Synthesis and Photoluminescent Properties of Dy³⁺-Doped and Dy³⁺/Eu³⁺ Co-Doped 50ZnO:40B₂O₃:5WO₃:Nb₂O₅ Glass

Abstract

Dy³⁺ single-doped and Dy³⁺/Eu³⁺ co-doped ZnO:B₂O₃:WO₃:Nb₂O₅ glass was successfully synthesized using the melt quenching method. The amorphous character of the prepared samples was confirmed by X-ray diffraction (XRD). The glass transition and crystallization temperatures were examined by differential scanning calorimetry (DSC). Raman spectroscopy was applied to investigate the glass microstructure. Physical properties like the density, molar volume, oxygen molar volume and oxygen packing density of the glass were also determined. The photoluminescence excitation (PLE) and emission (PL) spectra of the resultant glass types were measured. The obtained Dy³⁺ single-doped glass was characterized by strong luminescence at 482 and 574 nm, corresponding to the ⁴F_9/2 → ⁶H_15/2 (blue) and ⁴F_9/2 → ⁶H_13/2 (yellow) transitions, respectively, and weak luminescence at 663 nm and 753 nm due to the ⁴F_9/2 → ⁶H_11/2 (red) and ⁴F_9/2 → ⁶H_9/2 + ⁶F_11/2 (red) transitions. The luminescence results indicate that energy transfer from the Dy³⁺ to Eu³⁺ ions occurs in the proposed glass system. The emitted light from the Dy³⁺ single-doped glass was found to be yellow-orange. The Dy³⁺/Eu³⁺ co-doped samples emitted darker orange light. The obtained results show that the investigated types of glass have the potential to be used as orange light-emitting materials.

1. Introduction

For many years, luminescent materials doped with rare-earth (RE) elements have garnered significant attention because of their diverse applications [1,2]. These light-emitting materials are used in a variety of areas, including optoelectronic devices, displays, biomarkers, optical fibers and lasers [3,4,5,6,7]. A considerable part of the research in this domain is dedicated to rare-earth-ion-doped phosphors for solid-state lighting (SSL)-based devices, such as light-emitting diodes (LEDs) [8,9]. These materials offer several benefits compared to traditional incandescent or fluorescent lamps, including high brightness, efficiency, a long lifespan, reliability and reduced energy consumption [10]. This aspect is particularly vital considering that the electrical lighting industry accounts for nearly 19% of global electricity usage [11]. Consequently, the creation of innovative and environmentally friendly materials that are both efficient and durable is of utmost importance [2,12,13,14]. In this context, the significance of RE ions is considerable. These ions possess a distinctive capability to emit light due to their 4f-4f or 4d-4f transitions when excited at an appropriate wavelength [8,15]. Dy³⁺ single-doped materials can produce neutral white light emission. The luminescence spectrum of trivalent dysprosium ions reveals multiple characteristic lines, with two main bands appearing in the blue (470–500 nm) and yellow (570–600 nm) regions, corresponding to the transitions ⁴F_9/2 → ⁶H1_5/2 and ⁴F_9/2 → ⁶H_13/2, respectively [16,17]. White light emission can be achieved through the proper adjustment of the blue and yellow luminescence intensity ratio [15,18,19]. Nonetheless, there is often a preference for producing warm white light rather than a cold, artificial white hue [20]. To generate this warm light, red emitters such as Eu³⁺ ions can be utilized, as their strongest band related to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ occurs at approximately 615 nm, placing it within the red section of the visible light spectrum [21,22]. Furthermore, a suitable ratio of Eu³⁺/Dy³⁺ within the host matrix can facilitate color adjustment, resulting in the emission of warm white light. This phenomenon has been observed in various Dy³⁺/Eu³⁺-doped matrices, such as yttrium alumino bismuth borosilicate and CdO-GeO₂-TeO₂ glass [4,23], as well as phosphate glass [24]. Additionally, warm white light was achieved in lithium fluoride bismuth borate glass co-doped with Dy³⁺ and Eu³⁺ [25]. Research on Zn(PO₃)₂ glass doped with Eu³⁺ and Dy³⁺ ions shows the transition of white light emission from neutral white at 348 nm excitation to warm white at 445 nm [26]. On the other hand, when a pair of rare-earth (RE) ions have matched energy levels and are co-doped in a host, a process in which one RE is sensitized by the energy from the other ion may occur. This process is defined as energy transfer (ET), a common way to improve the luminescence properties of RE ion-doped materials [27]. Combining Dy³⁺ with Eu³⁺ tends to enforce the europium emission because when materials with Dy³⁺ and Eu³⁺ are excited with a UV light source, the Dy³⁺ ions absorb the incident light, and they non-radiatively transfer part of the absorbed energy to Eu³⁺ ions, causing emissions.

It is well known that binary ZnO-B₂O₃ glass has been attracting continuous scientific interest as a host for luminescence applications due to its high transparency from the visible to mid-infrared region of the spectrum, its relatively low melting temperature, and its good chemical and thermal stability. We have experience in the synthesis, structural analysis and luminescent properties of ZnO-B₂O₃ glass containing WO₃ and Nb₂O₅ doped with Eu³⁺ ions [28]. Our results have shown the existence of a charge transfer from the matrix (WO₃ and Nb₂O₅) to Eu³⁺, resulting in enhanced Eu³⁺ emission. We are also expecting the occurrence of such a charge transfer from the matrix to Dy³⁺ (which we established through spectral results). On the other hand, it is well known from the literature that in a Dy³⁺ and Eu³⁺ co-doped matrix, a non-radiative charge transfer from Dy³⁺ to Eu³⁺ active ions is observed, resulting in an increase in Eu³⁺ emission intensity. In this way, we are sensitizing Eu³⁺ in two ways—from the matrix and from Dy³⁺. The novelty and significance of this study is the composition of the matrix, which is suitable for hosting Eu³⁺ and Dy³⁺ active ions and can also serve as a host sensitizer of both ions. The aim of this work was to synthesize ZnO:B₂O₃:WO₃:Nb₂O₅ glass doped with different concentrations of Dy³⁺ ions and co-doped with Eu³⁺ ions, which could be used as a phosphor in LEDs. This study provides a detailed analysis of the thermal and luminescent properties and structural features of these prepared types of glass. Physical parameters such as the density, molar volume, oxygen molar volume and oxygen packing density, which provide some structural information, were also evaluated.

2. Results

2.1. XRD Data and Thermal Analysis

Two series of glass were obtained. Their compositions and respective names are listed in

Table 1. Molar compositions of the investigated Dy³⁺ single-doped 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅ glass (mol%).

Sample ID	ZnO	B2O3	WO3	Nb2O5	Dy2O3	Eu2O3
0.25Dy	50	40	5	5	0.25	-
0.5Dy	50	40	5	5	0.5	-
0.75Dy	50	40	5	5	0.75	-
1Dy	50	40	5	5	1	-

and

Table 2. Molar compositions of the investigated Dy³⁺/Eu³⁺ co-doped 50ZnO:40B₂O₃:WO₃:Nb₂O₅ glass (mol%).

Sample ID	ZnO	B2O3	WO3	Nb2O5	Dy2O3	Eu2O3
0.5Dy-0.25Eu	50	40	5	5	0.5	0.25
0.5Dy-0.5Eu	50	40	5	5	0.5	0.5
0.5Dy-0.75Eu	50	40	5	5	0.5	0.75
0.5Dy-1Eu	50	40	5	5	0.5	1

, respectively.

The measured X-ray diffraction patterns are shown in Figure 1a,b. The absence of sharp diffraction peaks in the spectra showed that prepared samples are glassy in nature.

The amorphous nature of the obtained materials was further confirmed by differential scanning calorimetry (DSC). The DSC curves for the two series of samples—Dy³⁺-doped and Dy³⁺/Eu³⁺ co-doped glasses—are shown in Figure 2a and Figure 2b, respectively.

The endothermic dips corresponding to the glass transition temperature (T_g) and the exothermic peaks due to the crystallization temperature (T_c) are observed. The estimated values of T_g and T_c are pointed out in the figure. As can be seen from the figure, the glass transition temperatures of all glasses are above 500 °C and do not vary significantly with composition, indicating that the addition of Dy₂O₃ and Eu₂O₃ does not significantly affect the structure of the matrix glass. All glasses are characterized by a high thermal stability, i.e., ΔT = T_c − T_g between 178 and 210 °C.

2.2. Physical Properties

Structural information of the glasses was gained by density (ρ_g) measurements, which provided the base for the determination of several physical parameters, such as molar volume (V_m), oxygen molar volume (V_o) and oxygen packing density (OPD). These were evaluated using the following relations:

$$V_m={\displaystyle \frac{\sum x_i{M}_i}{{\rho }_g}}$$

(1)

$$V_o=V_m\times \left({\displaystyle \frac{1}{\sum x_i{n}_i}} \right)$$

(2)

$$OPD=1000\times C\times \left({\displaystyle \frac{{\rho }_g}{M}}\right)$$

(3)

where x_i is the molar fraction of each component i, M_i is the molecular weight, ρ_g is the glass density, n_i is the number of oxygen atoms in each oxide, C is the number of oxygen atoms per formula units, and M is the total molecular weight of the glass compositions. The values obtained are listed in

Table 3. Values of physical parameters of 50ZnO:40B₂O₃:5WO₃:5Nb₂O_5:xDy₂O₃ (0 ≤ x ≤ 1) glass: density, ρ_g, molar volume, V_m, oxygen molar volume, V_o, oxygen packing density, OPD.

Sample ID	ρg	Vm	Vo	OPD
0.25Dy	3.887 ± 0.005	24.28	11.51	86.90
0.5Dy	3.923 ± 0.004	24.29	11.46	87.28
0.75Dy	3.926 ± 0.004	24.51	11.56	86.50
1Dy	3.949 ± 0.003	24.60	11.55	86.59

and

Table 4. Values of physical parameters of 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅:0.5Dy₂O₃:xEu₂O₃ (0 ≤ x ≤ 1) glass: density, ρ_g, molar volume, V_m, oxygen molar volume, V_o, oxygen packing density, OPD.

Sample ID	ρg	Vm	Vo	OPD
0.5Dy-0.25Eu	3.927 ± 0.004	24.49	11.46	87.28
0.5Dy-0.5Eu	3.956 ± 0.002	24.53	11.52	86.83
0.5Dy-0.75Eu	3.968 ± 0.003	24.68	11.53	86.71
0.5Dy-1Eu	3.991 ± 0.003	24.76	11.52	86.83

.

As seen from Table 3, the density of glass 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅ from the first series with increasing content of Dy₂O₃ increases with the Dy₂O₃ loading, which is mostly due to the increase in average molecular mass of the glasses [29]. Molar volume (V_m) is also an important physical property and normally follows a trend opposite to that of density. However, V_m also enhances with increasing Dy₂O₃ content, indicating an increasing inter-atomic spacing in the network due to the insertion of Dy³⁺ ions with high ionic radius (0.91 Å). Oxygen molar volume (V_o) and OPD are two parameters that provide information on the packing of the oxygen ions in the glass structure [30]. Lower V_o and higher OPD indicate a higher degree of network connectivity. In this series of glasses, the lowest V_o and the highest OPD are observed for the Dy0.5 glass, evidencing the highest cross-linking and bonding in its glass network. The highest value of V_o and the lowest OPD for Dy0.25, Dy0.75, Dy1 glasses indicate lower cross-linking density and the formation of less reticulated glass networks. For the second series of 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅ glasses, containing a constant Dy₂O₃ content of 0.5 mol% and increasing Eu₂O₃ concentration from 0.25 to 1 mol% (Table 4), both density and oxygen molar values increase with the Eu₂O₃ loading. V_o increases and OPD decreases for glasses having up to 0.75 mol% Eu₂O₃ (samples 0.5Dy-0.25Eu₂O₃, 0.5Dy-0.5Eu₂O₃ and 0.5Dy-0.75Eu₂O₃). With future increase in Eu₂O₃ content of 1 mol% (sample 0.5Dy-1Eu), the oxygen molar volume decreases and oxygen packing density enhances. The increasing glass density can be attributed to the increase in the average molecular mass of glasses [29]. As the molar volume (volume occupied by a mass of the glass equal to 1 mole) is strongly affected by the ionic radii of the incorporated ionic species in the glass, the increasing trend of V_m is due to the insertion of Eu³⁺ ions, which are known to have a high ionic radius (0.95 Å), resulting in the formation of an excess free volume, which increases the overall molar volume of these glasses [31]. The increasing V_o and decreasing OPD for glasses having up to 0.75 mol% Eu₂O₃ revealed insufficient cross-linking (high number of non-bridging oxygen atoms) of the borate network. Decreasing V_o and enhancing OPD for Dy0.5-1Eu glass suggest increased cross-linking in the glass network.

2.3. Raman Analysis

The structure of the obtained glasses was studied by Raman spectroscopy. Figure 3a shows the Raman spectra of 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅ glasses from the first series, containing increasing Dy₂O₃ content. As can be seen, all glass spectra contain a band at 960 cm⁻¹, a high-intensity band at 905–885 cm⁻¹, a shoulder at 805 cm⁻¹, a broad shoulder at 710–740 cm⁻¹, broad Raman activity in the range of 200–450 cm⁻¹ and a band at 126–132 cm⁻¹. Taking into account previous spectral investigations on glasses 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅:xEu₂O₃ (x = 0, 0.1, 0.5, 1, 2, 5, and 10 mol%) and (50-x)WO₃:25La₂O₃:25B₂O₃:xNb₂O₅ reported in refs. [28,32], we attribute the band at 960 cm⁻¹ in the Raman spectra of the first series glasses to the ν₁ mode of the isolated tetrahedral species, [WO₄]²⁻. This band becomes more intense with the addition of Dy₂O₃, indicating an increasing quantity of isolated [WO₄]²⁻ groups as a result of the WO₆ → WO₄ transformation. The intensive Raman peak at 905–885 cm⁻¹ is assigned to overlapping contributions from the asymmetric stretching mode ν₃ of [WO₄]²⁻ tetrahedra and the ν₁ stretching vibration of short Nb–O bonds in strongly deformed and isolated NbO₆ octahedra [28,32]. The broadening of this band with increasing Dy₂O₃ content indicates the formation of [WO₄]²⁻ tetrahedral and NbO₆ octahedral units with different distortions upon the addition of Dy₂O₃. The shoulder at 805 cm⁻¹ is a superposition of the symmetric stretching ν₁ mode of tetrahedral [NbO₄]³⁻ groups, and vibrations of B–O–Nb bonds [28,32]. The broad Raman shoulder at ca. 710–740 cm⁻¹ is attributed to the stretching of the bridging oxygen atoms linking the tungsten ion and niobium polyhedral units, via Nb–O–W bridges that overlap with the vibrations of chain-type metaborate units, [BØ₂O]⁻ [28,33]. In addition, the observed upshift to 740 cm⁻¹ indicates that metaborate units, [BØ₂O]⁻, are charge balanced by Dy³⁺ ions. The increasing intensity of this shoulder upon increasing Dy₂O₃ concentration indicates a growing number of Nb–O–W linkages and metaborate groups in the glass networks. The vibrational features in the lower-frequency region 200–450 cm⁻¹ correspond mainly to bending modes of [WO₄]²⁻ and [NbO₄]³⁻ tetrahedra coupled with the Zn–O (at ca. 235 cm⁻¹) and Dy–O vibrations (378 cm⁻¹) [28,34,35]. The band observed at 126–132 cm⁻¹ in all glass compositions was related to the out-of-plane W–O deformation mode [36].

Figure 3b presents the Raman spectra of the second series of 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅ glasses containing a constant Dy₂O₃ content of 0.5 mol% and increasing Eu₂O₃ concentration from 0.25 to 1 mol%. As can be seen from the figure, all glass spectra contain Raman bands at 133–144 cm⁻¹, at 305–283 cm⁻¹, at 700–720 cm⁻¹ and at 970 cm⁻¹. In the spectrum of glass 0.5Dy-1Eu, a band is also present at about 875 cm⁻¹ that is not well resolved.

Having in mind our previous spectral investigation on glasses 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅:xEu₂O₃ (x = 0, 0.1, 0.5, 1, 2, 5, and 10 mol%) reported in ref. [28], we ascribe the weak band at 970 cm⁻¹ to the ν₁ mode of the isolated tetrahedra, [WO₄]²⁻. The not-well-resolved band at 875 cm⁻¹ is due to a coupled mode including the ν₁ stretching vibration of short Nb–O bonds in strongly deformed and isolated NbO₆ octahedra and the asymmetric stretching mode ν₃ of [WO₄]²⁻ tetrahedra [28]. The band at 700–720 cm⁻¹ is a superposition of the peaks associated with the vibrational modes of the Nb–O–W linkages and metaborate units [28,33]. The band at 305–283 cm⁻¹ is connected with the overlapping Zn–O, Dy–O, and Eu–O stretching vibrations [28,35,37]. The low- frequency band at 133–144 cm⁻¹ is assigned to the out-of-plane W–O deformation mode [36]. As seen from Figure 3b, with increasing Eu₂O₃ content, the overall intensity of the Raman spectra for all glasses increases, reflecting enhanced crosslinking in the glass network [38]. In addition, the observed upshift of the band at 700 to 720 (vibrations of Nb–O–W and [BØ₂O]⁻) and the band at 133 to 144 (W–O deformation mode) in the Raman spectrum of the glass having the highest Eu₂O₃ content of 1 mol% (Figure 3b, spectrum 0.5Dy-1Eu₂O₃) indicates that the short-range order of niobate, tungstate and borate groups is influenced by the added Eu₂O₃. Most probably, the Eu³⁺ ions are situated in the local environment of these structural arrangements, charge balancing them via the formation of Nb/W–O–Eu and B–O–Eu bonding and thus increasing the reticulation of the glass network. This observation coincides well with the physical parameters obtained above and the data of the thermal analysis.

2.4. Photoluminescent Properties

The excitation spectra of Dy³⁺-doped 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅ glass, monitored at the 574 nm wavelength, are shown at Figure 4a. The sharp excitation peaks seen at 324 nm, 337 nm, 349 nm, 363 nm, 385 nm, 424 nm, 452 nm and 471 nm can be attributed to the transitions of ⁶H_15/2 → ⁶P_3/2, ⁶H_15/2 → ⁴I_9/2, ⁶H_15/2 → ⁴P_7/2, ⁶H_15/2 → ⁶P_5/2, ⁶H_15/2 → ⁴I_13/2, ⁶H_15/2 → ⁴G_11/2, ⁶H_15/2 → ⁴I_15/2, ⁶H_15/2 → ⁴F_9/2, respectively [39]. From the observed spectra, the intense band at 385 nm (⁶H_15/2 → ⁴I_13/2) was selected for photoluminescence measurements, as it shows the highest intensity.

These wavelengths are appropriate for excitation using blue LED chips (430–470 nm) and commercial near-ultraviolet light-emitting diodes (LEDs) (250–400 nm). In addition, it can be seen that the excitation spectra show a broad band below 320 nm, which corresponds to the ligand-to-metal charge transfer states (LMCT) of O²⁻ → Dy³⁺ from the oxygen 2p excited state to the Dy³⁺ 4f state [40], and O²⁻ → W⁶⁺ in WO₄ groups [41] and O²⁻ → Nb⁵⁺ in NbO_n groups (NbO_n = NbO₆, NbO₄) [42]. The overlapping of bands in this spectral region makes it impossible to distinguish the individual contribution of these transitions. The existence of the host lattice absorption in the excitation spectra indicates the presence of energy transfer from the WO₄ and NbO_n structural polyhedra to the active Dy³⁺ ions. This mechanism is known as host-sensitized luminescence [41,43]. Figure 4b shows the visible emission spectra of the Dy³⁺-doped 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅ glass under excitation at a wavelength of 385 nm. The obtained glasses are characterized by strong luminescence at 482 and 574 nm corresponding to the ⁴F_9/2 → ⁶H_15/2 (blue) and ⁴F_9/2 → ⁶H_13/2 (yellow) transitions, respectively, and weaker luminescence at 663 nm and 753 nm due to the ⁴F_9/2 → ⁶H_11/2 (red) and ⁴F_9/2 → ⁶H_9/2 + ⁶F_11/2 (red) transitions. From Figure 4b it is clear that the emission intensity strongly depends on the Dy³⁺ concentration and increases with increasing Dy₂O₃ content, reaching a maximum at 0.5 mol%, after which it gradually decreases. This fact may be attributed to the concentration quenching effect [44]. As the Dy³⁺ concentration rises, the distance between Dy³⁺ reduces, enabling non-radiative energy transfer between neighboring Dy³⁺ ions. The occurrence of crossing-relaxation phenomena between these ions reduces the jump from the ⁴F_9/2 energy level to ⁶H_J levels (J = 15/2, 13/2, 11/2 and 9/2) and ⁶F_11/2, leading to fluorescence quenching and weakening of the emission intensity.

To study the local symmetry around rare earth ions, dysprosium can be used as a spectroscopic probe. Among all the observed emission transitions, the blue emission transition (⁴F_9/2 → ⁶H_15/2) at 482 nm is an allowed magnetic dipole (MD) in nature and is independent of a local crystal field environment, while the yellow emission transition at 574 nm (⁴F_9/2 → ⁶H_13/2) is identified as an electric dipole (ED) and is forced by the crystal field environment in the vicinity of the Dy³⁺ ions. When the yellow emission is dominant in the spectrum, the Dy³⁺ ions are located at low-symmetry sites. Conversely, when the blue emission is stronger in the PL spectrum, the Dy³⁺ ions are located at high-symmetry sites (with inversion symmetry center) [45,46]. In our case, Dy³⁺ ions are located in the more asymmetrical environment, as evidenced by the greater intensity of the ⁴F_9/2 → ⁶H_13/2 transition compared to the ⁴F_9/2 → ⁶H_15/2 emission transition.

In our previous article, the same glass composition (50ZnO:40B₂O₃:5WO₃:5Nb₂O₅) with different Eu³⁺ doping concentrations (0–10 mol%) was obtained [28]. As an example, in Figure 4c,d can be seen the excitation and emission spectra of 0.5 Eu³⁺-doped glass. We established an increase in europium emission intensity due to the appearance of strong host lattice absorption, resulting in non-radiative energy transfer from the WO₄ and NbO_n structural polyhedra to the active Eu³⁺ ion. This fact, along with the present results concerning Dy³⁺-doped glass (Figure 4a,b), shows that 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅ glass matrix is suitable for hosting both active ions and can serve as a host sensitizer. Therefore, inspired by the above-mentioned observations, in the present study, we are synthesizing Dy³⁺/Eu³⁺ co-doped glass with different active ion concentrations, which will be excited simultaneously to verify the resulting color of emission.

Furthermore, Figure 5 shows the overlap between the excitation spectra of Eu³⁺-doped glass (black line), monitored at 612 nm wavelength and emission spectra of Dy³⁺-doped glass (red line) at 385 nm excitation. Within the wavelength range of 450–500 nm, a spectral overlap is observed, which indicates that Eu³⁺ can be sensitized by Dy³⁺ in the obtained Dy³⁺/Eu³⁺ co-doped 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅ luminescent glass [47].

Figure 6 shows the excitation spectra of the 0.5Dy³⁺/0.5Eu³⁺ co-doped 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅ glass at 612 nm (a) and at 574 nm (b) excitation wavelengths. When monitoring the most intensive transition for Dy³⁺ (⁴F_9/2 → ⁶H_13/2) at 574 nm, the excitation spectra of 0.5Dy³⁺/0.5Eu³⁺ co-doped glass consists of only Dy³⁺ bands (Figure 6b). In contrast, when monitoring the most prominent emission of Eu³⁺ (⁵D₀ → ⁷F₂) at 612 nm, the excitation spectrum consists of all Eu³⁺ transitions along with several low-intensity Dy³⁺ bands: ⁶H_15/2 → ⁶P_7/2 (349 nm), ⁶H_15/2 → ⁴G_11/2 (424 nm), and ⁶H_15/2 → ⁴I_15/2 (452 nm). The appearance of excitation wavelengths of Dy³⁺, when monitoring the Eu³⁺ emission at 612 nm, provides evidence that energy transfer from Dy³⁺ to Eu³⁺ occurs. As can be seen from Figure 6, for both Dy³⁺ and Eu³⁺, excitation peaks are located at around 363 nm.

Hence, in order to achieve better co-emission from Dy³⁺ and Eu³⁺ in the co-doped glass, the 363 nm wavelength was chosen as the excitation source.

As can be seen from Figure 4a, the maximum emission intensity is observed at a Dy³⁺ concentration of 0.5 mol%. Based on this observation, we synthesized co-doped glass with 0.5 mol% Dy³⁺ and varying Eu³⁺ concentrations ranging from 0.25 to 1.0 mol%. The emission spectra of 0.5Dy³⁺/xEu³⁺ (x = 0, 0.25, 0.5, 0.75, and 1.0 mol%) co-doped glass and single Eu³⁺-doped glass under 363 nm excitation wavelength are shown in Figure 7. The characteristic peaks located at 482, 574 and 663 nm belong to the Dy³⁺ transitions ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_11/2, respectively. In addition to the dysprosium, the ⁵D₀ → ⁷F₁, 5D₀ → ⁷F₂ and ⁵D₀ → 7F₄ emission peaks of europium are also present at 597 nm, 612 nm and 700 nm.

The intensity of europium emission peaks in the co-doped samples is most prominent at a concentration of 0.5 mol% Eu³⁺ ion (Figure 7). No further increase in intensity is observed at 1.0 mol%. The intensity of the Dy³⁺ transitions decreases progressively with increasing Eu³⁺ concentration, while the intensity of Eu³⁺ emission bands increases. This observation can be attributed to the fact that Dy³⁺ sensitizes Eu³⁺ and thus enhances the emission intensity of Eu³⁺. Additional evidence for the occurring energy transfer is that the single Eu³⁺-doped glass is characterized by lower emission intensity.

Figure 8 shows the energy level diagram of Dy³⁺ and Eu³⁺ ions within the obtained luminescent glass. Under 363 wavelength of excitation, the ground state of Dy³⁺ ions was excited to the ⁶P_5/2 excitation state and subsequently decays non-radiatively to the ⁴F_9/2 state. From there, they emit radiatively to the 6H_J levels (J = 15/2, 13/2, 11/2 and 9/2) and ⁶F_11/2. A portion of the excitation energy of the ⁴F_9/2 state is absorbed by Eu³⁺ ions and is promoted to the ⁵D₂ level, and after that, via non-radiative transitions, they populate the lower ⁵D_1,0 levels and further decay radiatively to the ⁷F_J (J = 0, 1, 2, 3) ground states.

The energy transfer mechanisms that can occur are illustrated in Figure 8 and are as follows [48]:

ET1: ⁴F_9/2 [Dy³⁺] + ⁷F₁ [Eu³⁺] → ⁶H_15/2 [Dy³⁺] + ⁵D₂ [Eu³⁺]

ET2: ⁴F_9/2 [Dy³⁺]+⁷F₀ [Eu³⁺] → ⁶H_13/2 [Dy³⁺] + ⁵D₀ [Eu³⁺]

To estimate the actual emission colors, the Commission International de l’Eclairage (CIE) 1931 chromaticity diagram was applied (Figure 9) [49]. The color calculator program SpectraChroma (Version 1.0.1) was employed to determine the chromaticity coordinates from the emission spectra (CIE coordinate calculator) [50].

Table 5. CIE chromaticity coordinates of Dy³⁺ and Eu³⁺ single-doped and Dy³⁺/Eu³⁺ co-doped 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅ glass.

Glass Composition	λex (nm)	Chromaticity Coordinates (x, y)
50ZnO:40B2O3:5WO3:5Nb2O5:0.25Dy2O3	385	(0.412, 0.446)
50ZnO:40B2O3:5WO3:5Nb2O5:0.5Dy2O3	385	(0.416, 0.452)
50ZnO:40B2O3:5WO3:5Nb2O5:0.75Dy2O3	385	(0.413, 0.448)
50ZnO:40B2O3:5WO3:5Nb2O5:1Dy2O3	385	(0.413, 0447)
50ZnO:40B2O3:5WO3:5Nb2O5:0.5Eu2O3	394	(0.649, 0.343)
50ZnO:40B2O3:5WO3:5Nb2O5:0.5Dy2O3:0.25Eu2O3	363	(0.442, 0.438)
50ZnO:40B2O3:5WO3:5Nb2O5:0.5Dy2O3:0.5Eu2O3	363	(0.464, 0.424)
50ZnO:40B2O3:5WO3:5Nb2O5:0.5Dy2O3:0.75Eu2O3	363	(0.485, 0.419)
50ZnO:40B2O3:5WO3:5Nb2O5:0.5Dy2O3:1Eu2O3	363	(0.502, 0.408)

displays the calculated values. The emission color of the single Dy³⁺ (0.25–1 mol%)-doped glass is yellow-orange. Their values are almost identical and appear indistinguishable in the figure. When the resulting 0.5 mol% Dy³⁺-doped glass is co-doped with Eu³⁺ at a concentration of 0.25–1 mol%, the emission color becomes darker orange, and for a single Eu³⁺-doped glass, the color is pure red. The obtained results show that the investigated glass has the potential to be used as orange-emitting materials, with tunable color output, depending on the rare earth doping proportion.

3. Materials and Methods

Glasses with nominal composition 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅:xDy₂O₃ (x = 0.25, 0.5, 0.75, 1 mol%) and 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅:0.5Dy₂O₃:xEu₂O₃ (x = 0.25, 0.5, 0.75, 1 mol%) were obtained by the conventional melt-quenching method, using commercial powders of reagent grade WO₃, ZnO, H₃BO₃, Nb₂O₅, Dy₂O₃ and Eu₂O₃ as starting materials. The homogenized batches were melted at 1250 °C for 30 min in a platinum crucible in air. The melts were cast into a graphite mold to produce bulk glass samples. Then, the glasses were transferred to a laboratory electric furnace, annealed at 490 °C (a temperature 10 °C below the glass transition temperature), and cooled to room temperature at a very slow cooling rate of about 0.5 °C/min. The phase of the samples was established by X-ray phase analysis with a Bruker D8 Advance diffractometer (Karlsruhe, Germany) using Cu Kα radiation in the 10° < 2θ < 80° range. The differential scanning calorimetry (DSC) was performed using NETZSCH Jupiter STA 449 F5 apparatus (Selb, Germany) with a Pt/Pt-Rh thermocouple in a platinum crucible with a cover. The sample was heated from room temperature up to 800 °C at a heating rate of 10 K/min under an atmosphere of argon. The density of the obtained glasses at room temperature was measured by Archimedes’ principle using toluene (ρ = 0·867 g/cm³) as an immersion liquid on a Mettler Toledo electronic balance (Parramatta, NSW, USA) of sensitivity 10⁻⁴ g. The Raman spectra were recorded on a Via Qontor Raman Confocal Microscope (Renishaw plc, Wotton-under-Edge, UK) with a laser wavelength of 532 nm (Nd:YAG-Laser, London, UK). Photoluminescence (PL) excitation and emission spectra at room temperature were measured for all glasses with a Spectrofluorometer FluoroLog3-22, Horiba, Jobin Yvon (Longjumeau, France).

4. Conclusions

In summary, 50ZnO:40B₂O₃:5WO₃:5Nb₂O₅ luminescent glasses containing Dy³⁺ and Dy³⁺/Eu³⁺ were prepared by the standard melt-quenching method. The densities are in the range of 3.887–3.991 g/cm³. The glass transition temperature for all glasses is over 500 °C, while the crystallization temperature, T_c, varies between 695 and 730°C. All glasses are characterized by a high thermal stability, i.e., ΔT = T_c − T_g between 178 and 210 °C. Raman analysis revealed that the glass network consists of [WO₄]²⁻ and [NbO₄]³⁻ tetrahedral units, NbO₆ octahedra, and metaborate units, [BØ₂O]⁻. The existence of Dy³⁺ → Eu³⁺ energy transfer was confirmed by the fact that in the co-doped glasses, intensity of the Dy³⁺ transitions decreases progressively with increasing Eu³⁺ concentration, while the intensity of Eu³⁺ emission bands increases. Depending on the rare earth doping content, the researched glasses may be used as orange light-emitting materials. The CIE coordinates may be modified by altering the dopant ratio and the obtained color shifted from yellow-orange to the dark orange light region.