Various Yb2+-containing fluoride glasses melting under a reductive atmosphere were prepared. The brightest white light emission was observed for an AlF3-based fluoride glass not containing Hf or Zr. The largest full width at half maximum of the white emission spectra was 202 nm. In addition, incorporation of chloride into the AlF3-based glass enabled efficient excitation with near-ultraviolet light corresponding to a GaN bandgap of 3.4 eV and the maximum internal quantum efficiency of Yb2+: AlF3-based fluoride glass was 42%.
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White light-emitting diodes (LEDs) have recently attracted interest as efficient light sources and have conventionally been fabricated with a yellow phosphor (YAG:Ce3+) and blue GaN chips . However, several problems with the light quality from these types of white LEDs have been reported, including poor color rendering, color shifts caused by changes in current, and an uncomfortable glare [2, 3]. Thus, much effort has been directed toward addressing these issues. Multicolor phosphor systems (e.g., red, yellow, green, and blue) excited with near-UV light have also been proposed . However, mixing different color phosphors causes low luminous efficacy and undesirable luminescence because the light emitted from blue phosphors is internally absorbed by red-green phosphors [3, 4]. In order to avoid this “re-absorption”, phosphors which have very weak absorption in blue regions and based on near-ultraviolet (UV) excitation are developed, such as yellow-emitting phosphors (Ca1 − x − y, Srx, Euy)7(SiO3)6Cl2 . Single-phased multi-activators with co-doped materials capable of generating white light under UV excitation are also developed, which are based on the luminescence and energy transfer between multi-activators, such as Eu2+/Mn2+ , Ce3+/Mn2+ , Ce3+/Eu2+ . In contrast, Yb2+ doped crystalline host have been recently demonstrated as a green phosphor, such as Yb2+:α/β-SiAlON [9, 10].
Glass phosphors emitting white luminescence have also been investigated, containing Ag/Eu3+ , Cu+/Eu3+ , Cu+/Sm3+ , Ce3+/Sm3+ , Tb3+/Sm3+ , Mn2+ without rare earth cation  in various glass. The glass phosphors can be formed in platelike shape directly without epoxy-resin, therefore they have advantages for practical use of LED to avoid aging of the coating layer of LED under long-term UV irradiation or thermal degradation [11, 12, 17].
On the other hand, Verwey et. al demonstrated Yb2+-doped SrF2–YF3–AlF3–MgF2 (SYAM) fluoride glass which emits wide-band luminescence at visible wavelengths; however, its peak excitation wavelength is less than 250 nm, and the glass exhibits a grayish color as a result of the melting process in a reductive atmosphere . While this glass phosphor have high quantum efficiency (QE), the excitation wavelength (deep-UV) is much shorter than visible wavelengths; thus, thermal losses corresponding to Stokes shifts are unavoidable in visible applications. Moreover, compositions of Yb2+:fluoride glasses for white LED have not been investigated yet. If Yb2+:fluoride glass can be efficiently excited at near UV light, re-absorption less, epoxy resin free, novel white LED will be realized.
We investigated the potential use of Yb2+:fluoride glasses excited with near-UV light in visible applications. First, suitable fluoride glass compositions containing highly reduced Yb ions that maintain high transmittance even under a reductive atmosphere were identified. Chloride ions were then incorporated into fluoride glasses to intensify white emissions, and the QEs of the modified glasses were evaluated.
Three fluoride glass systems were prepared with the starting compositions in the mole percentages as listed in Table 1. In addition, three types of glass were prepared for each glass system: (1) YbF3-doped glass (x = 0.2) melted under a reductive atmosphere, (2) YbF3-undoped glass (x = 0) melted under a reductive atmosphere to confirm the behavior of the host glass, and (3) YbF3-doped glass (x = 0.2) melted under an oxidizing atmosphere to obtain a reference sample that did not contain Yb2+ ions. Thus, a total of nine glass samples were prepared in the first phase of the study.
The concentration of YbF3 was experimentally determined in advance to avoid concentration quenching. All fluoride raw materials were high purity grade (Fe, Ni, Co, Cu < 50 ppb, O < 100 ppm) and obtained from Central Glass Co., Ltd. After the raw materials were mixed, NH4FHF (1% by weight) was added to each fluoride mixture, followed by heating at 350°C for 1 h in a glassy carbon crucible to eliminate oxide impurities. These batches were then melted at 1000°C for 1 h. During melting, the atmosphere in the furnace was chosen for different conditions: 5 l/min of 3% H2 + 97% Ar (H2/Ar) gas for the reductive atmosphere, and 10 l/min of 0.5% Cl2 + 99.5% N2 (Cl2/N2) gas for the oxidative atmosphere. After the batches had melted, the crucibles were cooled on a room temperature copper plate without casting the melts. The entire procedure, from mixing the raw materials to cooling the melt, was carried out in an N2-filled glove box directly equipped with an atmosphere controllable furnace.
The glass transition temperatures (Tg) and crystallization temperatures (Tx) of the glasses were measured using a Rigaku DSC8270 differential scanning calorimeter. The excitation and emission spectra of the obtained glasses were measured at 300 K using a Jasco FP-6500 spectrofluorometer. The transmittance spectra of the glasses were measured using a Hitachi U-4100 spectrophotometer. Finally, the QEs of the Yb2+ fluoride glasses were measured using a FP-6500 spectrofluorometer equipped with a JASCO ILF-533 integrating sphere. Samples (6 × 6 × 1.5 mm) were polished and placed into the integrating sphere. The diameter of the excitation beam on the sample was approximately 5 mm. The internal (ηin) and external (ηex) QEs were calculated  as,
where,, and correspond to the number of photons during excitation and the reflectance and emission wavelengths of the sample, respectively.
3. Results and discussion
Figure 1 shows photographs of the glasses (diameter: 35 mm and thickness: 6 mm) melted under an H2/Ar (reductive) atmosphere excited at different wavelengths. The glasses melted under the Cl2/N2 (oxidative) atmosphere did not exhibit any luminescence when excited with UV light (photograph not shown). As can be seen in Figs. 1(b) and 1(c), YbF3-doped Al glass excited with near-UV (365 nm) and deep-UV (254 nm) wavelengths exhibited strong white luminescence. The broad emission from the Yb2+ centers is due to 4f5d → 4f transitions ; therefore, these results indicate that the divalent Yb ions are stabilized in the Al glass. On the other hand, the intensity of white luminescence from the Al–Hf and Al–Zr glasses excited with near-UV light (365 nm) was lower than that of the Al glass, while the Al–Zr and Al–Hf glasses exhibited strong bluish luminescence following deep-UV (254 nm) excitation, suggesting the possible generation of other emission centers in the Zr- and Hf-containing glasses.
Figure 2 shows the excitation and emission spectra for the Yb2+:fluoride glasses excited with near-UV light (300 nm). The detail of the transitions and energy diagram for Yb2+ ions in CaF2 hosts have been described by Nicoara et al. . Each 5d level of the 4f135d(2F5/2) and 4f135d(2F7/2) orbitals splits in the crystal field into t2 and e levels ; therefore, at least four strong excitation bands are typically observed. With the Al glass, which is an amorphous host, five excitation peaks were also observed (Fig. 2). The emission spectrum of the Al glass covers the entire visible region, and the full width at half maximum of its white luminescence was 202 nm. The white emission intensity notably decreased with the addition of Hf and Zr. Note that because the excitation spectrum of the Al–Zr glass increased below 275 nm, the fluorescence properties of this material excited at short wavelengths were investigated and are presented below.
The excitation and emission spectra of the doped and undoped Al–Hf and Al–Zr glasses excited with deep-UV light (235 nm), which led to blue emissions, are presented in Fig. 3. Strong bluish-white light emissions centered around 450 nm were observed, even for the YbF3-undoped glasses. In addition, the emission intensities decreased with the incorporation of YbF3. Here, these bluish broadband emissions are attributed to the reduced species, Zr4+ and Hf4+. Fluorohafnate glasses melted under a 5% H2/Ar atmosphere were previously found to exhibit luminescence around 340–460 nm under X-ray excitation, and the presence of Hf3+ in these glasses was also suggested . In addition, the results of an electrochemical study of a Yb-doped fluorozirconate melt suggested the presence of both Yb2+ and Zr3+ . It is believed, on the basis of these reports that Zr3+ and Hf3+ exist in the Al–Zr and Al–Hf glasses melted under an H2/Ar atmosphere.
4. Estimates of reduction ratios for Yb3+ in Al, Al–Hf, and Al–Zr glasses
Because white luminescence in these glasses was considered to originate from Yb2+, the reduction ratio, Δ = Yb2+/(Yb2+ + Yb3+), was estimated by decreasing the absorbance of trivalent Yb3+ (2F7/2 → 2F5/2) around 972 nm and comparing the transmittance spectra of glasses melted under Cl2/N2 and H2/Ar atmospheres.
Figure 4 shows the transmission spectra of 6.0-mm-thick fluoride glasses melted under different atmospheres. It can be seen from Fig. 4(a) that a new absorption band below 400 nm appeared in the spectra of the three YbF3-doped glasses following melting under an H2/Ar atmosphere, while absorption edges appeared below 200 nm in the spectra of the three glasses melted under a Cl2/N2 atmosphere. These results suggest that the reduction of Yb3+ not only occurred in the Al glass, but also in the Al–Hf and Al–Zr glasses. Although the previously reported SYAM glass was grayish in color due to the reductive atmosphere , the transmittance of the Al glass in the visible region (>425 nm) was independent of the melting atmosphere. On the other hand, the transmittance of the Al–Hf and Al–Zr glasses slightly decreased when melted under H2/Ar. Therefore, the Al glass provided the best composition to maintain transmittance, even under reductive conditions.
The transmittance of the undoped fluoride glasses melted under an H2/Ar atmosphere can be seen in Fig. 4(b). The absorption edges in the spectra of the host glasses are less intense than those of the Yb-doped glasses, and the increased intensity of the absorption in the UV region in Fig. 4(a) indicates the generation of Yb2+. In addition, only the Al–Zr glass that melted under an H2/Ar atmosphere exhibited wavelength-independent absorption. This type of absorption is generally considered to occur because of aggregates of carbon black, metal, or lower valence cations such as Zr3+ . Aggregates of Zr3+ in the Al–Zr glass may be the main source of absorption because the Al and Al–Hf glasses did not exhibit any wavelength-independent losses. Notably, this wavelength-independent loss in the Al–Zr glass could be almost completely suppressed by incorporating 0.2 mol% YbF3.
Figure 5 shows the normalized transmittance at 972 nm, which corresponds to the Yb3+ (2F7/2 → 2F5/2) transition, from which the background losses described in Fig. 4(a) have been deducted. The reduction ratio Δ was estimated from the decrease in absorbance for the glasses melted under different atmospheres, as shown in the inset of Fig. 5. As a result, the Δ values were calculated to be 11 ± 1% for the Al glass, 9 ± 1% for the Al–Hf glass, and 10 ± 1% for the Al–Zr glass, considering the accuracy of the measured quantities of raw materials used. These results revealed that the percentage of Yb2+ ions in the different glasses was nearly the same. However, the intensity of the white luminescence of the Al–Hf and Al–Zr glasses was much lower than that of the Al glass, suggesting that energy transfer occurs between the Yb2+ and Hf3+/Zr3+ ions; thus, the co-existence of these reduced species quenches not only the white luminescence from Yb2+, but also the bluish luminescence from Hf3+/Zr3+.
Fluoride glasses are typically highly doped with ZrF4 or HfF4, particularly those used in optical fiber applications, to lower Tg (<300°C) and reproducibly obtain optical fibers. It is difficult in these glass systems to obtain efficient white luminescence of Yb2+ due to the presence of Hf3+/Zr3+. Tg and Tx of the Al, Al–Hf, and Al–Zr glasses produced in this study are listed in Table 1. As can be seen in the table, the elimination of Hf/Zr from the fluoride glass compositions resulted in high Tg and Tx values.
5. Enhancement of white luminescence of Al glass
As discussed above, the Al glass exhibited the highest white luminescence intensity of the prepared glass samples. Host glasses, such as those with fluoride, chloride, bromide, or iodide compositions, which generally have low phonon energies, have higher emission efficiencies due to the low non-radiative decay rates and high radiative emission rates of rare-earth ion levels [24, 25]. Thus, we primarily investigated the incorporation of non-fluoride halide ions (Cl–) into Al glass to intensify their white luminescence. The glass compositions used in this second phase of the study are listed in Table 2. The raw material, BaF2, was replaced with BaCl2 and SrCl2. The concentration of YbF3 (x = 0.2) and the reduction conditions were the same as those used to prepare the initial Yb-doped Al glass. Note that the “Al glass” in the first entry of Table 2 has exactly the same composition as that in Table1.
The excitation and emission spectra of Yb2+:Al glasses containing added chloride can be seen in Fig. 6. An excitation peak near 365 nm can be observed in each spectrum. Notably, as more fluoride ions are replaced with chloride ions (BaF2 → BaCl2; Al → Al-1 → Al-2), the intensity of the 365 nm peak is increased remarkably. This wavelength is close to the bandgap of GaN (3.4 eV); therefore, Yb2+:fluoride glasses can be excited by conventional GaN semiconductor light sources. In addition, when BaCl2 was substituted with the same amount of SrCl2 (Al-2 → Al-3), the excitation peak shifted to a longer wavelength. The chromaticity diagram corresponding to the emission spectra are shown in Fig. 7. The luminescence color of Al-2 shifted to blue, while that of Al-3 returned to white because the emission intensity of Al-3 in the green and red regions was improved remarkably.
The incorporation of non-fluoride halide ions in AlF3-based fluoride glasses has been discussed , and promotion of the reduction of rare earth ions has been confirmed experimentally. The reduction ratio Δ of the Al-3 glass was estimated to be 45% using the same procedure as that in Fig. 5. Because this value is approximately four times greater than that of the original Al glass, it can be concluded that the number of Yb2+ ions increased in the fluoride glass.
Xia et al. recently prepared Yb2+-containing silica glass (SiO2–Al2O3–Yb2O3) by heating the relevant mixture at 1950°C for 8 h under vacuum conditions . The excitation band of this glass was wider (up to 500 nm) than that of Al-3 glass; therefore, reabsorbance of the blue light emitted from Yb2+ may have led to a decrease in the luminescence efficacy. On the other hand, because the excitation band of the Al-3 glass was below 400 nm, the overlap of the excitation and visible emission bands was minimized in this glass.
6. Quantum efficiencies of Yb2+:Al glasses
Finally, the QEs of the Al glasses were measured and are listed in Table 3. The highest QE was obtained for the Al-3 glass, with ηin calculated to be 42% and ηex calculated to be 34%. On the other hand, the QE of Yb2+:β-SiAlON (a green phosphor) measured at an excitation wavelength of 480 nm was reported to be ηin = 28% and ηex = 9% using the same approach . Thus, the Yb2+-doped fluoride glass had moderately higher QE than the Yb2+-doped crystalline phosphor. Because these Yb2+ fluoride glasses demonstrated efficient visible emission under near-UV excitation, further improvements to the reduction ratio and optimization of the glass composition should enable the development of even more efficient white emission glasses.
We successfully prepared white luminescent Yb2+:AlF3-based fluoride glasses that were efficiently excited with near-UV light. Fluoride glasses containing Hf or Zr that melted under an H2/Ar atmosphere exhibited bluish white luminescence, but the co-existence of Yb2+ and Hf3+/Zr3+ resulted in the suppression of white luminescence. On the other hand, the white luminescence of Yb2+ was notably intensified by incorporating chloride through replacing BaF2 with BaCl2 and SrCl2. Although the highest reduction ratio (Δ = Yb2+/(Yb2+ + Yb3+)) was only 45% for the chloride-containing glass, ηin was 42% and ηex was 34%. Further improvements in the QE are required for practical visible applications; however, such enhancements can be achieved by analyzing the mechanism responsible for chloride incorporation.
Fluoride glass is a low melting, inorganic glass, and it can thus be used as an inorganic material to seal LEDs and optical fibers. In addition, although AlF3-based glass has a higher Tg than fluorozirconate glass, fiber drawing techniques for AlF3-based glass have been improved . Therefore, fiber light sources using divalent rare earth ion-doped fluoride glasses will also be possible in the near future.
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