Lead Borate Glasses and Glass-Ceramics Singly Doped with Dy3+ for White LEDs.

In this paper, some series of lead borate glasses and glass ceramics singly doped with Dy3+ ions were prepared and then studied using spectroscopic techniques. Our research includes mainly studies of the luminescence properties of received materials for white light. The luminescence bands associated with the characteristic 4F9/2→6H15/2 (blue), 4F9/2→6H13/2 (yellow) and 4F9/2→6H11/2 (red) transitions of trivalent dysprosium in lead borate systems are well observed. In particular, the Commission Internationale de I'Eclairage (CIE) chromaticity coordinates (x, y) were calculated in relation to potential applications for white light-emitting diodes (W-LEDs). Their values depend on the relative B2O3/PbO ratios and PbX2 contents (where X = Cl, F, Br) in glass composition. For glass-ceramics, the chromaticity coordinates are changed significantly under different excitation wavelengths.

1. Introduction

Rare earth doped inorganic glasses are promising amorphous materials for tricolor W-LEDs (white light emitting diodes), solid-state lasers emitting visible light or near-infrared radiation, and other optoelectronic device applications [1,2,3,4,5]. In particular, white light-emitting devices have attracted considerable interest for their use in diodes and liquid crystal monitor screens. In this field, inorganic glasses are particularly studied for generating white luminescence [6,7,8,9]. Different glass matrices were tested in order to find the best materials emitting white light. In particular, borate glasses containing various glass-network modifiers and rare earth ions acting as optically-active ions have aroused considerable attention for white-emitting laser sources.

Borate based glasses are excellent amorphous materials for numerous scientific and industrial applications [10]. Among the different oxide components, such as SiO2, GeO2 or P2O5, successfully used for glass synthesis, B2O3 is the best glass-former, which makes glass with high chemical durability and thermal stability, good transparency and rare earths solubility. Moreover, borate-based glasses mixed with different modifier oxides [11] and halides [12] have attractive optical and structural properties. These materials containing B2O3 are promising systems for optoelectronic devices, lasers and white light applications [13,14,15,16,17,18,19]. Simultaneously, boron oxide can be incorporated with either Bi2O3 [19] or PbO [20], with the dominant percent forming chemically and thermally stable glasses. Great attention has been paid to lead borate glasses due to their distinct advantages, including good radiation shielding properties, strong absorption in the UV region, large transmission windows and the relatively large refractive indices compared to pure borate glasses or alkali borate glasses [21,22]. Recent studies indicate that the properties of systems based on PbO–B2O3 containing rare earth ions are significant in the field of solid-state lasers, tri-color LEDs, reflecting windows and optoelectronic devices [23,24,25,26,27,28].

Among numerous rare earth-doped host materials, the inorganic Dy3+-doped glasses are really important systems for developing solid state lasers operating in the visible region [29]. Various lead-free [30,31,32,33,34] and lead-based [35,36,37,38,39,40] glass systems singly doped with dysprosium ions are also widely used to generate white light due to their simultaneous blue, yellow and red emissions. This behavior is associated with transitions originating from the state 4F9/2 to the lower lying states 6HJ/2 (J = 11, 13, 15). Based on the integrated intensities of the emission bands related to the 4F9/2→6H15/2 (blue) and 4F9/2→6H13/2 (yellow) transitions of trivalent dysprosium ions, the yellow to blue fluorescence intensity ratio can be calculated. The ratio Y/B (yellow to blue) for dysprosium ions is an important spectroscopic factor in evaluating the studied system for white light generation. What is more, dysprosium-doped borate-based glasses belong to promising matrices that can be used to generate white light [41,42,43,44,45,46,47], for which the ratio Y/B (Dy3+) oscillates within the limits of one. In general, oxide and oxyfluoride borate-based glasses with Dy3+ have become one of the most important classes of materials because of their potential application in numerous fields, but nowadays most of the studies are concentrated on generating white light.

Furthermore, lead borate-based glasses containing rare earth ions belong to interesting luminescent amorphous materials. The optical properties of glasses based on PbO-B2O3-Al2O3-WO3-Ln2O3 (Ln—rare earth ions) without or with PbX2 (X = Cl, Br or F) were presented and discussed in the reviewed work published by us ten years ago [48]. Structural and optical aspects for Ln3+ ions have been demonstrated in glass systems containing PbF2 [49]. The interesting results were also obtained for Dy3+-doped lead borate glass, where the stimulated emission cross-section (σem = 5.57 × 10−21 cm2 at λp = 573 nm) as well as the quantum efficiency (η = 70%) for the main 4F9/2→6H13/2 yellow transition are relatively large, suggesting its potential solid-state laser application [50]. Multicomponent glasses yPbX2-(1-y)PbO-B2O3-Al2O3-WO3-Dy2O3 were also thermally treated to fabricate glass-ceramic materials containing the nano- or microcrystal PbX2 (where X = Cl, Br or F), owing to its larger tendency to crystallize lead halide, especially lead fluoride, than oxides. Unexpectedly, the results obtained by us were completely different. Independently on PbX2, PbWO4 microcrystals with tetragonal unit cells (PDF-2 card no. P190708) were successfully formed in transparent lead borate glass-ceramics doped with Dy3+ ions [51].

This paper shows the results for lead borate glasses and glass-ceramics containing Dy3+ ions, and the applicability of these systems as white light emitters. Glass samples with Dy3+ were prepared, and in the next step, they were heat-treated in order to obtain transparent glass-ceramics. Based on the emission spectra measured for dysprosium ions in glasses and glass-ceramics, the chromaticity coordinates (x, y) were calculated in relation to their potential application in W-LEDs.

2. Materials and Methods

Glass systems singly doped with dysprosium ions containing various B2O3/PbO ratios were prepared by the traditional melt quenching technique from high-purity raw materials (99.99% Aldrich Chemical Co., St. Louis, MO, USA). The B2O3/PbO ratios are 2:1, 1:1, 1:2, 1:4, 1:5 and 1:8. Each glass composition in wt.% is presented in Table 1.

Table 1

Chemical compositions of Dy3+ singly doped lead borate glasses.

Glass No.	Ratio	Chemical Composition (wt.%)
	B2O3:PbO	B2O3	PbO	Al2O3	WO3	Dy2O3
(a)	2:1	60	30	6	3	1
(b)	1:1	45	45	6	3	1
(c)	1:2	30	60	6	3	1
(d)	1:4	18	72	6	3	1
(e)	1:5	15	75	6	3	1
(f)	1:8	10	80	6	3	1

During synthesis, the appropriate amounts of metal oxides were mixed in a glow-box in an atmosphere of dried argon and placed in a corundum crucible. The samples with B2O3/PbO ratios 1:4, 1:5 and 1:8 were melted at 850 °C, whereas the systems with B2O3/PbO ratios 2:1, 1:1 and 1:2 were melted for 1 h at 1250 °C.

Furthermore, glasses with 9 wt.% PbX2 (where X = Cl, F or Br) were synthesized for a 2:1 system of B2O3/PbO. Lead oxide was partially replaced by PbX2. To prepare glass samples, metal halides and metal oxides were mixed together in an agate ball mill. Similar to the oxide glass system, the samples were fabricated in a glow-box in an atmosphere of dried argon. The samples were melted in a corundum crucible at 850 °C for 1 h. From the prepared glass systems, the glass-ceramics were obtained. In order to fabricate transparent glass-ceramic materials, the lead borate glass systems with PbX2 (X = Cl, F, Br) were annealed at 450 °C for 5–15 h. Independently on PbX2, lead tungstate PbWO4 microcrystals with tetragonal unit cells were successfully formed in transparent lead borate glass-ceramic materials containing dysprosium ions.

Measurements were carried out on a PTI QuantaMaster QM40 spectrofluorimeter (Photon Technology International, Birmingham, NJ, USA) coupled with the optical parametric oscillator (OPO), pumped by the third harmonic of a Nd:YAG laser (OpotekOpolette 355 LD, OPOTEK, Carlsband, CA, USA). Double 200 mm monochromators were used to detect luminescence spectra. Then, the spectra with ±0.1 nm resolution were recorded using a multimode detector UVVIS PMT (R928). Decay curves were registered by a PTI ASOC-10 (USB-2500) oscilloscope with an accuracy of ±1 µs. The Raman spectra were measured using a Thermo Fisher ScientificTM DXRTM2xi Raman Imaging Microscope (Thermo Scientific, Waltham, MA, USA). The appropriate laser source with an excitation wavelength of 780 nm was used to obtain the Raman spectra. Measurements were performed at room temperature.

3. Results and Discussion

3.1. Glasses

Figure 1a shows the emission spectra for glass systems singly doped with dysprosium ions with different B2O3:PbO weight ratios.

Figure 1

Emission spectra for dysprosium-doped lead borate glass systems with different B2O3:PbO ratios (a), and the influence of B2O3:PbO ratio on spectroscopy parameters for dysprosium ions (b).

Independently on the excitation wavelengths 390 nm (4K17/2 level) and 450 nm (4I15/2 level), the spectrum consists of luminescence bands characteristic for Dy3+ ions. The spectra exhibit strong blue and yellow emission bands, which are assigned to the 4F9/2→6H15/2 and 4F9/2→6H13/2 electronic transitions of dysprosium, respectively. Moreover, a significantly weaker red emission band corresponding to the 4F9/2→6H11/2 transition is noticeable.

Figure 1b shows the dependence of the luminescence intensity ratios Y/B, related to the integrated emission intensities of 4F9/2→6H13/2 (yellow) and 4F9/2→6H15/2 (blue) transitions and the measured lifetimes for the 4F9/2 excited state of Dy3+, on B2O3:PbO ratios. Both spectroscopic parameters depend strongly on the amount of PbO in the glass composition. These aspects were examined in detail in the previously published work [52], and they are not discussed here. Among rare earths, materials doped with dysprosium ions, such as inorganic glass systems, are found to be an appropriate candidate for white light applications. Dysprosium ions reveal two intense luminescence bands in the blue and yellow spectral regions. These bands correspond to the 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions, as mentioned above. The proper combination of yellow and blue luminescence bands can produce white light. The yellow to blue luminescence intensity ratio can be adjusted by varying the glass-host [53], the pumping wavelengths [54] and the dysprosium ion concentration [55].

The ratio Y/B is influenced by the effect between the dysprosium ions and the nearest surroundings. In general, the yellow emission of Dy3+ ions is stronger if the Y/B factor is higher. Therefore, near-white light luminescent materials are possible obtainable by adjusting the yellow to blue intensity ratio of dysprosium. When the Y/B ratio is greater than unity, the degree of covalent character between the dysprosium ions and the surrounding ligands is higher, suggesting that the nature of the host environment is more disordered. Our previous experimental results clearly shown that the yellow to blue intensity ratio values decreased from 1.59 for lead germanate glass to 1.12 for lead borate glass [56]. Moreover, the Y/B ratio of the dysprosium ions changes with the reduction in the covalent character of heavy metal oxides in the following direction: GeO2→SiO2→P2O5→B2O3. Therefore, the lead borate glass-host was chosen for further research into W-LED applications. The studies on lead borate glass evidently show that the Y/B ratio of trivalent dysprosium ions seems to be 1.04 when the concentration of B2O3 is the highest (B2O3:PbO = 2:1), which is favorable for white light applications. The interesting results for lead borate glasses were also obtained in previous studies conducted by Jayasankar et al. [57], which confirm that the yellow to blue factor is more sensitive for glass hosts with lower Dy3+ ion concentrations.

In order to evaluate white light chromaticity coordinates, a CIE diagram is usually employed. This is regulated by the Commission Internationale de l’Éclairage, defined in 1931, and it expresses all colors by using the three primary colors of X, Y, and Z. The chromaticity color coordinates (x, y) can be calculated from the emission spectra [58].

The chromaticity color coordinates (x, y) of lead borate glass systems containing various B2O3/PbO weight ratios were calculated. The obtained values are given in Table 2. The coordinates for the glass samples singly doped with dysprosium ions are located in the green and green–yellow region of the CIE diagram shown in Figure 2. The presence of PbO in the glass systems causes the color coordinates to shift from the green region to the greenish–yellow for samples with the highest (80 wt.%) amount of lead oxide. However, the exception is the sample containing the highest amount of B2O3. In this case, the chromaticity coordinates are located nearest to the white point in the CIE diagram. Therefore, introducing PbO into the glass host is rather unfavorable. The less lead oxide there is in the glass systems, the closer the chromaticity coordinates approach to white. Moreover, glass materials containing Pb2+ ions are often eliminated because of their well-known hazardous effects. On the other hand, the systematic luminescent and energy transfer studies indicate that dysprosium-doped lithium lead alumino borate glasses are promising candidates for W-LED and laser applications [59].

Table 2

The CIE (chromaticity coordinates) value and the Y/B factor for Dy3+ ions in the studied glass samples with different B2O3:PbO ratios.

Glass No.	Ratio	Y/B	CIE (x, y)
	B2O3:PbO
(a)	2:1	1.04	(0.321, 0.352)
(b)	1:1	1.12	(0.319, 0.379)
(c)	1:2	1.18	(0.338, 0.386)
(d)	1:4	1.08	(0.331, 0.378)
(e)	1:5	1.19	(0.353, 0.391)
(f)	1:8	1.22	(0.375, 0.395)

Figure 2

Chromaticity coordinates in the CIE diagram for Dy3+-doped lead borate glasses with different B2O3:PbO relative ratios.

In order to gradually eliminate the lead oxide from the glass-host, lead halide borate glasses were synthesized. In this case, lead oxide was partially substituted with PbX2 (X = Cl, Br, F). At this moment, it should be mentioned that for mixed oxyhalide systems, the nominal starting composition and final glass composition are rather different due to the gases lost during the melting process, according to the chemical reaction PbX2 + H2O → PbO + 2HX↑. Previously published works suggest that the losses of fluorine or chlorine components increase when the PbX2 content in the glass composition is the greatest [60,61,62]. The presence of PbX2 in the mixed lead halide borate glasses was also evidenced by the Raman spectra measurements. Figure 3 presents the Raman spectra for glass samples in the absence and presence of PbX2 (X = Cl, Br, F).

Figure 3

The Raman spectra for lead borate glasses with PbX2 (X = Cl, Br, F).

In general, the local structure of lead borate glass was changed, from trigonal BO3 to pentaborate groups containing three- and four-coordinated boron atoms, when PbX2 was added to the base glass. This results in Raman bands at about 870 cm−1 and 925 cm−1, because of the greater number of borate units containing non-bridging oxygen (NBO). The Raman investigations clearly indicate that the intensities of these vibration bands associated with the BO4 units and NBO atoms increase with the presence of PbX2 (X = Cl, Br, F). They are assigned to the pentaborate groups [63]. Similar results were obtained earlier for the B2O3–BaF2–LiX (X denotes F, Cl or Br) glass systems [64]. The Raman bands near 1300 cm−1 are related to the maximal phonon energy of the glass host. For our lead borate glasses, the Raman band is decreased from 1300 cm−1 (glass without PbX2) to nearly 1277 cm−1 (glass with PbX2), suggesting the important role of lead halide in the formation of the glass host. This was also confirmed by thermal measurements and the calculation of bonding parameters from the absorption spectra of lead borate glasses doped with dysprosium ions [65]. The glass transition temperature Tg for glass without PbX2 is close to 440 °C, and this value is reduced for system with PbX2 (X = Cl, Br, F) in direction Br (390 °C) → Cl (375 °C) → F (340 °C), respectively. Similar effects were obtained for the bonding parameter δ. Its value can be positive or negative, suggesting the existence of more covalent or more ionic bonding between Dy3+ ions and their nearest surrounding. For lead borate glass system without PbX2, the bonding parameter is equal to δ = −0.77, indicating the ionic relation between rare earths and ligands.

The values of δ decrease when samples containing PbX2 (X = Cl, Br or F) are changed in the direction Br (−0.90) → Cl (−0.94) → F (−1.04). This indicates that the environment around the active ions (Dy3+) is more ionic [65]. Further studies revealed that the coordination sphere around the optically active ions (Er3+) is changed significantly when lead halide is introduced to the lead borate glass. The spectral bandwidth (FWHM) for the main 4I13/2→4I15/2 near-infrared laser transition of Er3+ is reduced in the following direction: Br → Cl → F. This suggests evidently that the glass structure around the optically active ions was changed during the substitution of PbO by PbX2, and the fraction of halide ions was successfully bridged with Er3+ [66].

Figure 4a presents the emission spectra for lead borate glasses containing lead halide and Dy3+ ions. Figure 4b shows schematically the Y/B factors and the measured lifetimes of 4F9/2 (Dy3+) for samples without or with PbX2 (X = Cl, Br, F). The spectra were also excited at 450 nm, and consist of emission bands characteristic of transitions of Dy3+ ions. Interestingly, all the bands exhibit an increase in emission intensity when the halide glass-modifiers are increased. However, from the perspective of white light emission, the most interesting is the ratio of yellow to blue bands. Our investigations indicate that the emission intensities of blue and yellow bands depend on the halide glass-modifier. From the calculated values, it was found that the yellow to blue factor of Dy3+ for glass samples containing lead fluoride is the best, and is near one (Y/B = 1.11), which confirms the possibility of white light emission. Similar results have also been described by Babu et al [67], whereby a Y/B factor equal to 1.05 was received for the oxyfluoride glass sample containing 1% dysprosium ions. For lead borate glass system with PbX2 (X = Cl, Br, F), the chromaticity coordinates (x, y) were also calculated. Table 3 presents all the results. Furthermore, the chromaticity coordinates show the location of the emission color in the chromaticity diagram given in Figure 5. Among the prepared samples, the lead borate glass with PbF2 presents CIE values nearest to the chromaticity coordinates of perfect white light (0.333, 0.333). In conclusion, it can be stated that lead borate glass systems with PbF2 are potentially useful for white light emission.

Figure 4

Emission spectra for dysprosium-doped lead borate glass systems containing PbX2 (a). influence of PbX2 (X = Cl, Br, F) on spectroscopy parameters for dysprosium ions (b).

Table 3

The chromaticity coordinates (x, y) for the Dy3+ ions in the studied glass samples containing PbX2 (where X = Cl, Br or F).

Glass No.	PbX2	Y/B	CIE (x, y)
(a)	without	1.04	(0.336, 0.381)
(b)	PbF2	1.11	(0.318, 0.352)
(c)	PbCl2	1.13	(0.328, 0.373)
(d)	PbBr2	1.32	(0.348, 0.421)

Figure 5

Chromaticity coordinates in CIE diagram for Dy3+-doped lead borate glass with PbX2 (X = Cl, Br, F).

3.2. Glass-Ceramics

Controlled heat treatment elicits the transformation process from precursor glasses to transparent samples of glass-ceramics (TGC). During the thermal treatment of mixed oxyfluoride systems, fluoride micro- or nanocrystals are usually formed. They are dispersed into oxide amorphous matrices. Rare earth ions are usually distributed in both crystalline and amorphous phases. This was presented for several transparent glass-ceramic materials [67,68,69,70]. The heat treatment process was also applied for lead borate glasses. With a controlled annealing time and temperature, lead borate glass-ceramic materials containing PbWO4 microcrystals and dysprosium ions were successfully fabricated and then characterized using various experimental techniques [67].

Figure 6 shows the emission spectra for lead borate glass-ceramics singly doped with dysprosium ions, which were measured under the various excitation wavelengths 310 nm, 360 nm and 390 nm.

Figure 6

Emission spectra for Dy3+ doped lead borate glass-ceramic materials under the various excitation wavelengths (a) 310 nm, (b) 360 nm and (c) 390 nm.

The spectrum obtained under excitation λexc = 310 nm consists of a broad emission band centered at 450 nm, which is characteristic for the crystalline phase PbWO4 [71,72]. With the excitation of the glass-ceramic sample at 360 nm or 390 nm, the observed bands located at about 480 nm and 573 nm are characteristic for Dy3+ ions, and correspond to the 4F9/2→6H15/2 (blue) and 4F9/2→6H13/2 (yellow) transitions, respectively. The relative luminescence intensities of the blue and yellow bands strongly depend on the excitation wavelengths 360 nm and 390 nm. In particular, the blue emission band, related to the 4F9/2→6H15/2 transition of dysprosium, is enhanced significantly when the glass-ceramic sample is directly excited at 360 nm. It can be inferred that the enhanced intensity of the blue band is the result of the superimposition of the PbWO4 microcrystals present in the glass-ceramic system and Dy3+ luminescence (details are given in [73]). This was also confirmed by the excitation spectra measurements. Figure 7 presents the excitation spectra recorded for glass-ceramics containing PbWO4.

Figure 7

The excitation spectra measured for the studied glass-ceramic materials.

When the spectrum was monitored at λem = 430 nm, an intense peak centered at about 310 nm is well observed. It is assigned to an exciton excitation, and strictly related to the presence of crystalline phase PbWO4 [74]. The spectrum monitored at λem = 573 nm consists of six bands, which originate from the transition from the 6H15/2 ground state of dysprosium ions to the following levels: 4F9/2, 4I15/2, 4G11/2, 4K17/2 and 6PJ/2 (J = 5 and 7). It is clearly seen that both the excitation bands due to the PbWO4 microcrystals and Dy3+ ions exist in the spectral range near 360 nm. Thus, the blue luminescent band corresponding to the 4F9/2→6H15/2 transition is enhanced and broadened, because PbWO4 crystallites and Dy3+ ions in the glass-ceramic sample are simultaneously excited at 360 nm.

In order to further discuss the potential application of the prepared glass-ceramic material in white light-emitting diodes, the chromaticity color coordinates are determined, and the results are given in Table 4. All the chromaticity coordinates for the TGC system excited at 310 nm, 360 nm and 390 nm were calculated from the luminescence spectra. The results for lead borate glass-ceramic materials revealed that the calculated chromaticity coordinates are completely different, depending critically on the excitation wavelengths.

Table 4

The chromaticity coordinates (x, y) for Dy3+ ions in the studied glass-ceramic system under various excitation wavelengths.

Glass-Ceramics No.	λexc (nm)	CIE (x, y)
(a)	310	(0.142, 0.081)
(b)	360	(0.305, 0.317)
(c)	390	(0.370, 0.416)

Figure 8 presents the calculated chromaticity coordinates for glass-ceramics. They shifted from blue (λexc = 310 nm) and yellow–green (λexc = 390 nm) to bluish–white light when the glass-ceramic sample was directly excited at 360 nm. Similar effects of excitation wavelengths on emission intensities and CIE coordinates have been observed for SiO2-KYF4 nano-glass-ceramics [75]. Kemere et al. [76] concluded that the chromaticity coordinates of the aluminosilicate oxyfluoride glass-ceramic materials co-doped with Dy3+/Eu3+ are not changed drastically, but the heat treatment process of precursor glasses can be effectively used to tune the color of the emitted light. Our studies suggest that lead borate glass-ceramics containing dysprosium ions are attractive for multicolor luminescence applications. These TGC materials can be used in optoelectronic devices, such as light-emitting diodes. In particular, they may be sources of white emission when pumped directly at 360 nm.

Figure 8

Chromaticity coordinates in CIE diagram for Dy3+ doped lead borate glass-ceramics under different excitation wavelengths.

4. Conclusions

To summarize, lead borate glasses and glass-ceramics singly doped with dysprosium ions were successfully synthesized via the melt-quenching technique. The received materials were studied using luminescence spectroscopy to confirm the possibility of white emission. It was proven that the Y/B factor of trivalent dysprosium ions for a system with the highest concentration of B2O3 (B2O3:PbO = 2:1) is close to 1.00. Moreover, the Y/B ratio value in the studied samples with PbX2 (X = Cl, Br, F) decreased from 1.32 to 1.11 with increases in the ionic character of lead halides from PbBr2 to PbF2. For all glass and glass-ceramic systems, the CIE values were calculated from the emission spectra in relation to the application of W-LEDs. The analysis of the CIE diagram indicates that the presence of lead oxide in borate glasses causes the color coordinates to shift from the green region to greenish–yellow for systems with the highest amounts of PbO. Among the prepared samples with PbX2, lead borate glass containing PbF2 presents CIE values nearest to the color coordinates of perfect white light. Furthermore, the tendency to shift the chromaticity coordinates from blue (λexc = 310 nm) and yellow–green (λexc = 390 nm) to bluish–white light was observed when the glass-ceramic sample was directly excited at 360 nm. Our studies demonstrate that the spectroscopic properties and chromaticity coordinates of lead borate systems can be effectively controlled by modification of the chemical composition of glass-hosts and changing the excitation wavelengths. There is an efficient way to obtain white light-emitting glasses and glass-ceramics. Based on the experimental and calculated results, we suggest the possibility of using these materials containing Dy3+ ions for future applications in white light generation.