environment for the activator ions. Resultantly, Figure 6 shows the partial energy level diagram of Dy³⁺ ions in the silicate host, which was constructed based on the excitation and emission spectra of Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ (x = 0.01, 0.02, 0.03, 0.04, and 0.05), as shown in Figure 5.

[figure(s) omitted; refer to PDF]

Figure 7(a) shows the CIE color coordinate ((x, y) = (0.17, 0.24)) for the host (Ba_1.3Ca_0.7SiO₄), whereas Figure 7(b) shows the corresponding CIE coordinates ((x, y) = (0.27, 0.33), (0.29, 0.34), (0.30, 0.33), (0.29, 0.34), and (0.31, 0.35)) for the Dy³⁺-doped silicate phosphors Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ when x is 0.01, 0.02, 0.03, 0.04, and 0.05, respectively. As shown in Figure 7(a), the color for the host falls in the blue space. In contrast, the color for the doped samples takes the position of the white region (Figure 7(b)). Here, the intensity ratio of yellow to blue (I_Y/I_B) was varied in the range of 0.54–0.68 when the dopant amounts were changed. This hypersensitive phenomenon results in the shift of CIE chromaticity coordinates to the vicinity of the white point (x = 0.33, y = 0.33). Hence, by manipulating the Dy³⁺ doping percentage in the silicate host, the emission color is tunable from blue to white, presenting the importance of doping engineering.

[figure(s) omitted; refer to PDF]

Finally, Figure 8 shows the PL decay curves for the silicate phosphors, Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ (x = 0.00, 0.01, 0.02, 0.03, 0.04, and 0.05). The decay curves were fitted using the second-order exponential equation [59, 60].$$\begin{array}{}\text{(3)}& It=I_0+A_1\exp -\frac{t}{{\tau }_1}+A_2\exp -\frac{t}{{\tau }_2},\end{array}$$where $It$ is the time (t)-dependent intensity, $I_0$, $A_1$, and $A_2$ are the three constants, and ${\tau }_1$ and ${\tau }_2$ are the decay times. The average lifetime $\tau$ is defined as $A_1{\tau }_1^2+A_2{\tau }_2^2/A_1{\tau }_1+A_2{\tau }_2$. Resultantly, the decay parameters are given in Table 3, based on the PL curve fitting. As shown in Figure 8, the 3 mol% Dy³⁺-doped silicate phosphor (Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ (x = 0.03)) showed both longer lifetime and lower decay rate (see the blue solid line). The longest average lifetime $\tau$ is 2.12 ms when Dy³⁺ ions were 3 mol %, confirming again that the “3 mol% of Dy³⁺ ions” should be the optimum condition in line with the PL data.

[figure(s) omitted; refer to PDF]

Table 3

PL decay curve parameters for the silicate phosphors, Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ (x = 0.00, 0.01, 0.02, 0.03, 0.04, and 0.05).

Ba1.3Ca0.7−xSiO4:xDy3+	$A_1$	${\tau }_1$ (ms)	$A_2$	${\tau }_2$ (ms)	$\tau$ (ms)
x = 0.00	22.83	0.68	2.36	2.63	1.24
x = 0.01	2.85	2.40	18.25	0.69	1.29
x = 0.02	15.92	0.65	2.46	2.27	1.22
x = 0.03	0.20	4.71	2.50	0.67	2.12
x = 0.04	4.42	2.42	13.14	0.67	1.63
x = 0.05	0.60	3.38	10.28	0.75	1.29

4. Conclusion

Single-phase silicate phosphors Ba_1.3Ca_0.7−xSiO₄:xDy³⁺ doped with Dy³⁺ ions (x = 0.01, 0.02, 0.03, 0.04, and 0.05) were well prepared using a gel-combustion method. Then, the structural, morphological, and optical properties were characterized in detail. XRD data confirm that the silicate phosphors have a hexagonal unit cell. However, when the host was doped with Dy³⁺ ions, the d-spacing was reduced from 0.291 nm (host) to 0.288 ± 0.001 nm (doped), indicating the enhanced interactions between the (110) crystallographic planes. Furthermore, the optical bandgap was also reduced from 3.85 eV (host) to 3.74 ± 0.09 eV (doped), which is in line with the XRD data. Morphological analysis showed that the doped samples exhibited more aggregated domains with connectivity and micropores depending on composition. Then, the PL spectra were characterized, confirming that the optimized doping condition is 3 mol % Dy³⁺ ions for the silicate (Ba_1.3Ca_0.7SiO₄) phosphor. Finally, the CIE coordinates fall into a white space based on the combined emission of blue, yellow, and red electromagnetic waves. Therefore, the dysprosium-doped silicate phosphors are well applicable to WLEDs for SSL and display devices.

Acknowledgments

This work was supported by both Jimma University in Ethiopia and University of the Free State in South Africa.

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