In this paper we report results of tunable lighting in Ce3+/Eu2+,3+ doped low silica calcium aluminosilicate glass. Optical spectroscopy experiments indicate that there is a red color compensation from Eu2+ and Eu3+ to the green emission from Ce3+, resulting in a broad and tunable emission spectra depending on the excitation wavelength. This result analysed in the CIE 1976 color diagram shows a close distance from the Plank emission and a correlated color temperature, varying from 5200 to 3500K. This indicates that our system can be easily excited by GaN based blue LEDs, being an interesting phosphor for white lighting devices.
© 2012 OSA
In the last decade many efforts have been done in order to develop new materials for white light (WL) sources, aiming the replacement of both fluorescent and incandescent lamps. It is already recognized that WL emission based on light emitting diodes (LEDs) presents several advantages when compared with traditional lamps such as low energy consumption and high durability [1–4]. There are basically two ways to get WL using LEDs : the first one is combining colors emitted from different LED chips and the second by using blue or ultraviolet (UV) LEDs to excite a phosphor material that has a broad emission band in the yellow region. The last one has attracted much interest due to easy fabrication, low cost, and the possibility to obtain higher brightness. In this context, several materials doped with luminescent ions have been studied in order to develop WL generators having LEDs as the primary sources [6–12].
Recently, we have shown that the Ce doped low silica calcium aluminosilicate glass (LSCAS) presented the formation of Ce3+ in the glass structure, that excited by a LED at 405 nm showed a broad emission band in the yellow region [13, 14]. The melting under vacuum atmosphere allowed the removal of OH molecules from the glass and the formation of reduced oxidation state of the luminescent doping [15, 16]. These, allied to the garnet like structure of the chosen composition with a highly distorted crystal filed around the luminescent ions permitted to obtain a broad, intense and red shifted Ce3+ emission [13, 14]. Then, our hypothesis in this work is the possibility of red color compensation by co-doping this material with Eu2+, 3+ ions. This may be achieved since the Eu ions excited by UV or violet radiation can produce emission bands in the orange-red region of the spectrum . In this way, an increase in the color rendering index (CRI) may be obtained, improving the color balance to produce ideal WL with the Ce-Eu co-doped glass.
Therefore, in this work a LSCAS glass co-doped with CeO2 and Eu2O3 was prepared in vacuum atmosphere condition. For comparison, single doped samples with CeO2 or Eu2O3 were also studied. Our results showed that the material is able to produce tunable WL, representing an important step toward the emulation of natural day light for artificial lighting.
2. Experimental Details
2.1. Sample preparation
The glass samples, in wt.%, were prepared with high purity oxides. The composition was 41.5% of Al2O3 (5N), 47.4% of CaO (5N), 7.0% of SiO2(5N), 2.1% of MgO (5N). The optimized doping in terms of a broad luminescence were 2.0% of CeO2(4N) plus 0.5% of Eu2O3 (4N). For comparison, single doped samples with 2.0% of CeO2 or 0.5% of Eu2O3 were also prepared. The melting was performed under vacuum atmosphere at 1600 °C for 2 h, what contribute to remove OH- molecules from the glass structure. The quenching was performed inside the vacuum camera with the temperature reduced to about 400 °C, followed by an annealing procedure reaching the room temperature in about 3 hours. This experimental condition was shown to be successful to obtain a high ratio of Ce3+ and Eu2+ oxidation states in the glass. After the synthesis, the glass presented yellow coloration with excellent homogeneity and transparency. For the measurement, the samples were optically polished until reaching a thickness around 2.0 mm.
2.2. Spectroscopic characterization
The following spectroscopic experiments were performed: optical absorption (OA); optical excitation (OEx) and emission (OEm); time-resolved luminescence (TRL). In addition, thermal lens (TL) spectrometry was used to measure the fluorescence quantum efficiency.
The OEx experiments were carried out using a 450 W Xe+ lamp and a H10D Horiba-Jobin Yvon monochromator. The OEm was collected by an optical fiber and analyzed by a Triax 320 Jobin Yvon monochromator with a 600-grooves/mm grating and a cooled charge-coupled-device (CCD) detector. The emission spectra as a function of excitation was obtained by scanning the excitation wavelength from 230 to 440 nm, with 2 nm steps and recording the optical emission for each excitation.
TRL experiments were carried out in order to observe the behavior of radiative decay time. The samples were excited using an OPO laser pumped by the third harmonics of a pulsed Nd3+:YAG laser manufactured by Spectra Physics, model Quanta-Ray GCR 130. It delivered pulses of 10 Hz with 10 ns of time length. The emission of the sample was analyzed by an Oriel f-125 monochromator with a grating of 400 grooves/mm and detected by an Instaspec V ICCD camera calibrated with an Hg fluorescent lamp.
The fluorescence quantum efficiency measurements were performed using the TL technique in the mode mismatched experimental configuration, described elsewhere . The excitation laser was an argon ion laser Coherent Inova 300 at 457 nm and the thermal lens effect induced by the pump laser was detected by a HeNe laser (probe beam) at 632.8 nm.
3. Results and discussion
Figure 1 shows the OA spectra from LSCAS samples doped with 0.5 wt.% of Eu2O3, 2 wt.% of CeO2 [13, 14] and also the co-doped one, with 2 wt.% of CeO2 and 0.5 wt.% of Eu2O3. The Eu-LSCAS glass exhibits an absorption spectrum with a broad band centered at 340 nm, associated to the Eu2+ 8S7/2 → eg transition, and a second one in the region of 260 nm, from 8S7/2 → t2g transition, that is overlapped with the glass band gap. Due to the forbidden f-f transition relative to the Eu3+ ion, its absorption band was not observed. As previously reported, the OA spectrum for Ce3+:LSCAS sample shows an intense absorption band centered at 340 nm, attributed to the 4f→2D3/2 electronic transition . The band centered at 280 nm is related to the electronic transition 4f→2D5/2. The Ce3+ ions present transitions that are spin and dipole permitted, resulting in higher signal as compared with those of dipole allowed of the Eu2+. The OA of Ce3+/Eu2+,3+:LSCAS sample shows an apparent influence of the two doping ions. It can be seen that there is a superposition of the band at 340 nm originating from the europium and cerium ions, with a greater influence of the cerium band. Besides, there is a greater contribution of Eu2+ ion in the regions centered at 260 and 420 nm.
As already reported in literature, LSCAS doped with Ce3+ ions have interesting luminescent proprieties for application like phosphors in the green spectral region when excited by a violet radiation [13, 14]. This glass, when doped with Eu2+,3+ ions and excited in the UV-blue region, emits a broad band centered in the orange-red region . Due to this fact, a LSCAS glass co-doped with Ce3+ and Eu2+ shows an emission centered on the yellow band, as a consequence of the emissions contribution from both ions.
Figure 2 shows the contribution of the cerium and europium ions for OEx and OEm spectra, from 300 to 430 nm and from 450 to 650 nm, respectively. It can be seen in Fig. 2(a), for the LSCAS glass doped with europium, that the excitation spectrum for the emission at 550 nm shows a broad band from 300 to 420 nm, with a maximum located at 320 nm. This indicated that the Eu2+ ions contribute for the spectral displacement to the yellow. Figure 2(b) shows the contour plot of emission for different excitation. As can be seen, the broad emission band coming from the Eu2+ remains almost unchanged, centered at 600 nm for all the excitation in this region. However, for the excitations from 390 to 410 nm, it is possible to observe emission coming from the Eu3+ ions, located in the region of 610 nm. This excitation region corresponds to the f-f transition, 7F0 →5D2, which was not observed in the absorption spectrum. Figure 2(c) exhibits the emission spectra and the photography of the samples under excitation at 405 nm. This emission presents a broad band centered at 600 nm from Eu2+ ion and a peak at 610 nm originated from the Eu3+ emission. Due to this emission, the sample present a reddish color as seen by the picture. In Fig. 2(a), the OEx spectrum for Ce3+:LSCAS glass is also shown, in which the emission centered at 550 nm shows a broad band from 350 to 430 nm, with the maximum located at 410 nm. As reported in previous works [13, 14], the contour plot in Fig. 2(b) shows a variation of the emission as a function of the excitation, shifting the maximum emission from 460 to 540 nm. Figure 2(c) shows the Ce3+:LSCAS emission spectrum for an excitation at 405 nm. A broad emission band with a maximum at 525 nm can observed, so the sample shows a greenish yellow color emission.
The excitation spectrum of the co-doped sample, recording the emission at 550 nm, shows a broad band centered at 405nm as a consequence of the contribution of the two ions. The counter plot shows an emission band centered at 525 nm for excitation in the region from 300 to 350 nm. By changing the excitation from 350 to 430 nm, the emission shows a band with maximum at 550 nm due to contribution of red emission from Eu2+ ions. Emission peaks at 610nm due to the Eu3+ ions are also present in the sample. It is possible to observe, in this excitation region, that the emission of this sample is driven by Ce3+ and Eu2+,3+ ions. Looking at the emission spectrum of this sample with excitation at 405 nm it is possible to observe a similarity between this emission and the one from single doped cerium glass. However, the co-doped sample presents an increase in the luminescence intensity in the region from 530 to 650 nm, showing a peak at 610 nm.
Figure 2(c) shows the yellow emission under excitation at 405nm. It can be noted that the emission of the sample containing the two ions has the maximum centered at 530nm, close to that of Ce3+:LSCAS, so it is proved that the Ce3+ ions has a higher influence on the emission as compared to Eu2+ ions. This is because the concentration of cerium ions is greater than that of europium and in addition, the Eu2+ transitions are electric dipole allowed, while those of Ce3+ are dipole and spin permitted .
Figure 3 shows the absorption coefficient and the excitation spectrum for emission at 590 nm for the LSCAS glass co-doped with 2 wt.% of CeO2 and 0.5 wt% of Eu2O3. It can be observed that the excitation band, responsible for the broad emission in the yellow region, adjusted in the absorption spectrum, showing two bands, one around 250nm, responsible for charge transfer in the Eu3+-O system, and around 420nm, originated from the combination of Eu2+ and Ce3+ ions absorptions, with an absorption coefficient of 6 cm−1, that is responsible for the 590 nm emission. Another characteristic is that the excitation spectrum centered at 420 nm is not coincident with the maximum absorption band centered at 340 nm. The reason for this behavior is that the cerium sites with OA at 350nm and responsible for the blue emission at 490nm present a higher absorption cross section than the sites with OA at 420nm, which are responsible for the greenish yellow emission. Despite of this difference, the sites responsible for the greenish yellow emission show a higher intensity, indicating that they are more efficient than the blue one . By exciting the sample at 415 nm a broad emission band can be observed, centered at 550 nm, which ranges from 450 to 750 nm. This band, originated from the Ce3+ and Eu2+ ions, presents some overlapped peaks at 575, 615 and 710 nm from Eu3+ ion, indicating that the three ions contribute to the emission. It is in fact a quite broad emission, providing a red color compensation from Eu2+ and Eu3+ to the green emission from Ce3+. In other words, depending on the excitation wavelength, this result shows that it is possible to achieve a broad and tunable emission spectrum with this co-doped glass.
Figure 4 shows the lifetime curves for the LSCAS glass sample doped with 1.0 wt.% of CeO2 and 0.5 wt% of Eu2O3 under excitation at 400 nm. It can be noted that the lifetime for the emission at 480 nm is around 50 ns. As can be verified by the contour plot in Fig. 2(b) and 2(c), under 400 nm excitation the emission at 480 nm is driven by Ce3+ ions. This lifetime is characteristic of Ce3+ ion, as observed in different materials found in the literature [6, 20] and also observed in LSCAS samples doped with Ce3+ ions. This result indicates that there is no energy transfer from europium to cerium ions in this material. By observing the emission at 580 nm, a non-exponential behavior of the lifetime is seen. In this case, there is an overlap between the emissions of the two ions, taking place in short and longer time intervals. The short lifetime is due to Ce3+ ion and the longer duration emission is characteristic of that from Eu2+ ions, which has been shown in the literature to have a lifetime around 800 ns [21–23]. In our sample, this emission lifetime was 290ns, calculated by using the integration method as described in ref . For the analysis by observing the emission at 610 nm, for longer lifetime, an exponential behavior is observed. This is due to the fact that both Ce3+ and Eu2+ ions stopped to emit and only Eu3+ ion still keep emitting with a lifetime of 1.5 ms. This value is similar to those found in literature [22, 24]. As the measured lifetimes for both ions do not show any difference for the LSCAS doped with Ce3+ or Eu2+,3+, the results indicated no evidence of energy transfer mechanism between Ce3+ and Eu2+,3+ ions in the co-doped glass.
Figure 5(a) shows the x,y coordinates in the CIE 1931 color diagram for different emissions of the LSCAS glass doped with 2.0 wt.% of CeO2 and 0.5 wt.% of Eu2O3, submitted to excitations from 370 to 430 nm, with steps of 5 nm. It can be observed that by increasing the excitation wavelength, the emission region shifts from the green towards the yellow region. An important measurement to assess a light source characteristic in terms of its ability to simulate natural light source, is the closest distance between its emission coordinate and the coordinates of the Planck’s emission spectrum. This distance has been analyzed with the use of the CIE 1976 u’v’ color diagram. It presents linearity between the points of the diagram, in terms of difference of color perception by human eyes [13, 25, 26]. Figure 5(b) shows the closest distance Du’v’ between the Planck’s spectrum correlated color temperature (CCT) and the coordinates u’v’ for each emission obtained by different excitations. It can be verified that by increasing the excitation wavelength, the Du’v’ distance ranges from 0.074 to 0.047. This decrease is followed by a corresponding CCT one from 5000 to 3500 K. Other characteristic that can be seen in Fig. 5(b) is that the CCT and Du’v’ curve presented two peaks at 392 and 415 nm, due to Eu3+ ion emission. These results confirm that this glass can provide color tuning, depending on the excitation pump. As seen in Fig. 5(a), the emission spectra when combined with blue or violet emissions, easily obtained from commercial GaN-based LED, can generate different colors, with CCT from 3500K to 5000K.
Table 1 shows the color properties of some materials known in the literature compared with the LSCAS glass co-doped with cerium and europium [13, 25, 26]. It is verified that the Du’v’ distance for an excitation at 415 nm has a color temperature of 3850 K close to TL84 standard light source, which has a color temperature of 4000 K. Another characteristic of this glass compared with the emission spectrum of other materials already reported in the literature, like Ce:YAG and Ce:Sr3SiO5, that are known for their high emission efficiency, is that our glass shows lower Du’v’ distance with low CCT. This is an important property, because it is known that many efficient luminescent materials reported in the literature as possible candidates for WL-LEDs have higher color temperatures because of the lack of emission in the red region. An interesting characteristic of our Ce3+-LSCAS sample co-doped with europium ions for color compensation in the red region is that the europium impurities with valence 3 + , which exist along with Eu2+ ions, also contribute to the color compensation.
The quantum efficiency of Ce3+/Eu2+,3+:LSCAS measured by the known thermal lens technique  shows that the LSCAS glass co-doped with cerium and europium reaches the fluorescence quantum efficiency of 26%. Comparing this value with other Ce doped glasses reported in literature , it can observed that this glass presented a higher value and a broader emission band.
In conclusion, the results of this work showed that LSCAS glass co-doped with cerium and europium is a potential phosphor material to produce white lighting, when excited by commercially available GaN based blue our violet LEDs. The emission coordinates for this material combined with LEDs were analyzed in the CIE 1931 x-y chromatic diagram, showing that by exciting the glass at 415 nm, the observed emission is very close to the ideal WL position. The spectroscopic results also indicate that our system can be used as tunable light source to simulate the different light emissions along the day, with good quality due to its broad emission band once excited in the violet and blue spectral regions. This is a step towards a new class of WL sources for artificial lighting with improved color rendering indexes and lower energy consumption.
The authors are thankful to CAPES/COFECUB Brazil/France cooperation Grant No. 565/07, Fundação Araucária, CNPq, FINEP and CNRS-UCBLyon1 for the financial support of this work.
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