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A single-phased tunable color conversion phosphor for WLED was prepared by coactivating Ca9Y(PO4)7 with Eu2+ and Sm3+ ions. The structure, UV-visible and photoluminescence spectra of the phosphor were studied as a function of annealing atmosphere and concentration ratio of Eu2+/Sm3+. The coexistence of Eu2+ and Sm3+ was achieved by annealing the phosphor in a reducing atmosphere. The as-obtained phosphors showed a blended emission of blue-green and orange-red light upon near-UV excitation. White light emission was realized by adjusting the concentration ratio. Energy transfer and dominant interaction between Eu2+ and Sm3+ was also studied.
©2012 Optical Society of America
1. Introduction
Solid-state lighting using LEDs, with its impressive economic-saving, environment-friendly, and many other promising features, is expected to replace conventional light sources [1]. There are several approaches to generate white light from inorganic III-Nitride LED sources and phosphors, and each combination has its advantages and disadvantages [2]. In recent years, a great amount of effort has been devoted to improve the performance of III-Nitride LED [3–13], as well as phosphors [14–16]. The widely used commercial WLEDs, which consist of a blue LED and a yellow phosphor (Y3Al5O12:Ce3+) have a high luminous efficacy and low fabrication cost, but a low color rendering index (CRI) because of weak emission in the red region [17,18]. One of the solutions to this drawback is the use of a combination of near-UV/blue LEDs and trichromatic phosphors. Therefore, near-UV/blue light excited color-conversion phosphors have attracted increasing interest in recent years. Most recently, single-phased multicolor-emitting phosphors have been widely investigated [19–39]. As compared to the blend of multiphase phosphors, single-phased phosphors do not have the disadvantages of incongruous physical and chemical properties and reabsorption of emission light by the different phosphors [40].
To obtain single-phased phosphors, the most widely used strategy is to dope a host lattice with a couple of activators, such as (Eu2+, Mn2+)- [19–32], (Ce3+, Mn2+)- [33,34], and (Ce3+, Eu2+)-couples [35–39]. For these phosphors, the excitation energy is firstly absorbed by one of the activators (as well as a sensitizer of the other activator), for example Eu2+ in the (Eu2+, Mn2+)-couple [19–32], and then the excitation energy is partly converted to high energy blue-green light emission, and partly transferred from Eu2+ to Mn2+, resulting in low energy red light emission [19–32]. There are some other strategies to achieve the goal. Saradhi et al. [41] developed a single-phased phosphor containing both Eu2+ and Eu3+ ions, but the control of Eu2+/Eu3+ ratio was a daunting task. A combination of Eu2+ and Sm3+ has been reported to activate glass-ceramics silicate host lattices [42,43]. The effect of Sm3+ codoping on the Eu2+ persistent luminescence in some aluminates has been investigated [44–46]. Although Sm3+ can also be reduced to Sm2+, the reduction probability of Sm3+→Sm2+ is much less than that of Eu3+→Eu2+, because there is significant divergence in the standard reduction potential, i.e. −0.36 V for Eu3+/Eu2+, and −1.55 V for Sm3+/Sm2+ [47]. It is reasonable to expect that white light emission might be achieved by coactivating a host lattice with Eu2+ and Sm3+.
As well-known, the excitation and emission of Sm3+ are mainly of f-f transition, the wavelengths hardly change with the host lattice, while the excitation and emission of Eu2+ are of f-d transition, which depend strongly on the host lattice. The emission of Eu2+ ranges from UV to red region in different hosts. Therefore the choosing of host lattice is important for Eu2+ to give a expected emission. In this work, Ca9Y(PO4)7 (CYPO) was chosen as the host lattice and Eu2+ and Sm3+ as the activators. CYPO doped with Eu and /or Sm was synthesized by a conventional solid-state reaction under different atmospheres. We investigated the effects of the annealing atmosphere on the oxide states of Eu and Sm, the photoluminescence (PL) properties of the phosphors and their dependence on the concentration ratio of Eu/Sm, and the energy transfer and dominant interaction between Eu2+ and Sm3+ ions.
2. Experimental
CYPO doped with Eu and/or Sm was synthesized by a conventional solid-state reaction under different atomspheres. Stoichiometric amounts of starting materials, Y2O3 (99.99%, Aldrich), Eu2O3 (99.99%, Aldrich), Sm2O3 (99.99% Aldrich), CaCO3 (99 + %, Aldrich) and NH4H2PO4 (99.999%, Aldrich), were mortared and fired at 1300 °C for 8 h in CO-atmosphere. Then, the products were powderized and annealed at 1200 °C for 2 h in air (referred to as CYPO-air) or in 5%H2/95%N2 atmosphere. Unless otherwise specified, all the samples were annealed in 5%H2/95%N2 atmosphere.
Powder X-ray diffraction (PXRD) measurement was carried out on a D/MAX 2500 (Rigaku, Japan) diffractometer with Rint 2000 wide angle goniometer and Cu Kα1 radiation (λ = 1.54056 Å) at 40 kV and 100 mA. The diffraction patterns were scanned over an angular range of 10°-70° (2θ) with a step length of 0.02° (2θ). PL spectra were collected on a PTI fluorescence spectrophotometer (Photon Technology International, USA) equipped with a 60 W Xe-arc lamp as the excitation light source. The PL decay of Sm3+ was measured with a phosphorimeter attached to the main system with a 25 W Xe flash lamp, and the PL decay of Eu2+ was measured using the third harmonic (355 nm) of a pulsed Nd:YAG laser (5 ns, 10 Hz). Diffuse reflectance (DR) spectra were measured on a V-670 UV-Vis spectrophotometer (JASCO, Japan). All measurements were performed at room temperature in air.
3. Results and discussion
Structure and tunable PL properties of CYPO:Eu, Sm
The crystal structure and phase purity of the powders were verified by PXRD. Figure 1 indicates that the patterns of all the samples are identical, and can be well indexed to JCPDS card #46-0402. From the PXRD patterns, the lattice parameters of the samples were calculated and listed in Table 1 . The results revealed that the host lattice expanded slightly after doping with Eu and/or Sm and also expanded when the annealing atmosphere was changed from air to H2/N2. The former might result from the ionic radius difference between Y and doped Eu and Sm, and the later might due to the formation of oxygen vacancy defect in the samples annealed in H2/N2.
Fig. 1 PXRD patterns of CYPO with and without Eu/Sm doped that were annealed in different atmospheres.
Table 1. Lattice Parameters of CYPO:x%Eu, y%Sm That Were Annealed in Different Atmospheres
The PL excitation and emission spectra of CYPO:1.0%Eu and CYPO:1.0%Sm annealed in different atmospheres are shown in Fig. 2 . As viewed, broad excitation and emission band due to Eu2+ ions 4f7↔4f65d transitions were observed when CYPO:Eu was annealed in H2/N2, whereas sharp-line emission corresponding to the f-f transitions of Eu3+ ions within 4f6 configuration were observed when CYPO:Eu was annealed in air. The emission peaks at 536 nm and 558 nm were due to the transitions from 5D1 energy level to 7FJ levels. Those at (579.5 nm, 581 nm), (591 nm, 597.5 nm), (614 nm, 617.5 nm), (653.5 nm) and (685.5 nm, 691 nm, 700 nm, 707.5 nm) were transitions from 5D0 level to 7F0, 7F1, 7F2, 7F3 and 7F4 sublevels within 4f6 configuration, respectively. The excitation band centered at 270 nm in the excitation spectrum was the charge transfer band of O2-→Eu3+, and the sharp lines in the range of 300-475 nm were intrinsic excitation of Eu3+ ions from 7F0 energy level to high energy levels. In contrast, the spectra of CYPO:Sm annealed in air and H2/N2 were identical in every aspect, except for a very small difference in the intensity. The excitation spectra consisted of narrow lines originating from the f-f transitions of Sm3+ ions within 4f5 configuration. The emission peaks at (563.5 nm, 569 nm), 602 nm, 649 nm, and 711 nm were transitions from 4G5/2 to 6F5/2, 6F7/2, 6F9/2, and 6F11/2 levels within 4f5 configuration, respectively. It can be concluded that the H2/N2 atmosphere reduced Eu from a + 3 to + 2 oxidation state but did not change the valence of Sm (+ 3). This result was also confirmed by the UV-visible DR spectra shown in Fig. 3 . The characteristic absorption band observed in the range of 420-270 nm, which corresponds to the 4f7-4f65d transitions of Eu2+ ions, was only observed in the Eu-contained compounds that were annealed in H2/N2.
Fig. 2 PL excitation and emission spectra of (a) and (b) CYPO:Eu; (c) and (d) CYPO:Sm that were annealed in different atmospheres.
As seen in Fig. 2, the excitation spectra of CYPO:Eu2+ and CYPO:Sm3+ have a significant overlap within the range of 300-450 nm, which indicates a possibility for exciting Eu2+ and Sm3+ simultaneously at the overlap. This was confirmed by the PL excitation and emission spectra of CYPO samples coactivated with Eu2+ and Sm3+ (see Fig. 4(a) ). Under 405 nm excitation, the emission spectra of CYPO:Eu2+, Sm3+ contains a broad band in the range of 425-650 nm originated from Eu2+ ions, and a series of sharp peaks at 564 nm, 602 nm, 649nm and 711 nm due to Sm3+ ions. To investigate the tunability of the phosphor, we studied the PL emission spectra of CYPO:x%Eu, y%Sm as a function of the Eu/Sm (x/y) concentration. The results shown in Fig. 4(a) indicate that the intensity ratio of Eu2+ emission to Sm3+ emission varied systematically with x/y. With the increasing of the ratio of y/x, the intensities of the sub peaks at 564 nm, 602 nm and 649 nm increase accordingly; meanwhile, the intensity of the emission band at 490 nm decreases. The contrastive variation of the blue-green emission and orange-red emission makes it possible to tune the color of the final emission. It can be seen from the Commission Internationale de I’Eclairage (CIE) coordinates (x, y) in Fig. 4(b) that the color of the emission shifted gradually from blue-green to white, and eventually to orange-red with the changes in the Eu/Sm concentration.
Fig. 4 (a) PL emission spectra and, (b) CIE coordinates of CYPO:x%Eu, y%Sm as a function of x/y ratio.
Energy transfer and dominant interaction between Eu2+ and Sm3+
Figure 4(a) also indicates that the PL intensity does not change proportionally to the concentrations of Eu/Sm. For example, when the concentration of Eu2+ was fixed at 1.0% and the concentration of Sm3+ was increased from 0% to 1.0%, the Sm3+ emission increased while the Eu2+ emission gradually decreased and vice versa. This suggested that there might be energy transfer between the Eu2+ and Sm3+ ions. It is known that if energy is transferred from a donor to an acceptor, the temporal decay of the donor increases.
We studied the temporal decay of Eu2+ as a function of the concentration of co-doped Sm3+, and vice versa. The results are shown in Fig. 5 . The PL decay data of Eu2+ and Sm3+ can be approximately fitted to a single-exponential function
where It and I0 are the intensity at time t and t = 0, respectively, τ is the lifetime. The determined lifetime revealed that when Sm3+ ions were introduced, the PL lifetime of Eu2+ was shortened from 2.00(2) μs in CYPO:(1.0%)Eu to 1.10(1) μs in CYPO:(1.0%)Eu, (1.0%)Sm, and on the other side, when Eu2+ ions was introduced, the lifetime of Sm3+ was also shortened from 1.20(1) ms in CYPO:(1.0%)Sm to 1.12(1) ms in CYPO:(1.0%)Eu, (1.0%)Sm. In the Eu2+ and Sm3+ codoped samples, if Eu2+ and Sm3+ work independently, and there is no energy transfer between them, the PL lifetime of both activators will be as same as in the solo doped samples. While if there is energy transfer, for example from Eu2+ to Sm3+, the decay of Eu2+ excitation state will be accelerated by this energy transfer channel, and consequently the PL lifetime of Eu2+ will be shortened. The shorten of the lifetime of Eu2+ and Sm3+ confirmed the energy transfer between the Sm3+ and Eu2+ ions.Fig. 5 (a) PL decay data of Eu2+ as a function of Sm3+ concentration and, (b) PL decay data of Sm3+ as a function of Eu2+ concentration.
According to Dexter and Uitert [48–50], different types of interactions might involve in the energy transfer between a donor and an acceptor. If the energy transfer is controlled by the exchange interaction, the following equation should be obeyed:
ID0 and ID are the PL intensities of the donor in the absence and presence of an acceptor, respectively, and CA is the concentration of the acceptor. If the multipolar interaction takes control, the PL intensity of the donor and the concentration of the acceptor should followwhere n = 6, 8, and 10 corresponding to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions, respectively. To study the dominant interaction in the energy transfer between Eu2+ and Sm3+ ions, we fixed the concentration of Eu2+ ions to observe the evolution of the PL intensity of Eu2+ ion as a function of Sm3+ concentration, as well as the reverse. The correlation between ID0/ID and CA are plotted in Figs. 6 and 7 . Linear relations can be observed in Figs. 6(a), 6(b), 7(a) and 7(b), which indicates that either exchange interaction or dipole-dipole interaction might control the energy transfer between Eu2+ and Sm3+ ions.Fig. 6 Plots of PL intensity of Eu2+ in CYPO:(x = 1.0%)Eu, y%Sm as a function of Sm3+ concentration: (a) follow Eq. (2); and follow Eq. (3) with (b) n = 6, (c) n = 8, and (d) n = 10.
Fig. 7 Plots of PL intensity of Sm3+ in CYPO:x%Eu, (y = 0.5%)Sm as a function of Eu2+ concentration: (a) follow Eq. (2); and follow Eq. (3) with (b) n = 6, (c) n = 8, and (d) n = 10.
It is also known that the dominant interaction is strongly dependent on the separation between the donors and the acceptors. The dipole-dipole interaction corresponds to long-range energy transfer, while the exchange interaction occurs only if the separation is short enough to allow a direct overlap of their wavefunctions [48–50]. In order to confirm which interaction dominates the energy transfer between the Eu2+ and Sm3+ ions, we estimated the average separation REu-Sm between Eu2+ and Sm3+ ions using the formula suggested by Blasse [51]:
where N is the number of sites available for the dopant, x and y are the concentration of the dopant, and V is the unit cell volume of the host lattice. In our case, the cell volume of CYPO was approximately used for all the samples in the calculations due to the fact that a small amount of doping changes this value slightly. The results shown in Table 2 indicate that the separation between Eu2+ and Sm3+ ions is so far that the overlap of the wavefunctions is hard to take place. Based on this fact, we would like to suggest that the dipole-dipole interaction is dominant in the energy transfer between Eu2+ and Sm3+.Table 2. Average Separation REu-Sm Between Eu2+ and Sm3+ Ions in CYPO: x%Eu, y%Sm Phosphors
4. Conclusion
The synthesis and PL properties of CYPO coactivated with Eu2+ and Sm3+ ions were studied. The coexistence of Eu2+ and Sm3+ ions in CYPO can be realized by annealing the phosphors in a weak reducing atmosphere. Upon near-UV excitation, the phosphors showed intensive luminescence consisting of blue-green emission and orange-red emission owing to the presence of Eu2+ and Sm3+, respectively. By adjusting the dopant concentration ratio between Eu2+ and Sm3+, the emission could be tuned. Energy transfer between Eu2+ and Sm3+ ions was evidenced, and the dominant interaction was determined to be of dipole-dipole type.
Acknowledgments
This study was supported by the Korea Research Foundation Grant funded by the Korean Government (No. 2010-0022540).
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