Synthesis and Luminescence Properties of Eu2+-Doped Sr3MgSi2O8 Blue Light-Emitting Phosphor for Application in Near-Ultraviolet Excitable White Light-Emitting Diodes

Abstract

In this study, [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors were sintered at 1200–1400 °C for 1–5 h by using the solid-state reaction method. The crystallinity and morphology of these phosphors were characterized through X-ray diffraction analysis and field-emission scanning electron microscopy, respectively, to determine their luminescence. The photoluminescence properties, including the excitation and emission properties, of the prepared phosphors were investigated through fluorescence spectrophotometry. The α-Sr₂SiO₄, Sr₂MgSi₂O₇, and Sr₃MgSi₂O₈ phases coexisted in the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors, which were synthesized at low temperatures. The particles of these phosphors had many fine hairs on their surface and resembled Clavularia viridis, which is a coral species. Transmission electron microscopy and energy dispersive X-ray spectroscopy indicated that the fine hairs contained the Sr₂SiO₄ and Sr₂MgSi₂O₇ phases. However, when the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors were sintered at 1400 °C, the Sr₃MgSi₂O₈ phase was observed, and the Eu²⁺-doped Sr₃MgSi₂O₈ phase dominated the only broad emission band, which had a central wavelength of 457 nm (blue light). The emission peaks at this wavelength were attributed to the 4f⁶5d¹–4f⁷ transition at the Sr²⁺(I) site, where Sr²⁺ was substituted by Eu²⁺. The average decay time of the synthesized phosphors was calculated to be 1.197 ms. The aforementioned results indicate that [Sr_0.99Eu_0.01]₃MgSi₂O₈ can be used as a blue-emitting phosphor in ultraviolet-excited white light-emitting diodes.

1. Introduction

White light-emitting diodes (W-LEDs) have replaced conventional incandescent and fluorescent lamps for general illumination. Historically, artificial lighting is energy-intensive, with incandescent lamps exhibiting a luminous efficiency of only 2% and quartz halogen and fluorescent lamps reaching 4% and 15%, respectively, with most of the energy input converted to waste heat. In contrast, solid-state lighting based on W-LEDs currently attains ∼32% luminous efficiency. W-LEDs are a novel high-efficiency lighting system and fourth-generation illumination source with many advantages, including a long lifetime, high rendering index, high luminosity efficiency, low energy consumption, chemical stability, thermal stability, and eco-friendliness [1,2,3]. W-LEDs have superior luminescence characteristics relative to other lighting sources [4]. W-LEDs have many applications in various domains, such as lighting [5], biomedicine [6], communication [7], liquid crystal displays (as backlight sources) [8], and architecture [9]. However, there are several important luminescence parameters that characterize and determine the quality of W-LEDs, including luminous efficacy (LE), color rendering index (CRI), and correlated color temperature (CCT) [10,11].

Two main methods are currently used for producing W-LEDs. The first and most commonly adopted method involves producing W-LEDs by using a blue light-emitting diode chip and yellow light-emitting YAG: Ce³⁺ phosphor; however, the W-LEDs produced using this method have low CRI values (70 to 80) and a CCT value of 7750 K, because the light produced by them does not contain a red component [12,13]. The low CRI value of W-LEDs at a low color temperature limits their possible applications; however, many efforts have been made to overcome this disadvantage. W-LEDs produced using the second method of red (R), blue (B), and green (G) phosphors emit “warm” white light with a high CRI. Phosphor materials that can be effectively excited by ultraviolet or blue light to emit strong R, G, and B light have attracted considerable research attention [14,15,16].

M₃MgSi₂O₈ (M = Ca, Sr, Ba) phosphors were first reported in 1957 [17]. Alkali earth silicates are crucial hosts for rare-earth-doped phosphors because of the inherent advantages of these silicates, such as excellent chemical and thermal stability as well as the low price of high-purity silicate [18]. Klasensetal investigated the photoluminescence (PL) properties of Pb²⁺-, Mn²⁺-, Tl⁺-, and Sb³⁺-activated M₃MgSi₂O₈ (ternary silicates). In addition to the Pb²⁺-activated M₃MgSi₂O₈, none of the other silicates could emit light efficiently. Moreover, Klasensetal found that a substantial amount of Ca²⁺ in Ca₃MgSi₂O₈ can be replaced by Ba²⁺, whereas only a slight amount of Ba²⁺ in Ba₃MgSi₂O₈ can be replaced by Ca²⁺ [17].

Europium oxide (Eu₂O₃) is a highly useful doping material. When added to host materials as an activator, Eu₂O₃ has different ionic states and causes synthesized phosphors to produce different emission colors. Most Eu₂O₃-doped materials synthesized in the atmosphere behave as Eu³⁺-activated phosphors and emit red [19,20,21] or near-infrared [22] radiation. When Eu₂O₃-doped materials are synthesized in a reducing atmosphere, Eu³⁺ ions are reduced to Eu²⁺ ions, which results in the formation of Eu²⁺-activated phosphors that emit blue light [23,24] or green light [25]. Many Eu²⁺-activated materials and relevant synthesis methods have been developed to investigate highly efficient blue or green phosphors. A study that examined mixtures of Eu²⁺-activated Ba₃MgSi₂O₈ and Ca₃MgSi₂O₈ found that Ba₃MgSi₂O₈ has a higher PL emission intensity and shorter peak emission wavelength (437 nm) than does Ca₃MgSi₂O₈ (peak emission wavelength of 475 nm).

In the present study, we synthesized Eu₂O₃-doped Sr₃MgSi₂O₈ phosphors by using the solid-state reaction method at high temperatures, and investigated the crystal structure and PL properties of these phosphors. The effects of the synthesis temperature and time on Eu₂O₃-doped Sr₃MgSi₂O₈ phosphors were investigated. When Eu₂O₃-doped Sr₃MgSi₂O₈ was synthesized in a reducing atmosphere, Eu³⁺ ions were reduced to Eu²⁺ ions, and the synthesized phosphors emitted strong blue light. [Sr_1−xEu_x]₃MgSi₂O₈ might be a promising blue phosphor for RGB-W-LEDs.

2. Experimental

2.1. Preparation of the [Sr_1−xEu_x]₃MgSi₂O₈ Phosphors

In this study, [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors were synthesized using the solid-state reaction method. The raw materials used in this synthesis were SrCO₃ (Sigma-Aldrich, St. Louis, MO, USA, 99.99%), MgO (Sigma-Aldrich, USA, 99.99%), SiO₂ (Sigma-Aldrich, USA, 99.99%), and Eu₂O₃ (Sigma-Aldrich, USA, 99.99%) powders. These powders were mixed and ground in deionized water for 1 h by using the ball-milling method. ZrO₂ balls with a diameter of 5–8 mm were used to grind the powders. The powder mixture was then dried at 120 °C for 24 h in an oven. After drying, the mixture was ground in an agate mortar for 1 h and then calcined at 850 °C for 2 h. The mixture was placed in alumina crucibles and put in the tubular furnaces. Then, a vacuum was created in the tubular furnaces by using the mechanical pump. Finally, the reducing gas (4 vol% H₂/96 vol% N₂) was led into the tubular furnaces, and the mixture was sintered at 1200–1400 °C for 1–10 h in a reducing atmosphere.

2.2. Measurements

The crystalline structures of the prepared [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors were investigated using a ceramic X-ray diffraction (XRD) source that emitted CuKα radiation (λ = 1.5406 Å). The microstructures of the phosphors were analyzed through field-emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM). The PL spectra and PL excitation (PLE) spectra were obtained using a Hitachi F-7000 spectrofluorometer with a 150-W xenon lamp as the light source. The luminance and International Commission on Illumination [Commission Internationale de l’Eclairage (CIE)] coordinates were measured using the CS-100A Konica Minolta chroma meter. All the measurements were performed at room temperature.

3. Results and Discussion

The XRD patterns of the prepared [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors were obtained to verify their crystal structures. Figure 1 shows the diffraction peaks of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered at 1300 °C for different durations. These phosphors exhibited diffraction peaks at 2θ values of 22.7°, 28.1°, 30.4°, 31.9°, 32.8°, 38.9°, 40.4°, 46.5°, 48.2°, 50.1°, 51.8°, 58.1°, 59.5°, and 60.8°. These characteristic peaks suggest that the aforementioned phosphors had a monoclinic structure (a ≠ b ≠ c, α = β = γ = 90°, P2₁/a space group). In addition, the 2θ values of 24.9°, 31.1°, 35.4°, 43.9°, 45.1°, and 60.7° indicated the presence of the Sr₂MgSi₂O₇ phase (JCPDS No. 75-1736) and α-Sr₂SiO₄ phase (JCPDS No. 39-1256). No Eu₂O₃ compound was found in the phosphors. As displayed in Figure 2, in the phosphors, each Si atom was surrounded by four oxygen atoms, which resulted in the formation of a four-coordination [SiO₄] tetrahedral structure. Moreover, each Mg atom was surrounded by six oxygen atoms, which resulted in the formation of a [MgO₆] octahedron. A Sr atom could occupy three available sites, which were located in different crystallographic environments.

The Sr(I), Sr(II), and Sr(III) sites exhibited ten-coordination, eight-coordination, and nine-coordination, respectively. Eu²⁺-doped [Sr_1−xEu_x]₃MgSi₂O₈ phosphors were obtained by reducing Eu³⁺ ions to Eu²⁺ ions in a reducing atmosphere during the sintering process. The ionic radius of Sr²⁺ is 1.01 Å, which is close to that of Eu²⁺ (1.12 Å). Mg²⁺ and Si⁴⁺ have smaller ionic radii (0.72 and 0.40 Å, respectively) than does Sr²⁺. Therefore, the diffraction peaks of Eu₂O₃ were not observed, which demonstrated that Eu²⁺ ions could be doped into the [Sr_0.99Eu_0.01]₃MgSi₂O₈ lattice because of the similar ionic radii and valence of Sr²⁺ and Eu²⁺. As displayed in Figure 1, the intensity of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ signal increased as the sintering time increased from 1 to 10 h. Moreover, the intensities of the α-Sr₂SiO₄ and Sr₂MgSi₂O₇ signals decreased with sintering time. Because the α-Sr₂SiO₄ and Sr₂MgSi₂O₇ phases were formed within short sintering times or relatively low sintering temperatures, the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors exhibited better crystalline structures at longer sintering times.

Figure 3 displays the FE-SEM images of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered at 1300 °C for different durations. When the sintering time was 1 h, the synthesized [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors exhibited a special surface morphology. The particles of these phosphors appeared similar to Clavularia viridis, which is a coral species, and exhibited many fine hairs on their surface. The number of fine hairs on the particle surface decreased as the sintering time increased from 1 to 6 h. In addition, to understand the microstructure of the fine hair, the prepared [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors were subjected to HR-TEM and energy dispersive X-ray spectroscopy (EDS) analyses (Figure 4). At a sintering time of 1 h, the atomic percentages of Sr, Mg, Si, and O in the fine hairs were 25.1%, 27.9%, 1.8%, and 45.2%, respectively. On the basis of this information and the XRD results (Figure 1), we infer that the Sr₂SiO₄ and Sr₂MgSi₂O₇ phases were present in the fine hairs at a sintering time of 1 h. The element distribution images of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors are shown in Figure S1. The resulting presence of Sr, Si, and Mg can be found, and the element content was similar to the HR-TEM/EDS result (Figure 4). At a sintering time of 5 h, the fine hairs contained Sr, Mg, Si, and O, which indicates that the Sr₂MgSi₂O₇ phase was present in the fine hairs at a sintering time of 5 h, almost the same as the detected atomic percentage and nominal compositions in quantity. The SEM images of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered for different durations, whose BET specific surface area were 18.4 m²/g, 13.5 m²/g, 9.4 m²/g, 7.2 m²/g, 5.8 m²/g, and 2.5 m²/g, respectively, as shown in Figure 3a–f.

Figure 5 displays the PLE and PL spectra of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered at 1300 °C for 5 h. The Eu²⁺ excitation band of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors can be fitted into two Gaussian components with peaks at 280 and 350 nm, which correspond to the 4f⁷(⁸S_7/2)→4f⁶5d¹(t_2g) electron transition of Eu²⁺ [26]. Figure S2 shows the PLE spectra of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered for different durations. These spectra exhibit two broad bands ranging from 240 to 320 nm and from 330 to 410 nm, with peaks at 280 and 350 nm, which are assigned to the transitions between the ground state 4f⁷ and the crystal-field split state 4f⁶5d¹. As the sintering time increased, the excitation intensity increased and reached a maximum value at a sintering time of 5 h. The aforementioned results demonstrate that as the sintering duration increased from 1 to 5 h, the crystallinity (Figure 1), particle morphologies and sizes (Figure 3), and PLE intensities of the phosphors increased.

Figure S3 shows the PL spectra of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered at 1300 °C for different durations. The emission spectra corresponding to 280 nm excitation contain a single band at around 457 nm. As displayed in Figure S2, the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors exhibited the highest emission peak intensities when the sintering duration was 5 h, the Sr₃MgSi₂O₈ has a space group of P21/a, and the unit cell contains three Sr sites: one 12-coordinated Sr(I) site and two 10-coordinated Sr(II, III) sites [27]. The broad band at around 457 nm is attributed to the 4f⁶5d–4f⁷ transition at the Sr²⁺(I) site, where Sr²⁺ is substituted by Eu²⁺ [28,29]. The electronic mechanism of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors is shown in Figure 6. The 4f⁶5d–4f⁷ transition belongs to the electronic dipole-allowed transition, based on the Laporte selection rule. Kim et al. indicated that the 570 nm band to Eu²⁺ ions at the Sr²⁺ (II, III) sites occurs at high Eu²⁺ doping concentrations in Sr₃MgSi₂O₈ [28]. Figure S2 does not indicate an emission peak at 570 nm; thus, only Eu²⁺ ions substituted Sr²⁺ at the Sr²⁺(I) site. The full width at half maximum (FWHM) of the broad band of emission peaks were approximately 50, 46, 43, 41, and 40 nm as the sintered for 1 to 5 h. This result was caused by the electron on the outer 5d-orbital of the atom, while the emission peak of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors was easily influenced by the external environment.

Figure 7 displays the fluorescent decay curves of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors excited at 280 nm and monitored at 457 nm. These data fit well with a double-exponential curve. The aforementioned curves indicate the possible interactions between Eu²⁺ ions and suggest that these ions occupied the cationic sites (Sr²⁺). To calculate the luminescence lifetimes, all the fluorescent decay curves were fitted using the double-exponential equation of Sahu et al. [30], which is expressed as follows:

I = A₁exp(−t/τ₁) + A₂exp(−t/τ₂)(1)

where I is the PL intensity, A₁ and A₂ are the fitting parameters, and τ₁ and τ₂ are the decay constants of the exponential components.

On the basis of the aforementioned equation, the average luminescence lifetimes (τ*) of a rare-earth ion can be calculated using the following equation [31]:

τ* = (A₁τ₁² ₊ A₂τ₂²)/(A₁τ_{1 +} A₂τ₂)(2)

The average luminescence lifetimes of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors were calculated to be 3.406, 3.191, and 1.143 ms for the sintering durations of 1, 2, and 5 h, respectively. The parameter τ* decreased with sintering time. This phenomenon might be attributed to the energy transfer between the Eu²⁺ ions located at the Sr²⁺ sites [32].

Figure 8 shows the CIE chromaticity results of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors as a function of the sintering duration. The CIE (1931 chromaticity) diagram can be used to describe the color purity of the luminescent emissions of phosphors. In this study, a CIE chromaticity diagram was obtained for an excitation wavelength of 280 nm. The color coordinates of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered for 1, 2, 3, 4, and 5 h were (0.1659, 0.1382), (0.1612, 0.1256), (0.1593, 0.1211), (0.1549, 0.1111), and (0.1527, 0.1006), as displayed in Figure 8. The CIE chromaticity diagram indicates that as the sintering duration increased from 1 to 5 h, the emissions of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors changed from being light blue to navy blue. Thus, a sintering temperature of 1300 °C and a sintering duration of 5 h are optimal settings for the synthesis of a blue phosphor. The aforementioned results indicate that sintering duration is the main factor affecting the crystalline structure and PL properties of [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors.

Images of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered for different durations under ultraviolet (UV) light irradiation are shown in the inset of Figure 8 and in Figure S4. The brightness of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors increased with sintering duration. The phosphors sintered at 1300 °C for 5 h were very bright.

Sintering temperature affects the PL properties and structure of phosphors. Therefore, we attempted to determine the optimal sintering temperature for preparing [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors. XRD patterns of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered using the solid-state method at temperatures of 1200, 1250, 1300, 1350 and 1400 °C for 5 h are depicted in Figure 9. Figure 9a shows the diffraction peaks of the phosphor sintered at 1200 °C. This phosphor exhibited main diffraction peaks at 2θ values of 22.7°, 28.1°, 30.4°, 31.9°, 32.8°, 38.9°, 40.4°, 46.5°, 48.2°, 50.1°, 51.8°, 58.1°, 59.5°, and 60.8°. This set of XRD peaks is similar to that observed for Sr₃MgSi₂O₈ (JCPDS No. 10-0075). In addition, the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphor contained the Sr₂MgSi₂O₇ (JCPDS No. 75-1736) and α-Sr₂SiO₄ (JCPDS No. 39-1256). The intensity of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ signal increased with sintering temperature from 1200 to 1400 °C. Moreover, the intensities of the Sr₂MgSi₂O₇ and α-Sr₂SiO₄ signals decreased with sintering temperature.

The aforementioned results indicate that the row material of SrCO₃ decomposed into SrO and CO₂, then SrO reacted with SiO₂ to form Sr₂SiO₄, and finally SrO and MgO reacted with SiO₂ to form the Sr₂MgSi₂O₇ and Sr₃MgSi₂O₈ phases. When the sintering temperature was lower than 1000 °C, the following reaction occurred:

SrCO₃ → SrO + CO₂(3)

When the sintering temperature was between 1000 and 1200 °C, the following reaction occurred [33,34]:

2SrO+SiO₂ → Sr₂SiO₄(4)

When the sintering temperature was between 1200 and 1300 °C, the following reaction occurred [35]:

2SrO + MgO + 2SiO₂ → Sr₂MgSi₂O₇(5)

At 1450 °C, the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphor melted. Consequently, the crystalline structures and PL properties of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors were not examined at sintering temperatures higher than 1450 °C.

The findings for the crystal structure of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphor sintered at 1400 °C was fitted using the following parameters: a = 5.341 Å, b = 9.700 Å, and c = 7.184 Å (Sr₃MgSi₂O₈ phosphors). Subsequently, Rietveld refinement was conducted on the XRD data of this phosphor (Figure 10). The final refinement convergence was achieved when χ² = 5.42, which is marginally higher than the optimal value χ² value of <2. This result was due to the coexistence of the Sr₂MgSi₂O₇ (2θ = 29.7° and 30.2°) and α-Sr₂SiO₄ (2θ = 35.4°, 43.9°, 45.1°, and 60.7°) phases in the aforementioned phosphor. The remaining diffraction peak of 2θ values, in addition to those mentioned above, were assigned to the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phase. It demonstrated that the Sr²⁺ ions were substituted by Eu²⁺ ions in the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors.

Figure 11 and Figure 12 depict the PLE and PL spectra of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered at different temperatures. As the sintering temperature increased, the PLE intensity also increased, and the maximum PLE intensity was achieved when the sintering temperature was 1400 °C (Figure 11). As depicted in Figure 12, the PL intensity of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors increased with sintering temperature. The [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphor sintered at 1400 °C exhibited the highest PL intensity, and the broad and asymmetric band with an FWHM value of 38 nm was observed at around 457 nm. The FWHM of the broad band of emission peaks were approximately 38, 40, 43 and 45 nm as the sintered temperature decreased from 1400 °C to 1200 °C. The blue emission band of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors at 457 nm was attributed to the 5d–4f electron transition of Eu²⁺.

Figure 13 shows the Eu 3d XPS spectra of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered at different temperatures. The results shows that there is no Eu²⁺-related peaks at the sintered temperature of 900 °C (Figure 13a), and the Eu²⁺ peak appeared at the sintering temperature of 1400 °C (Figure 13b). The Eu 3d XPS spectra of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered at 1400 °C is shown in Figure 14, revealing the Eu 3d peak deconvolution of the electron binding energies of Eu³⁺ 3d_3/2 (1164 eV), Eu²⁺ 3d_3/2 (1155 eV), Eu³⁺ 3d_5/2 (1134 eV), and Eu²⁺ 3d_5/2 (1125 eV). This result demonstrated that the Eu³⁺ ions are successfully reduced to Eu²⁺ ions at a 1400 °C sintering temperature. In general, Eu³⁺→Eu²⁺ reduction requires a higher temperature in the reducing atmosphere.

Figure S5 displays the fluorescent decay curves of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors excited at 280 nm and monitored at 457 nm. The data fit well with a double-exponential curve. The average luminescence lifetimes of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered at 1200, 1300, and 1400 °C were calculated from Equation (2) to be 1.074, 1.144, and 1.197 ms, respectively. The parameter τ* decreased with sintering temperature. This result demonstrates that energy transfer occurred between the Eu²⁺ ions located at the Sr²⁺ sites [32].

Figure 15 shows the CIE chromaticity coordinates and photographs of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered at different temperatures. The CIE chromaticity diagram was obtained for an excitation wavelength of 280 nm. When the sintering temperature was increased from 1200 to 1400 °C, the CIE chromaticity coordinates shifted from a light blue region (x = 0.1659, y = 0.1382) to an ultramarine blue region (x = 0.1494, y = 0.0942). Therefore, the optimal sintering temperature in the production of blue phosphors is 1400 °C. Images of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered at different temperatures under UV light irradiation are displayed in the inset of Figure 15 and in Figure S6. The brightness of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors increased with sintering temperature. The highest brightness occurred at a sintering temperature of 1400 °C.

4. Conclusions

In this study, Eu²⁺-doped [Sr_1−xEu_x]₃MgSi₂O₈ phosphors were prepared in a reducing atmosphere by using a solid-state reaction method, and the photoluminescence properties of these phosphors were investigated. The optimal sintering temperature and duration for the preparation of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors was found to be 1400 °C and 5 h, respectively. The blue emission of these phosphors at 457 nm is attributed to the 5d–4f electron transition of Eu²⁺. In addition, the average decay time of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered at 1400 °C for 5 h was calculated to be 1.197 ms. The CIE chromaticity coordinates of the phosphors sintered at 1400 °C were (x = 0.1494, y = 0.0942), and this point lies in an ultramarine blue region in the CIE chromaticity diagram. [Sr_0.99Eu_0.01]₃MgSi₂O₈ is promising as a blue phosphor in RGB-W-LEDs.

Author Contributions

Experiment, C.-Y.L., C.-C.W. and H.-H.L.; design, C.-F.Y.; data curation, C.-Y.L.; analysis, C.-Y.L. and H.-H.L.; writing—review and editing, C.-C.W. and C.-F.Y. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors acknowledge the financial support of the Ministry of Science and Technology (MOST 110-2628-E-143-001, MOST 109-2221-E-390-023, MOST 110-2622-E-390-002, and MOST 110-2221-E-390-020). This research is also supported in part by Fuzhou University of International Studies and Trade under Grant Nos. FWB19004, 2019-SG-2, 2019020079010, 2018KYTD-05, FWXXS19093, FJTPY-2020009, and BRJF-01.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures

Figure 1. XRD patterns of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered for different durations: (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, (f) 6, and (g) 10 h.

Figure 2. Crystal structure of the [Sr0.99Eu0.01]3MgSi2O8 phosphors.

Figure 3. FE-SEM images of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered for different durations: (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) 6 h.

Figure 4. HR-TEM images of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered for different durations: (a) 1 and (b) 5 h. The EDS results are shown in the inset.

Figure 5. PLE and PL spectra of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered for 5 h.

Figure 6. The electronic mechanism of the [Sr0.99Eu0.01]3MgSi2O8 phosphors.

Figure 7. Decay times of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered for different durations.

Figure 8. CIE chromaticity diagram of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered for different durations. Images of the phosphors are shown in the inset.

Figure 9. XRD patterns of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered at different temperatures: (a) 1200, (b) 1250, (c) 1300, (d) 1350, and (e) 1400 °C.

Figure 10. Results of the Rietveld refinement performed for the [Sr0.99Eu0.01]3MgSi2O8 phosphor sintered at 1400 °C.

Figure 11. PLE spectra of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered at different temperatures.

Figure 12. PL spectra of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered at different temperatures.

Figure 13. XPS spectra of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered at different temperatures. (a) 900 °C and (b) 1400 °C.

Figure 14. XPS spectra of the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered at 1400 °C.

Figure 15. CIE chromaticity diagram for the [Sr0.99Eu0.01]3MgSi2O8 phosphors sintered at different temperatures.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/nano12152706/s1, Figure S1: SEM/EDS mapping images of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphor sintered at 1300 °C for 1 h. (a) SEM images, (b) Sr element, (c) Si element and (d) Mg element. Figure S2: PLE patterns of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered at different times; Figure S3: PL patterns of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered at different times; Figure S4: The photo-images of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered at different times under UV light irradiation. (a) 1 h, (b) 2 h, (c) 3 h, (d) 4 h and (e) 5 h, respectively; Figure S5: Decay times of [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered at different temperatures; Figure S6: The photo-images of the [Sr_0.99Eu_0.01]₃MgSi₂O₈ phosphors sintered at different temperatures under UV light irradiation. (a) 1200 °C, (b) 1250 °C, (c) 1300 °C, (d) 1350 °C and (e) 1400 °C, respectively.

References

1. Ki, H.K.; Won, B.I.; Ho, S.J.; Hyoung, S.Y.; Duk, Y.J. Luminescence properties and energy transfer of site-sensitive Ca_6-x-yMg_x-z(PO₄)₄:Eu_y²⁺, Mn_z²⁺ phosphors and their application to near-UVLED-based white LEDs. Inorg. Chem.; 2009; 48, pp. 11525-11532.

2. Yu, J.; Guo, C.F.; Ren, Z.Y. Photoluminescence of double-color-emitting phosphor Ca₅(PO₄)₃Cl:Eu²⁺, Mn²⁺ for near-UVLED. Opt. Laser Technol.; 2011; 43, pp. 762-766. [DOI: https://dx.doi.org/10.1016/j.optlastec.2010.12.008]

3. Lin, C.C.; Liu, R.S. Advances in phosphors for light-emitting diodes. J. Phys. Chem. Lett.; 2011; 2, pp. 1268-1277. [DOI: https://dx.doi.org/10.1021/jz2002452] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/26295420]

4. Nair, G.B.; Dhoble, S.J. A perspective perception on the applications of light-emitting diodes. Luminescence; 2015; 30, pp. 1167-1175. [DOI: https://dx.doi.org/10.1002/bio.2919] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/26014269]

5. Park, J.Y.; Jung, H.C.; Raju, G.S.R.; Jeong, H.J.; Moon, K.B.; Kim, H.J.; Lee, K.Y. Solvothermal synthesis and luminescence properties of the novel aluminum garnet phosphors for WLED applications. Curr. Appl. Phys.; 2013; 13, pp. 441-447. [DOI: https://dx.doi.org/10.1016/j.cap.2012.09.001]

6. Huang, S.; Chen, Y.; Wei, X.; Yin, M. Synthesis and luminescence properties of NaSrPO₄:Eu²⁺, Tb³⁺, Mn²⁺ for WLED. J. Nanosci. Nanotechnol.; 2014; 14, pp. 4574-4578. [DOI: https://dx.doi.org/10.1166/jnn.2014.8052]

7. Arunkumar, S.; Venkataiah, G.; Marimuthu, K. Spectroscopic and energy transfer behavior of Dy³⁺ ions in B₂O₃-TeO₂-PbO-PbF₂-Bi₂O₃-CdO glasses for laser and WLED applications. Spectrochim. Acta A Mol. Biomol. Spectrosc.; 2015; 136, pp. 1684-1697. [DOI: https://dx.doi.org/10.1016/j.saa.2014.10.067]

8. Kakinuma, K. Technology of Wide Color Gamut Backlight with Light-Emitting Diode for Liquid Crystal Display Television. Jpn. J. Appl. Phys.; 2006; 45, 4330. [DOI: https://dx.doi.org/10.1143/JJAP.45.4330]

9. Bandi, V.R.; Grandhe, B.K.; Jang, K. Luminescent properties of a new green emitting Eu²⁺ doped CaZrSi₂O₇ phosphor for WLED applications. J. Lumin.; 2011; 131, pp. 2414-2418. [DOI: https://dx.doi.org/10.1016/j.jlumin.2011.06.004]

10. Wang, B.; Liu, Y.-G.; Huang, Z.; Fang, M. Energy transfer and thermal stability of Ce³⁺, Tb³⁺ co-doped Ca₃Si₂O₄N₂ phosphors for white light-emitting diodes. Chem. Phys. Lett.; 2017; 690, pp. 31-37. [DOI: https://dx.doi.org/10.1016/j.cplett.2017.10.038]

11. Wang, B.; Liu, Y.; Chen, J.; Mi, R.; Xia, Y.; Huang, Z.; Fang, M.; Mei, L. Photoluminescence properties and application of yellow Ca_0.65Si₁₀A_l2O_0.7N_15.3:xEu²⁺ phosphors for white LEDs. Solid State Sci.; 2017; 64, pp. 84-90. [DOI: https://dx.doi.org/10.1016/j.solidstatesciences.2016.12.018]

12. Xia, Z.; Xu, Z.; Chen, M.; Liu, Q. Recent developments in the new inorganic solid-state LED phosphors. Dalton Trans.; 2016; 45, pp. 11214-11232. [DOI: https://dx.doi.org/10.1039/C6DT01230B] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/27302569]

13. Li, G.; Lin, C.C.; Chen, W.T.; Molokeev, M.S.; Atuchin, V.V.; Chiang, C.-Y. Photoluminescence Tuning via Cation Substitution in Oxonitrido silicate Phosphors: DFT Calculations, Different Site Occupations, and Luminescence Mechanisms. Chem. Mater.; 2014; 26, pp. 2991-3001. [DOI: https://dx.doi.org/10.1021/cm500844v]

14. Tang, W.J.; Chen, D.H.; Wu, M. Luminescence studies on SrMgAl₁₀O₁₇:Eu, Dy phosphor crystals. Opt. Laser Technol.; 2009; 41, pp. 81-84.

15. Yu, H.; Lai, Y.W.; Gao, G.M.; Kong, L. Photoluminescence and energy transfer studies on Eu²⁺ and Ce³⁺ co-doped SrCaSiO₄ for white light-emitting-diodes. J. Alloy. Compd.; 2011; 509, pp. 6635-6639. [DOI: https://dx.doi.org/10.1016/j.jallcom.2011.03.116]

16. Wu, H.Y.; Hun, Y.H.; Wang, Y.H.; Kang, F.W.; Mou, Z.F. Investigation on Eu³⁺ doped Sr₂MgSi₂O₇ red-emitting phosphors for white-light-emitting diodes. Opt. Laser Technol.; 2011; 43, pp. 1104-1110. [DOI: https://dx.doi.org/10.1016/j.optlastec.2011.02.006]

17. Klasens, H.A.; Hoekstra, A.H.; Cox, A.P.M. Ultraviolet fluorescence of some ternary silicates activated with lead. J. Electrochem. Soc.; 1957; 104, 93. [DOI: https://dx.doi.org/10.1149/1.2428519]

18. Luo, X.; Cao, W.; Sun, F. The development of silicate matrix phosphors with broad excitation band for phosphor-converted white LED. Sci. Bull.; 2008; 53, pp. 2923-2930. [DOI: https://dx.doi.org/10.1007/s11434-008-0392-4]

19. Liu, J.; Chen, K.N.; Tzou, W.C.; Houng, M.P.; Ho, Y.I.; Yang, C.F. Developments of crystal structures and photoluminescence properties of Sr_0.85Eu_0.15Al₁₂O₁₉ green phosphors using different synthesis parameters. Sens. Mater.; 2017; 29, 473. [DOI: https://dx.doi.org/10.1142/S0217984921410190]

20. Lin, C.Y.; Yang, S.H.; Lin, J.L.; Yang, C.F. Effects of the Concentration of Eu³⁺ Ions and Synthesizing Temperature on the Luminescence Properties of Sr_2−xEu_xZnMoO₆ Phosphors. Appl. Sci.; 2017; 7, 30. [DOI: https://dx.doi.org/10.3390/app7010030]

21. Liu, J.; Wu, C.C.; Yang, C.F.; Liou, L.S. Effect of Eu₂O₃ Concentration on the Properties of Red-Light-Emitting Sr_1.5Ca_0.5SiO₄ Fluorescent Materials. Nano; 2019; 14, 1950110. [DOI: https://dx.doi.org/10.1142/S1793292019501108]

22. Sun, J.Y.; Sun, Y.N.; Zhu, J.C.; Zeng, J.H.; Du, H.Y. Near-infrared downconversion in Eu²⁺ and Pr³⁺ co-doped KSrPO₄ phosphor. Chin. Phys. B; 2013; 22, 057803. [DOI: https://dx.doi.org/10.1088/1674-1056/22/5/057803]

23. Chandrakar, P.; Baghel, R.N.; Bisen, D.P.; Chandra, B.P. Characterization and luminescence properties of CaMgSi₂O₆:Eu²⁺ blue phosphor. Luminescence; 2015; 30, 1034. [DOI: https://dx.doi.org/10.1002/bio.2855] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/25665136]

24. Suda, Y.; Kamigaki, Y.; Miyagawa, H.; Takeda, T.; Takahashi, K.; Hirosaki, N. Effects of Eu²⁺ on the luminescence and afterglow that arise from defects in β-SiAlON: Eu²⁺. J. Phys. D Appl. Phys.; 2021; 54, 065102. [DOI: https://dx.doi.org/10.1088/1361-6463/abc199]

25. Liu, J.; Wang, Y.T.; Lin, C.Y.; Yang, C.F. Effect of Synthesis Temperature on the Crystalline Structures and Photoluminescence Properties of the Green-light Ca_1.975Eu_0.025MgSi₂O₇ Phosphors. Cryst. Growth Des.; 2020; 18, 3154. [DOI: https://dx.doi.org/10.1021/acs.cgd.0c00027]

26. Liu, L.Y.; Wang, D.J.; Mao, Z.Y.; Liu, Y.H.; Li, X.Z.; Lu, Q.F. Fluorescence enhancement of single-phase red-blue emitting Ba₃MgSi₂O₈:Eu²⁺,Mn²⁺ phosphors via Dy³⁺ addition for plant cultivation. Optoelectron. Lett.; 2009; 5, 26. [DOI: https://dx.doi.org/10.1007/s11801-009-8133-8]

27. Moore, P.B.; Araki, T.; Miner, A. Atomic arrangement of merwinite, Ca₃Mg[SiO₄]₂, an unusual dense-packed structure of geophysical interest. Am. Mineral. J. Earth Planet. Mater.; 1972; 57, 1355.

28. Kim, J.S.; Jeon, P.E.; Choi, J.C.; Park, H.L.; Mho, S.I.; Kim, G.C. Warm-white-light emitting diode utilizing a single-phase full-color Ba₃MgSi₂O₈: Eu²⁺, Mn²⁺ phosphor. Appl. Phys. Lett.; 2004; 82, 2931. [DOI: https://dx.doi.org/10.1063/1.1695441]

29. Butler, K.H. Fluorescent Lamp Phosphors: Technology and Theory; Pennsylvania State University Press: University Park, PA, USA, 1980.

30. Sahu, I.P.; Bisen, D.P.; Brahme, N.; Sharma, R. Luminescence properties of Eu²⁺, Dy³⁺-doped Sr₂MgSi₂O₇, and Ca₂MgSi₂O₇ phosphors by solid-state reaction method. Res. Chem. Intermed.; 2015; 41, pp. 6649-6664. [DOI: https://dx.doi.org/10.1007/s11164-014-1767-6]

31. Annadurai, G.; Kennedy, S.M.M. Synthesis and photoluminescence properties of Ba₂CaZn₂Si₆O₁₇: Eu³⁺ red phosphors for white LED applications. J. Lumin.; 2016; 169, pp. 690-694. [DOI: https://dx.doi.org/10.1016/j.jlumin.2015.03.007]

32. Rudnicka, D.; Deren, P.J. Preliminary spectroscopic properties of K₄SrSi₃O₉ doped with Eu³⁺. Opt. Mater.; 2013; 35, 2534. [DOI: https://dx.doi.org/10.1016/j.optmat.2013.07.018]

33. Qiao, Y.; Zhang, X.; Ye, X.; Chen, Y.; Guo, H. Photoluminescent properties of Sr₂SiO₄: Eu³⁺ and Sr₂SiO₄: Eu²⁺ phosphors prepared by solid-state reaction method. J. Rare Earths; 2009; 27, 323. [DOI: https://dx.doi.org/10.1016/S1002-0721(08)60243-4]

34. Tshabalala, M.A.; Dejene, F.B.; Shreyas,; Pitale, S.; Swart, H.C.; Ntwaeaborwa, O.M. Generation of white-light from Dy³⁺ doped Sr₂SiO₄ phosphor. Phys. B; 2014; 439, pp. 126-129. [DOI: https://dx.doi.org/10.1016/j.physb.2013.11.022]

35. Sahu, I.P. Effect of charge compensator ions (R⁺ = Li⁺, Na⁺ and K⁺) on Sr₂MgSi₂O₇: Dy³⁺ phosphors by solid-state reaction method. Appl. Phys. A; 2016; 122, 855. [DOI: https://dx.doi.org/10.1007/s00339-016-0379-y]