Synthesis and Luminescent Properties of Eu³⁺ Doped Complex Borosilicate Glasses

1. Introduction

Over recent decades, the development and analysis of fluorescent materials doped with rare earth ions have garnered significant interest within the field of optoelectronics research. This growing attention stems from their wide array of applications in the creation and advancement of novel optical materials. Glasses, in particular, stand out as some of the most thoroughly studied engineering materials due to their versatility and adaptability achieved through composition modification. Additionally, they hold immense potential in innovations for optical communication and solid-state laser technologies [1]. Among the various types of glasses, borosilicate glasses have captured researchers’ attention because of their remarkable properties. These include high chemical resistance, a high crystallization ability, lower thermal expansion, elevated softening temperature, and excellent mechanical strength. They are cost-effective and readily accessible. Such properties pave the way for extensive industrial applications in areas such as display technologies, solar energy systems, and MEMs technology.

In advanced technologies, trivalent rare earth ions play a pivotal role as active constituents in numerous optical materials. Their importance is owed to the presence of multiple absorption and emission bands that result from transitions between distinct energy levels. Notably, trivalent europium (Eu³⁺) ions stand out as effective spectroscopic probes due to their simple energy-level structure, which features a non-degenerate ⁷F₀ ground state and ⁵D₀ excited state. This makes Eu³⁺ ions instrumental in studying the structure and chemical bonding nature within host matrices. Moreover, the strong ⁵D₀–⁷F₂ electronic transition exhibited by these ions establishes them as efficient activators for generating intense red emission, particularly suited for display devices [1].

The study of borosilicate glasses doped with Eu₂O₃ oxide has been limited. Bi-containing borosilicate glasses doped with different amounts of Eu₃O₂ (1–5 mol%) were synthesized. The influence of the concentration of Eu₂O₃ on the physical, optical and luminescent properties of the glasses was studied. It was found that the glasses show the strongest emission at a wavelength of 613 nm and when excited by 465 nm. The color of the emission is reddish-orange. The optimal concentration of Eu₂O₃ in these glasses, at which the highest emission intensity is achieved, is 4.0 mol% [2]. Eu³⁺-doped glasses with the composition 74.5B₂O₃+10SiO₂+5MgO+R+0.5Eu₂O₃ [R=10 (Li₂O/Na₂O/K₂O)] are considered as potential candidates for red lasers, as well as for red color centers in displays [3]. Zinc-borosilicate glasses (with high ZnO content) doped with different amounts of Eu₂O₃ (0.2, 0.5, 1, 1.5, 2 mol %) were obtained. The glasses are characterized by intense red emission upon excitation with a wavelength of 395 nm. “Quenching” of the luminescence is observed at a concentration of Eu₂O₃ above 1 mol %. [4,5]. Thermally stable borosilicate glasses doped with Eu₂O₃ with the composition 35B₂O₃.20SiO₂.(15-x)Al₂O₃.15ZnO.15Na₂CO₃.xEu₂O₃ (x = 0.5 ÷ 2.5 mol%) were synthesized.These glasses showed red emission, the intensity of which increased with increasing concentration of Eu³⁺ ions up to 2.5 mol% [6]. The effect of changing the concentration of B₂O₃ and Al₂O₃ in the composition of the glass Na₂O-Gd₂O₃-B₂O₃-SiO₂-Al₂O₃-Eu₂O₃ on the luminescent properties of the incorporated Eu³⁺ ions was studied. It was shown that with the addition of Al₂O₃, a [BO₄] → [BO₃] transformation occurs. The amorphous network becomes ‘‘loose’’, resulting in an increase in the space around the rare earth ions embedded in the glass matrix, an increase in their quenching concentration, and an improvement in the emission intensity of Eu³⁺ [7]. Physical, optical, and luminescent properties of Eu³⁺-doped potassium borosilicate glasses (KBSi:Eu³⁺) have been studied. The density and molar volume of the glasses increase with increasing Eu₂O₃ oxide content. After excitation with 394 nm, KBSi:Eu³⁺ glasses emit highly intense reddish-orange light and could find application in various photonic devices such as solid-state lasers and light-emitting diodes [8]. The optical and luminescent properties of Eu³⁺ doped (55-x)B₂O₃:10SiO₂:25Y₂O₃:10CaO:xEu₂O₃, x = 0 ÷ 2.5 mol% were studied [9]. The color coordinates of the glasses were found to be in the red region. The resulting glass is a potential candidate for red laser emission at 613 nm. Sm³⁺/Eu³⁺ co-doped thermally stable zinc-alumino-borosilicate glasses and alkaline-earth-alumino-borosilicate glasses were investigated. They were shown to be suitable candidates for application as red components of white light emitting diodes [10,11]. Dy³⁺/Eu³⁺ co-doped luminescent glasses SiO₂-B₂O₃-ZnO-La₂O₃-BaO (SBZLBA) were synthesized. When excited at 386 nm, the glasses exhibit three distinct emission regions: blue, yellow, and red, allowing for color-tunable luminescence ranging from cool white to neutral white and ultimately warm white by varying the excitation wavelength and Eu³⁺ doping concentration. Results indicate that Dy³⁺/Eu³⁺ co-doped SBZLBa glasses are promising materials for white-light emitting devices [12]. Dy³⁺/Eu³⁺ co-doped white-light emitting CaO-B₂O₃-SiO₂ glasses have been obtained. The glasses exhibit good thermal stability, and the luminescence color can be tuned by controlling the relative concentrations of Dy³⁺ and Eu³⁺ ions and the excitation wavelength. White light was achieved upon excitation at 387 nm when the concentrations of Dy³⁺ and Eu³⁺ were 4% and 2%, respectively [13].

It should be noted that in many cases the emission intensity of Eu³⁺ ions is higher when they are embedded in matrices containing tungsten oxide, compared to their intensity in other matrices. The significance of tungstates is determined by the occurrence of non-radiative charge transfer from WO_n groups to the active Re³⁺ (Nd, Sm, Eu, Tb, Dy) ion, which leads to an increase in the intensity and efficiency of the emission. There are very few studies in the literature on borosilicate glasses containing tungsten oxide, for example compositions: B₂O₃-SiO₂-ZnO-Na₂O-WO₃ [14] and 20B₂O₃–10SiO₂–10CaO-(60-x)Bi₂O₃/xWO₃, x=0 to 20 wt% [15], which are mainly devoted to structural properties. There are no data on borosilicate glasses containing tungsten that are doped with Eu³⁺ ion.

This work aims to investigate the influence of WO₃ on the physical parameters and luminescent characteristics of Eu³⁺ doped complex (52.5-x/2)B₂O₃:(12.5-x/2)SiO₂:25La₂O₃:5ZnO:5CaO:0.5Eu₂O₃:xWO₃, x= 0, 5 (mol%) glasses.

2. Results and Discussion

2.1. XRD Data and Thermal Analysis

The amorphous nature of the prepared materials was confirmed by X-ray diffraction analysis. Typical diffraction patterns of glasses obtained are shown in Figure 1. The photographic images (insets, Figure 1) show that transparent bulk glass specimens were obtained.

(52.5-x/2)B₂O₃:(12.5-x/2)SiO₂:25La₂O₃:5ZnO:5CaO:0.5Eu₂O₃:xWO₃, x= 0, 5 (mol%) glasses have been also investigated by DSC analysis in order to obtain information for some thermal parameters and for structural changes that take place due to the compositional changes. The glass transition temperature, T_g, has been determined, since it is connected with both the strength of inter-atomic bonds and glass network connectivity. A higher T_g corresponds to a more rigid structure, whereas the glasses having a loose-packed structure have a lower T_g [16]. Figure 2 compares the DSC curves of the glasses investigated in this work.

The endothermic dips corresponding to the glass transition temperature (T_g) are observed. As one can see, both glasses are characterized with high values of the glass transition temperature (T_g)—684.4 °C and 677.1 °C for WO₃-free and for WO₃-containing glass respectively, that is an indication of the formation of well packed glass structure. On the other hand, the addition of WO₃ into the base glass causes a slight decrease in the glass transition temperature values. We explain the observed reduction in T_g as a result of increasing non-bridging atoms with the addition of WO₃ [17].

2.2. DR-UV–Vis Spectra

UV–Vis spectroscopy was also applied for the characterization of the prepared material. Figure 3 shows the diffuse reflectance spectra of glasses obtained.

In the spectrum of WO₃-free glass one symmetrical band at 250 nm is observed which is due to the presence of unavoidable trace iron impurities in the raw materials for glass preparation [18]. The optical absorption spectrum of the glass containing WO₃ displays one symmetrical high intensive band at 260 nm that can be assigned to the ligand–metal charge transfer (LMCT) from oxygen ligands to W⁶⁺ of distorted and isolated WO₄ tetrahedra. It is well known that UV-vis DRS for the isolated WO₄ reference compounds only possess a single ligand to-metal charge transfer (LMCT) band in the general region of 218-274 nm [19]. The exact location of this band maximum depends on the extent of distortion of the isolated WO₄ structure. For example, K₂WO₄ has a relatively undistorted isolated WO₄ unit and possesses a LMCT band at 223 nm, whereas Zr(WO₄)₂ consists of a distorted isolated WO₄ unit and exhibits a LMCT band at 274 nm. The absence of any absorption in the visible range indicates that W⁵⁺ species do not present in the investigated glasses [20]. Optical band gap values (E_g) evaluated from the UV–Vis spectra can give information about the structural arrangement of the glasses under investigation (inset of the Figure 3). The plot of transformed Kubelka–Munk function versus the energy of light (Tauc plot) provides band gap energies, E_g of 3.52 eV for WO₃-containing and 3.54 eV for WO₃-free glass samples respectively. According to the literature in glasses the variation of E_g may be attributed to the network structural changes. Generally, it is accepted that in metal oxides, creation of non-bonding orbitals with higher energy than bonding ones shifts valence band to higher energy which results to E_g decreasing [21]. Therefore, the increase in concentration of the NBOs (non-bridging oxygens) ions reduces the band gap energy. The lower band gap energy value for the WO₃ containing glass shows that introduction of WO₃ leads to the increase in number of non-bridging oxygen species in the glass structure.

2.3. Density, Molar Volume, Oxygen Packing Density and Oxygen Molar Volume

Higher number of non-bridging oxygen atoms in the network of WO₃-containing glass reveled by E_g values obtained are in line with the observed variation in density and various physical parameters established. The measured density of WO₃-free glass is 3.792 ± 0.002 g/cm³, while the density of WO₃-containing glass increases to 3.973 ± 0.004 g/cm³. This rise is attributed to substituting the lighter B₂O₃ (molecular weight 69.62 g/mol) and SiO₂ (molecular weight 60.08 g/mol) with the heavier WO₃ (molecular weight 231.84 g/mol) [22]. The molar volume also increases upon incorporating WO₃, changing from 35.38 cm³/mol in WO₃—free glass to 35.87 cm³/mol in WO₃—containing sample, which suggests that WO₃ expands the glass network. This expansion is linked to the difference in ionic radii between W⁶⁺ (0.6 Å) and B³⁺ (0.23 Å), and Si⁴⁺ (0.26 Å) creating additional free volume [23,24,25]. Oxygen molar volume (Vₒ) and OPD provide insight into how oxygen ions are packed within the glass structure [26]. Lower Vₒ and higher OPD values typically indicate a more tightly connected network. The WO₃ -containing glass shows a higher Vₒ (13.21 cm³/mol) and lower OPD (75.69 g atom/l) compared to WO₃-free glass (13.15 cm³/mol and 76.03 g atom/l, respectively), implying that WO₃ addition increases the concentration of non-bridging oxygens (NBOs) resulting in the formation of less packed and more disordered glass network.

2.4. Photoluminescent Properties

The excitation spectra of the prepared Eu³⁺ doped glasses are shown in Figure 4. All data were obtained at room temperature by monitoring the most intensive characteristic emission of Eu³⁺ ions at 612 nm wavelength, corresponding to ⁵D₀ →⁷F₂ transition. As can be seen from the figure, a broad, continuous band below 350 nm is observed, along with several narrow peaks distributed across the 350–600 nm wavelength range. Generally, the broadband is due to ligand to metal charge transfer transitions (LMCT) from oxygen 2p orbital to the empty 4f orbital of europium (O²⁻ → Eu³⁺) and from and O²⁻ → Zn²⁺ inside the ZnO_n (ZnO_n= ZnO₄) host absorbing groups [27,28,29,30,31]. Additionally, in the glass containing tungsten oxide (50B₂O₃:25La₂O₃:10SiO₂:5CaO:5ZnO:5WO₃:0.5Eu₂O₃) this band is also due to the transition from O²⁻ → W⁶⁺ inside the WO_n (WOn = WO₄ and WO₆) groups [32].

The presence of the excitation band of ZnO_n and WO_n, recorded at the emission wavelength of Eu³⁺ at 612 nm, suggests the existence of non-radiative energy transfer from the glass matrix to the luminescent rare-earth ion [32,33]. As can be observed from Figure 4, the glass containing WO₃ exhibits higher charge transfer absorption intensity. Consequently, WO₃ is expected to contribute significantly to the non-radiative energy transfer towards the luminescent Eu³⁺ ions, leading to stronger emission from the glass containing tungsten oxide. This mechanism is commonly referred to as host-sensitized luminescence. The distinct sharp peaks observed in the region above 350 nm are attributed to 4f–4f electron transitions from the ground state to excited energy levels, specifically ⁷F₀→ ⁵H₃ (∼318 nm), ⁷F₀ → ⁵D₄ (∼360 nm), ⁷F₀ → ⁵G₂ (∼376 nm), ⁷F₁ → ⁵L₇ (∼381 nm), ⁷F₀ → ⁵L₆ (∼393 nm), ⁷F₀ → ⁵D₃ (∼413 nm), ⁷F₀ → ⁵D₂ (∼463 nm), ⁷F₀ → ⁵D₁ (∼523 nm) and ⁷F₁ → ⁵D₁ (∼531 nm), ⁷F₀ → ⁵D₀ (∼576 nm). The highest excitation was observed at ⁷F₀→⁵L₆ (393 nm). Therefore, the emission spectra measurements were conducted under excitation at 393 nm. The comparison between the LMCT band and the 4f–4f transitions reveals that the narrow Eu³⁺ peaks exhibit higher intensity. This means that the efficient excitation by near-UV and blue LED chips can be obtained, since Eu³⁺ 4f–4f transitions are typically weak because they are partially forbidden by Laporte’s selection rule [34].

The emission spectra of Eu³⁺ doped borosilicate glasses (Figure 5) were recorded at room temperature using 393 nm excitation wavelength. The five narrow characteristic emission peaks, originating from the radiative transitions of Eu³⁺ ions from the ⁵D₀ excited state to the lower-lying ⁷F₀, ⁷F₁, ⁷F₂, ⁷F₃, ⁷F₄ ground states are observed at 578 nm, 591 nm, 612 nm, 652 nm and 701 nm [35]. As shown in Figure 5 the emission intensity exhibits a strong dependence on the composition and increases with the incorporation of tungsten in glass matrix. This behaviour can be related to non-radiative charge-transfer processes between the glass host and the luminescent Eu³⁺ ions.

Additional evidence for this energy-transfer mechanism is provided by the absence of the typical broad WO₃ band in the 400-600 nm [36] spectral region, indicating that the excitation energy absorbed by tungstate groups is efficiently transferred non-radiatively to the Eu³⁺ ions. The absence of ZnO emission bands [37] in the same region is also suggesting energy transfer, but to a lesser extent. Among all the observed emission bands, the most intense one, centered at 612 nm, originates from the forced electric-dipole (ED) ⁵D₀→⁷F₂ transition, which is highly sensitive of the local crystal-field environment surrounding the Eu³⁺ ions, followed by the magnetic-dipole (MD) ⁵D₀→⁷F₁ at 591 nm transition, which is insensitive to the surrounding ligands [27,35]. The dominance of the ED transition over the MD transition indicates that Eu³⁺ ions occupy non-centrosymmetric sites within the glass host. Moreover, the ratio of these emission intensities, commonly referred to the asymmetry ratio R=(⁵D₀ → ⁷F₂)/(⁵D₀ →⁷F₁), provides insight into the degree of local asymmetry surrounding around the Eu³⁺ ions as well as the strength of Eu–O covalence in various Eu³⁺-doped materials [38,39]. Lower values of the asymmetry parameter correspond to higher local site symmetry around the active ion, lower Eu–O covalency and emission intensity. The increase in R value is due to the increase in asymmetry and covalency between the Eu³⁺ ion and the ligands and leads to a higher emission intensity [40]. The R values of the synthesized glasses are listed in Table 1 along with other data reported in the literature for Eu³⁺-doped glasses and the commercial used phosphor material.

Compared to our previous synthesized glasses containing boron and tungsten oxides, the values of the asymmetric ratio R are similar [17,41,42,43], but compared to other borate or silicate oxide glass compositions [44,45,46,47,48,49,50] the values are higher. The relatively higher R values observed in the present glasses indicate that Eu³⁺ ions occupy low-symmetry crystallographic sites and provide evidence of the high Eu³⁺-O²⁻ covalency. The higher asymmetry ratio, 4.42, obtained for the tungsten containing glass composition 50B₂O₃:25La₂O₃:10SiO₂:5CaO:5ZnO:5WO₃:0.5Eu₂O₃, suggests stronger emission intensity compared to the WO₃ free glass. In addition, the ⁵D₀→⁷F₁ transition of Eu³⁺ exhibits splitting into three emission peaks. This behaviour is attributed to crystal-field-induced splitting, where a single electronic transition gives rise to multiple emission components [53]. Moreover, the presence of the ⁵D₀→⁷F₀ transition, which is highly sensitive to the local crystal field and normally forbidden under standard Judd–Ofelt theory, further confirms that Eu³⁺ ions reside at non-centrosymmetric sites with C₂ᵥ, Cₙ, or Cₛ symmetry within the glass matrix [54]. Further to evaluate the luminescent properties and the perceived emission color, the Commission Internationale de l’Éclairage (CIE) 1931 chromaticity diagram was used [55]. The chromaticity coordinates of the synthesized borosilicate glasses were calculated from the photoluminescence spectra (Figure 5) using SpectraChroma software (Version 1.0.1, CIE coordinate calculator) [56]. The obtained values are almost identical and are located within the red region of the CIE diagram (Figure 6) and are summarized in Table 2. The calculated coordinates are very close to the NTSC standard for red light (0.670, 0.330) as well as to the chromaticity coordinates of the commercial red phosphor Y₂O₂S:Eu³⁺ (0.658, 0.340) [57].

3. Materials and Methods

Two glass samples of 52.5B₂O₃:12.5SiO₂:25La₂O₃:5CaO:5ZnO:0.5Eu₂O₃ and 50B₂O₃:10SiO₂:25La₂O₃:5CaO:5ZnO:5WO₃:0.5Eu₂O₃ compositions (in mol%) were obtained by applying of the melt quenching method, using reagent grade La₂O₃, H₃BO₃, SiO₂, CaO, ZnO and WO₃ as raw materials. The homogenized batches were melted at 1400 °C for 2 hours in a curundum crucible in air. The melts were cast into preheated graphite mold to get bulk glass samples. Then the glasses were transferred in a laboratory electric furnace annealed at 500 °C for 1hour and then were cooldown to room temperature at a very slow cooling rate of about 0.5 °C/min. The phase formation of the samples was established by x-ray phase analysis with a Bruker D8 Advance diffractometer, using Cu Kα radiation in the 10 < 2θ < 60 range. The glass transition (T_g) temperatures of the glasses were determined by differential scanning calorimetry (DSC) using a Netzsch 404 Pegasus instrument, 2021 Selb, Germany, at a heating rate of 10 K/min in an Ar flow of 10 mL/s, using corundum crucibles with lids. The density of the obtained glasses at room temperature was measured by the Archimedes principle using toluene (ρ = 0.867 g/cm³) as an immersion liquid on a Mettler Toledo electronic balance of sensitivity 10⁻⁴ g. From the experimentally evaluated density values the molar volume (V_m), the molar volume of oxygen (V_o) (volume of glass in which 1 mol of oxygen is contained) and the oxygen packing density (OPD) of glasses obtained were estimated, using the following relations respectively:

$${\mathbf{V}}_{\mathbf{m}}={\displaystyle \frac{\mathrm{\Sigma }{\mathbf{x}}_{\mathbf{i}}{\mathbf{M}}_{\mathbf{i}}}{{\mathsf{\rho }}_{\mathbf{g}}}}$$

(1)

$${\mathbf{V}}_{\mathbf{o}}={\mathbf{V}}_{\mathbf{m}}\times \left({\displaystyle \frac{1}{\mathrm{\Sigma }{\mathbf{x}}_{\mathbf{i}}{\mathbf{n}}_{\mathbf{i}}}}\right)$$

(2)

$$\mathbf{O}\mathbf{P}\mathbf{D}=1000\times \mathbf{C}\times \left({\displaystyle \frac{{\mathbf{\rho }}_{\mathbf{g}}}{\mathbf{M}}}\right)$$

(3)

where x_i is the molar fraction of each component i, M_i the molecular weight, ρ_g the glass density and n_i is the number of oxygen atoms in each oxide, C is the number of oxygen per formula units, and M is the total molecular weight of the glass compositions. The optical spectra of the powder samples at room temperature were recorded with a spectrometer (Evolution 300 UV–vis Spectrophotometer) employing the integration sphere diffuse reflectance attachment. The samples were measured in the wavelength (λ) range of 200–1100 nm with a magnesium oxide reflectance standard used as the baseline. The uncertainty in the observed wavelength is about ±1 nm. The Kubelka–Munk function (F(R∞)) was calculated from the UV–Vis diffuse reflectance spectra. The band gap energy (E_g) was determined by plot (F(R∞) hν)^1/n, n = 2 versus hν (incident photon energy). Photoluminescence(PL) excitation and emission spectra at room temperature for all glasses were measured with a Spectrofluorometer FluoroLog3-22, 2014 (Horiba JobinYvon,).

4. Conclusions

In this study the influence of WO₃ on the physical parameters and luminescent characteristics of Eu³⁺ doped complex borosilicate glasses (52.5-x/2)B₂O₃:(12.5-x/2)SiO₂:25La₂O₃:5ZnO:5CaO:0.5Eu₂O₃:xWO₃, x = 0, 5 (mol%) was established. The XRD results confirmed the amorphous nature of the glasses. The optical absorption spectra contained a major band corresponding to W⁶⁺ ions in tetrahedral positions. The decrease in both the optical band gap and the glass transition temperature, and variation in the physical parameters indicated the decrease in the degree of polymerization (increasing number of non-bridging oxygen atoms) with the addition of 5 mol% WO₃ into the base complex borosilicate glass. The positive effect of WO₃ on the luminescence intensity of the Eu³⁺ doped complex borosilicate glass was established. The high luminesce intensity ratio (R) = (⁵D₀-⁷F₂)/(⁵D₀-⁷F₁) and color chromaticity coordinate (0.652, 0.348) support that, Eu³⁺ doped borosilicate glass, having 5 mol. % WO₃ is a suitable candidate for visible red emission applications.