The radiative emission properties of the Dy3+ ions in an oxyfluoride glass and glass-ceramics have been studied for the generation of white light. The x-ray diffraction pattern of the glass-ceramics shows the formation of CaF2 fluorite-type nanocrystals in the glass matrix after a suitable thermal treatment of the precursor glass, whereas time-resolved optical measurements show the incorporation of the Dy3+ ions in the CaF2 nanocrystals. Intense white light has been observed when the samples are excited with 451 nm laser light. From the visible emission spectra, yellow to blue intensity ratios and the chromaticity color coordinates have been determined. All the color coordinates are found to lie in the white light region of the chromaticity color diagram.
© 2011 OSA
White light-emitting diodes (LEDs) show high potential for replacement of conventional incandescent and fluorescent lamps because of their advantages such as long lifetime, energy saving, reliability, safety and environment-friendly characteristics . Although the majority of the research has been focused on full color phosphors for the preparation of white LEDs, glasses and glass-ceramics doped with rare earth (RE) ions could be considered as an alternate approach due to their potential advantages such as lower production cost, simpler manufacture procedure, free from halo effect, environment-friendly characteristics and so on .
Different works focused on RE3+-doped fluoride glasses have succeeded in generating white light, although codoping with at least two different RE3+ ions and/or simultaneous IR or UV-IR laser excitations at different wavelengths have been necessary [3–5]. Moreover, one of the most spectacular, but complex, color-display devices developed by Downing et al.  uses laminated stacks of individual fluorozirconate glass layers, doped with a different RE3+ ions being excited using different combinations of diode lasers appropriate for each doping ion. So far, many works have been reported on the white light generation in RE3+ ions-doped oxyfluride nanocrystalline glass-ceramics. However, the main problem comes when taking into account that they contain CdF2 and/or PbF2 [7–9], designated as specified toxic substances by the Restriction Hazardous Substance (RoHS) in 2006 . The results presented here try to simplify the methods for generation of white light since a single RE3+ ion (Dy3+) at a very low concentration of 0.1 mol% and an alternative host material free of PbF2 and CdF2 such as glass and glass-ceramic containing CaF2 fluoride-type nanocrystals are used.
The special interest in Dy3+ visible luminescence is due to the existence of two intense bands in the blue and yellow wavelength regions that, combined, will emit white light. Transparent glass-ceramics containing RE3+-doped CaF2 nanocrystals have been recently reported and demonstrated having potential applications in photonics [10–14]. CaF2 is an important optical raw material with high solubility of RE3+ ions, is highly transparent from 0.13 to 9.5 μm and gives a better matching of refractive index with the aluminosilicate glass, leading to the possibility of making transparent glass-ceramics containing CaF2 nanocrystals . The present work deals with the preparation and luminescence study of a low-concentrated (0.1 mol%) Dy3+-doped oxyfluoride glass and two temperature-induced glass-ceramics containing CaF2 nanocrystals. The main goal of these matrices doped with Dy3+ ions is to accomplish white light as a sum of blue and yellow emissions under blue laser excitation, at a unique excitation wavelength and in single, low concentration-doped glass and nanocrystalline glass-ceramics.
2. Experimental details
The as-made glass of composition (in mol%) 45 SiO2-20 Al2O3-10 CaO-24.9 CaF2-0.1 DyF3 was prepared using the melt quenching technique at a temperature of 1350 °C for 1h using a platinum crucible. The density of the glass (d = 2.656 g cm−3) was determined by Archimedes method using distilled water as an immersion liquid. The refractive index (n = 1.557) was measured on an Abbe refractometer at sodium wavelength (589.3 nm). The density and refractive index of the glass are closer to those (d = 2.783 g cm−3, n = 1.55) reported for a similar glass matrix doped with Eu2+ ions .
Monitoring the glass composition, preparation and the heat treatment allows one to obtain nano-crystalline structures embedded in the glassy phase with superior mechanical, chemical and optical properties compared to the precursor glass. The thermal properties of the glass have been measured by DTA at a heating rate of 10 °C/min under N2 atmosphere (Bruker. axs K.K, TG-DTA 2020). The DTA spectrum of the glass, presented in Fig. 1(a) , shows that glass transition temperature Tg = 630 °C and the onset of two crystallization temperatures Tx1 = 660 °C and Tx2 = 900 °C where the former is due to precipitation of CaF2 and the later is due to bulk crystallization of the glass . The large difference of 240 °C between the crystallization temperature (Tx1) for CaF2 and the bulk crystallization (Tx2) indicates that it is easy to precipitate CaF2 nanocrystalline phase from the glass matrix, whereas the small Tx1-Tg value would indicate a high tendency of devitrification of the precursor glass. Hence, the samples were annealed at 650 and 700 °C for 4 h to precipitate Dy3+-doped CaF2 nanocrystals, which are hereafter referred as GC1 and GC2, respectively. In GCs Dy3+ ions substitute the Ca2+ ions since they have similar ionic radius.
Structural differences between the precursor glass and the glass-ceramics were measured by X-ray diffraction using the Cu-Kα radiation and are shown in Fig. 1(b). The XRD spectrum of glass contains two broad curves typical of structures without long-range order such as the amorphous precursor glass. However, for the glass-ceramics there are a several sharp peaks showing a diffraction pattern of a crystalline structure that can be identified as the CaF2 crystalline phase. From the XRD peak widths and using the Scherrer formula, an average size of the nanocrystals have been evaluated for GC1 and GC2 and are found to be around 10 and 18 nm, respectively.
The visible emission spectra were measured by exciting at 451 nm using an OPO pumped by the third harmonic (355 nm) of a Nd:YAG laser and detected by a photomultiplier tube through a 75 cm monochromator. Luminescence decay curves have been measured using a digital storage oscilloscope interfaced to a personal computer.
3. Results and discussion
When excited with a blue laser at around 451 nm in resonance with the 6H15/2→4I15/2 absorbing transition, an intense white light from the 4F9/2 level is observed by the naked eyes. The different emission transitions composing this white light are given in Fig. 2 along with the Dy3+ ions partial energy level diagram. The emission spectra consists of two intense bands at blue (484 nm) and yellow (574 nm) regions and two more weak bands at 664 and 753 nm corresponding to 4F9/2→6HJ (J = 15/2, 13/2, 11/2 and 9/2) transitions, respectively, in which the yellow band corresponding to the 4F9/2→6H13/2 is the hypersensitive transition (ΔL = 2 and ΔJ = 2). It is worth noting that there is significant change in the relative intensities of the emission bands with the change in heat treatment temperatures that can be understood as a fingerprint of the progressive incorporation of Dy3+ ions in the CaF2 nanocrystals. The branching ratios (relative intensities) of different emissions from the 4F9/2 level have been determined for the present systems and for the yellow emission it increases from glass (0.518) to GC1 (0.538) and then to GC2 (0.552) which clearly indicates the gradual change in local environment of the Dy3+ ions in the samples with annealing.
These changes are quantified as the yellow to blue (Y/B) intensity ratio. As can be seen, the Y/B ratios increases from 1.52 (glass) to 1.58 (GC1) and then to 1.68 (GC2). The change in Y/B ratio is attributed to the change in the environment of Dy3+ ions (incorporation of Dy3+ ions in to the CaF2 nanocrystals) in the material as it involves hypersensitive transition which is strongly influenced by the local environment of the Dy3+ ions . This is supported by the fact that when Dy3+ ion is located at a lower symmetry local site (without an inversion centre), yellow emission transition is often stronger in its emission spectra . The variation of Y/B values (inset of Fig. 3(b) ) clearly indicates that it is possible to vary its values by changing the heat treatment conditions.
Luminescence decay curves of the 4F9/2 level have been measured for glass and glass-ceramic samples exciting at 451 nm and monitoring the emission at 575 nm. Decay curves of all the samples are slightly non-exponential in nature (Fig. 3(a)). In moving from glass to glass-ceramics, non-exponential nature of the decay curves increases along with decrease in lifetimes, again given an experimental indication of the progressive incorporation of Dy3+ ions in the CaF2 nanocrystals. The average or effective lifetimes  of the 4F9/2 level have been extracted from the non-exponential decay curves and are found to be around 820, 815 and 794 micro-s for glass, GC1 and GC2, respectively. The increase in non-exponential nature of the decay curves along with decrease in lifetimes from glass to glass-ceramics is attributed to enhanced energy transfer processes between Dy3+ ions incorporated in the CaF2 nanocrystals. The concentration of Dy3+ ions in the nanocrystalline phase is more compared to the precursor glass which increases the energy transfer between the ions. However, in the present case, the concentration of Dy3+ ions is kept low enough to minimize the energy transfer processes and to avoid the possible formation of DyF3 clusters in the glass-ceramic samples .
The blue and yellow emissions of Dy3+ ions can produce white light through an appropriate combination [16,20] and can find application as a potential solid state color display material in which generation and control of colors is possible . The white light emissions of the present materials have been analyzed in the frame of chromaticity coordinate of colors and are found to be (x = 0.39, y = 0.41), (x = 0.38, y = 0.42) and (x = 0.39, y = 0.42) for glass, GC1 and GC2, respectively. The color coordinates are shown in the CIE 1931 chromaticity diagram (Fig. 3(b)). As can be seen, the coordinates of color for all the three present samples are within the white light zone. The CIE color coordinates reported in this work are close to those (x = 0.33, y = 0.38) reported for NGP:Dy3+ phosphor  and nearer to those (x = 0.32, y = 0.33) of commercial pc-LED (Blue LED + YAG Ce, HT-P278BPV) . These results indicate that the present materials have the potential to be used in solid state color display devices.
The quality of the light source is also evaluated in terms of the correlated color temperature (CCT), which illustrates the temperature of a closest Plankian black-body radiator to the operating point on the chromaticity diagram . The CCT can be calculated using the color coordinates by the McCamy empirical formula ,
The CCT values obtained for the present Dy3+-doped glass, GC1 and GC2 are 4350 K, 4240 K and 4125 K, respectively. They are only slightly more than the warm CCT (i.e., CCT<4000 K) . The emission approaches warm white with increase in heat treatment temperature of the samples. The CCT values of the present materials are in between to those of fluorescent tube (3935 K) and day light (5500 K) .
Oxyfluoride glass and transparent glass-ceramics doped with 0.1 mol % Dy3+ ions have been prepared and characterized. The XRD spectra confirm the formation of CaF2 nanocrystals in the glass matrix. Both the glass and glass-ceramics emit white light when excited with 451 nm. Yellow to blue intensity ratios of the visible emissions are found to vary with change in heat treatment conditions. Chromaticity coordinates of color have been calculated for the visible emissions and are found to be in the white light zone for all the samples. The CCT values of the present materials are in between fluorescent tube and day light. The main goal of these matrices is the ability to address white light as a sum of blue, yellow and red emissions under laser excitation with commercially available diode lasers or an Argon laser, at a unique excitation wavelength and in low concentration single RE3+ ion-doped glass and glass-ceramics.
The work was supported by the PKNU research program (2008-09) and Mid-career Researcher Program through NRF grant funded by the MEST (No. 2009-0078682). DAE-BRNS, India (No. 2007/34/25-BRNS/2415, Dt. 18-01-2008), Ministerio de Ciencia e Innovación of Spain (under an International Complementary Action with India PCI2006-A7-0638, the National Material Program MAT 2007-65990-103-02 and MAT2010-21270-C04-02 and MALTA Consolider-Ingenio 2010 CSD2007-00045) and the EU-Feder funds.
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