In this paper, a broad combined orange-red emission from Eu2+- and Eu3+-doped low-silica calcium aluminosilicate (LSCAS) glass is reported. Spectroscopic results demonstrate that it is possible to tune the emission wavelength by changing the excitation wavelength in the UV-Vis region. The color coordinates for the emission spectra were calculated, and using the Commission Internationale de l’Éclairage 1931 and 1976 chromatic diagrams, it is possible to note that they are dependent on the excitation wavelength. In addition, the (u’, v’) color coordinates for the investigated LSCAS samples are close to the Planckian spectrum in the cold region between 2000 and 2600K. Our results show that the Eu:LSCAS system can be used in a white light phosphor when mixed in aggregate with phosphors using green-yellow luminescent ions.
© 2012 OSA
In the last few years, there has been a growing interest in the search for new phosphor materials for white light (WL) generation using light-emitting diodes to replace incandescent and halogen lamps. The ideal WL device should have long durability, environmental benefits and a high color rendering index (CRI) that is close to that of the black body spectrum. With the development of light-emitting diodes (LEDs) that emit in the blue and UV regions, WL devices have been produced by coupling them with yellow-emitting phosphors; for example, blue GaN-based LEDs have been coupled with Ce3+:YAG  or Ce3+,Li+:Sr3SiO5 [2, 3]. Unfortunately, these WL phosphors do not exhibit a high CRI, mainly due to their low emission in the red spectral region. To solve this problem, many red-emitting components (Eu3+, Sm3+ and Pr3+) have been added as dopants or co-dopants in different structures [2, 4]. However, this alternative is not always efficient, as in the case of Ce3+,Li+:Sr3SiO5 co-doped with Pr3+, in which practically no luminescence due Pr3+ was observed . Another inconvenience is the long luminescence lifetime and narrow emission lines of most rare earth ions due to the forbidden 4f −4f transitions, which are not suitable for fast WL devices.
Some rare earth ions are known to be incorporated into matrix hosts as divalent ions that exhibit a transition metal-like behavior. Among these, Eu2+ ions have attracted significant attention as a potential material for phosphors, principally due to their strong emission maximum dependence (near the blue, green and red regions of the visible region) on the material structure [5–9]. In addition, Eu2+-doped materials exhibit a broad absorption band from the UV up to the blue region due to their 4f-5d transition to be sensible to the crystal field and their covalency, which makes it possible for them to be excited by different light emitting diodes.
Beginning in the last decade, low-silica calcium aluminosilicate glass (LSCAS) containing different dopants has been prepared to study their thermo-optical and spectroscopic properties. Since then, Nd3+:LSCAS, Yb3+:LSCAS and Er3+:LSCAS glasses have been tested in a cavity, and laser emission was observed at 1077 , 1037  and 1540 nm , respectively. In addition, when doped with Ti3+, a long lifetime, broadband emission in the red region, a high fluorescence quantum efficiency and optical gain were observed, indicating that this glass system is candidate for use in tunable lasers [13, 14]. Recently, europium ions were reported to be present in LSCAS glasses in two valence states, where the ratio Eu3+/Eu2+ was approximately 7:3 . Over the past few years, we have studied Ce3+:LSCAS as a promising material to generate white smart light [16, 17]. Our previous spectroscopic results showed that this Ce-doped glass exhibits two broad emission bands centered at 475 and 540 nm, whose intensities can be tuned by changing the excitation wavelength. Moreover, we observed that the same emission can be achieved over a correlated color temperature (CCT) range from 3200 K to 10000 K, and a CRI of approximately 75 can be obtained by changing the optical path length of the sample .
Here, we report a broad combined orange-red emission band of Eu2+- and Eu3+-doped LSCAS glass. Spectroscopic characterization (absorption, excitation, luminescence and lifetime) indicated that the Eu2+ orange emission can be combined with the Eu3+ red emission for white light phosphors.
2. Experimental details
The glass samples, in wt%, were prepared using high-purity oxides of 41.5-x Al2O3 (5N), 47.4 CaO (5N), 7 SiO2 (5N), 2.1 MgO (5N) x Eu2O3 (4N), where x = 0.2, 0.5, 1.0, 2.0, 4.0 and 6.0. The mixture was melted under vacuum atmosphere at 1600 °C for two hours. The details regarding the glass preparation can be found elsewhere .
Optical absorption spectroscopy in the UV-Vis region was performed using a Perkin Elmer Lambda 900 spectrophotometer. The optical excitation experiments were performed using a 450 W Xe+ lamp and a H10D Jobin Yvon monochromator. The optical emission was collected by an optical fiber and analyzed by a Triax 320 Jobin Yvon monochromator with a 600 grooves/mm grating that was coupled to a Peltier cooled charge-coupled-device (CCD) detector. The excitation/emission contour plots were obtained by scanning the excitation wavelengths from 200 to 520 nm in 5-nm steps. Additional excitation spectra were recorded using a Hamamatzu Photomultiplier at different wavelengths corresponding to the maximum of the Eu2+ or Eu3+ emissions. The obtained excitation spectra were normalized using the lamp emission spectra.
Time-resolved luminescence experiments were performed to obtain the temporal behavior of the emission decay. The samples were excited with the third or fourth harmonics of a pulsed Nd3+:YAG laser (Spectra Physics, model Quanta-Ray GCR130), which delivers 10-ns pulses at a 10-Hz repetition rate. The emission of the samples was analyzed by an Oriel f-125 monochromator with a grating of 400 grooves/nm calibrated using a Hg fluorescent lamp and detected by an Instaspec V ICCD.
The luminescence quantum efficiency (η) of the samples was measured using dual-beam mode-mismatched Thermal Lens Spectroscopy (TLS). In this experimental setup, an Ar+ laser at 457 nm was used to excite the sample, and a HeNe laser at 632.8 nm was used to probe the thermal lens (TL) effect. For details on the TLS methodology, see .
3. Results and discussion
Figure 1 shows the absorption spectra for 0.5 wt% Eu2O3-doped LSCAS and undoped LSCAS glasses. The eg absorption band of Eu2+, centered at approximately 340 nm (29400 cm−1), is clearly observed, and a superposition between the charge transfer absorption of the host glass and the t2g absorption band of Eu2+ at higher energy (> 33330 cm−1) is also observed. However, no evidence of the f-f transitions due Eu3+ is exhibited.
Contour plots of the excitation versus emission spectra obtained from LSCAS glasses doped with different Eu2O3 concentrations (0.2, 0.5, 1.0, 2.0, 4.0 and 6.0 wt.%) are shown in Fig. 2 . The excitation was performed from 220 to 520 nm, and the emission was observed in the range between 475 and 765 nm. It is possible to observe in the combined spectra an intense luminescence between 575 and 640 nm and at approximately 700 nm, corresponding to 5D0 → 7FJ (J = 0, 1, 2, 3) and 5D0 → 7F4 emission transitions of the Eu3+ ions, respectively. These lines are observed primarily over a wide excitation band from 220 to 300 nm, and the sharp lines at 395 and at 465 nm, corresponding to the f-f transitions 7F0 → 5D3,2, are not observed in the absorption spectrum. The UV excitation region is characteristic of the Eu3+-O charge transfer energy (CTE), as noted by the absorption spectrum in Fig. 1. This UV absorption results in a red emission characteristic of the Eu3+, indicating a possible energy transfer between the host-Eu2+ and the Eu3+.
Simultaneous to the intense red emissions mentioned above, a broad emission band can also be observed from 450 to 750 nm under a broad excitation range from 250 to 475 nm, which can be assigned to the (4f65d) → 4f7 transition of the Eu2+ ions. This feature is more pronounced in the 0.5 wt.% Eu2O3-doped LSCAS glass and disappears for concentrations higher than 4 wt% Eu2O3. The excitation spectrum at 550 nm (18180 cm−1) and the emission spectrum at 350 nm (28570 cm−1) for the 0.5 wt.% Eu2O3 doped sample are shown in Fig. 3(a) . Under excitation at this wavelength, only the broad Eu2+ orange emission band is observed, which is centered at 584 nm (17125 cm−1) with a full width half maximum (FWHM) of 5450 cm−1. This band is broader than that observed in Eu:Al2O3-La2O3-SiO2 (ALSO) glass (4865 cm−1), whose emission peak position is at approximately 455 nm (21980 cm−1) . This significant red shift indicates that the Eu2+ ions are subject to a high crystal field [8, 9], similar to that observed for the Ce3+:LSCAS. In this case, an emission band centered in the yellow region was observed, similar to the typical spectrum of a garnet structure [12, 13].
Figure 3(b) shows the excitation spectrum that was monitored at 618 nm (16180 cm−1) and the emission spectrum measured using excitation at a wavelength of 395 nm (25320 cm−1). These excitation and emission peaks can be better understood by examining the partial schematic energy levels diagram depicted in Fig. 3(c). The t2g and eg Eu2+ excitation bands are shifted towards longer wavelengths in the visible spectral region, with a higher shift than that observed in other europium-doped oxide systems [5, 6]; they also overlap with the well-known f-f transitions of Eu3+. While examining the emission spectrum, note that both Eu2+ and Eu3+ are responsible for the orange-red emissions in this glass when the excitation wavelength is at 395 nm: the broad band emission of Eu2+-doped LSCAS glass is assigned to the allowed 4f65d → 4f7 transitions, and the fluorescence emission peaks of Eu3+ are due 5D0 → 7FJ (J = 0, 1, 2, 3 and 4), which are numbered in part (c). Time-resolved luminescence measurements were performed in 0.5 wt.% Eu2O3-doped LSCAS glass by excitation at 323 nm to examine both the fast (Eu2+) and slow (Eu3+) components, as shown in Fig. 4 . The eg band position (lower energy) suggests that Eu2+ ions possess cubic symmetry . Furthermore, a very large Stokes shift of approximately 1000 cm−1 is observed in this glass. This shift can be interpreted by the glass optical basicity, as suggested by Wang et al., who investigated Eu-doped borate glasses  and suggested that in a system with high optical basicity, there is a red shift in the Eu2+ absorption bands. The optical basicity for the LSCAS system was recently determined to be 0.83 ; this value is greater than those obtained from soda-lime (~0.6) , phosphate (~0.5)  and borate (0.48-0.59)  glasses, which reinforces the assumption of Wang et al.
The decay curve of the fast Eu2+ fluorescence observed in Fig. 4(b) is non-exponential, primarily due to the distribution of Eu2+ sites in the host, as previously observed for Yb3+- and Ce3+-doped LSCAS glasses [11, 15, 16]. By integrating the curve in (b), an average lifetime of 1.0 μs was determined. This lifetime value is reasonable for the allowed transition, 4f65d1 → 4f7 (or 5d → 4f for Eu2+), and it is in agreement with the characteristics of other Eu2+-doped systems found in the literature . Considering the long Eu3+ luminescence at 620 nm, the dynamic is well-fitted by a single exponential function, yielding a lifetime of approximately 1.1 ms.
The lifetime values for both Eu2+ and Eu3+ for the 0.5 wt.% Eu2O3-doped LSCAS glass (5 × 1019 ions/cm3) are near the value found in the literature for fluoro-phosphate glass with (mol%) 10Sr(PO3)2, 10 MgF2, 30 CaF2, 15SrF2 and 35 AlF3 doped with 5 × 1019 ions/cm3 of Eu2O3 . Although Eu3+ and Eu2+ emit in the same orange-red regions, no evidence of any energy transfer mechanism was observed by the time-resolved luminescence study.
To assess the Eu2O3-doped LSCAS glass emission in the orange-red region and their dependence on the excitation wavelength, their emission spectra for excitation at 394, 405 and 465 nm were deconvoluted using the three color matching functions established by the Commission Internationale de l’Éclairage (CIE) in 1931, known as the CIE 1931 x-y color matching functions . Figure 5 shows these calculated coordinates for each excitation wavelength in the (x, y) chromaticity diagram. Note that the coordinates are strongly dependent on the excitation wavelength and that the coordinates are located in the orange-red region. By combining the glass emission with the 405 nm component of the UV LED used to excite phosphors, the color coordinates can be displaced along the dotted line.
An important parameter for evaluating a visible light source is the previously mentioned correlated color temperature (CCT), which is a specification of the color appearance of the light emitted relating its color to the color of light from an ideal black-body radiator reference source at a particular temperature. The black-body spectrum is considered an ideal spectrum due its similarity with the day light spectrum. The CCT is determined by evaluating the distance from the color coordinates to the Planckian locus. However, this distance can only be calculated in a perceptual uniform color diagram known as the CIE 1976 (u’, v’) color space, which is a transformation of the CIE 1931. Therefore, the distance from the Planckian locus to the (u’, v’) color coordinate (Duv) indicates how close the tested light source is to the ideal source and its CCT . Table 1 summarizes the color coordinates, Duv and CCT, for three different excitations. The excitations at 394 and 465 nm both match the Eu3+ absorption lines, and their red emission lines are responsible for the low CCT values. Under 405 nm excitation, the Eu:LSCAS glass exhibits broader emission due to Eu2+, which displaces the color coordinates to the orange and consequently increases the CCT. The Duv values indicate that the color coordinates are near the Planck locus. Note that the Duv calculated values in Table 1 do not consider the contribution of the excitation source; this if the Duv was considered, it could assume the lowest values that are closest to the Planckian locus.
The 0.5 wt.% Eu2O3-doped LSCAS glass exhibits the highest luminescence quantum efficiency (approximately 50%) among the samples studied. Note that our measured η value does not distinguish the contributions of the Eu3+ and Eu2+ emissions. When the sample is excited at 457 nm, the entire emission band is integrated and the average emission wavelength is considered to determine η. The resulting η value for our system is considerably higher than the η = 8% reported in the literature for Eu2+:SrBA glass  and also the η = 30% reported in the literature for Eu2+:Sr2SiS4 phosphor .
In this work, a systematic study was performed on the excitation and time-resolved emission luminescence of Eu-doped LSCAS glasses. Our results showed that the coexistence of Eu2+ and Eu3+, which is induced by preparing the sample under vacuum atmosphere, provides a broad combined orange-red emission when the sample is excited in the UV-blue region. In addition, this emission can be tuned according to the excitation wavelength used. The emission peak shifting towards longer wavelength is interpreted in terms of the larger basicity than in other glass systems. The emission peak position near the orange region indicates that this is a potential material for use in phosphors, which is supported by the good values of Duv, which indicate that the color coordinates are near the Planckian curve.
The authors thank CAPES/COFECUB Brazil/France cooperation Grant No. 565/07, Fundação Araucária, FUNDECT, CNPq, FINEP and CNRS-UCBLyon1 for their financial support.
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