Abstract
This paper reports the synthesis and luminescence properties of blue emitting Li(Ca1−xEux)PO4 (x = 0.005, 0.01, 0.03) phosphors. These phosphors were prepared by a modified sol-gel/Pechini method. The particle size ranged from 1 to 1.5 μm with a narrow size distribution. X-ray diffraction analysis shows that the phases present are dependent on the post-synthesis annealing temperature and x. A single phase of this phosphor is obtained at x ≤ 0.01 and an annealing temperature of 800°C. These phosphors are excited efficiently in the range 280-410 nm and the photoluminescence emission spectra consist of a strong broad blue band centered at ∼470 nm. Furthermore, the quantum efficiencies range from 58% to 73% depending on the annealing conditions.
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The common approach for producing white-emitting LEDs has been to use a blue-emitting InGaN LED with the yellow-emitting phosphor Y3Al5O12:Ce3+ (YAG:Ce) and a red emitting phosphor for color correction.1,2 Recently, white LEDs in which a near UV-LED is combined with blue, green, and red phosphors have been investigated due to less current droop and improved binning of the UV-LEDs, and a better control over color rendering index and color temperature through manipulation of phosphor blends.3–5 These phosphors require strong absorption in the near-UV range, which can be obtained from dipole-allowed 4f-5d transitions in rare earth activated ions such as Ce3+ and Eu2+ in a suitable host.6,7 The most promising phosphors examined for this approach have been (Sr,Ba)2SiO4:Eu2+,8–11 β-SiAlON:Eu2+,12 Ba2MgSi2O7:Eu2+13 for the green–emitting component, Sr3SiO5:Eu2+,14 CaAlSiN3:Ce3+15 for the yellow/orange-emitting component and (Sr,Ca)2Si5N8:Eu2+16 and CaAlSiN3:Eu2+17 for the red-emitting component.
For blue-emitting phosphors excited by near UV-LEDs, ABPO4 and halophosphate activated with Eu2+ families have been studied.18–20 In our previous report, LiCaPO4:Eu2+ phosphors were found to have superior luminescence properties for UV-LED application among ABPO4:Eu2+ (A = Li, Na, K, B = Ca, Sr, Ba) families because of their high quantum efficiency and acceptable thermal stability.20
Investigations of these phosphors date back to the 1960's, first reported by Wanmaker et al.21 They were activated by Cu and had strong excitation bands in the deep UV range. Wanmaker et al. reported X-ray diffraction powder patterns of LiCaPO4, currently indexed as JCPDS 14-0203. However, Lightfoot et al.22 argued that the Wanmaker's powder pattern is not correct because of the presence of a second phase of Ca3(PO4)2 due to the loss of Li under the high temperature reaction. Lightfoot reported a single phase LiCaPO4 produced by solid-state reaction, and the corresponding diffraction pattern is indexed as JCPDS 79-1396. LiCaPO4 has a hexagonal structure with space group P31c and lattice parameters: a = 0.752 nm and c = 0.996 nm. The Ca2+ ions have six short contacts (0.231-0.254 nm) to oxygen and two longer contacts (0.276 and 0.290 nm), completing an irregular eight coordinated geometry.22
However, most reports on LiCaPO4:Eu2+ show mixtures of LiCaPO4 with appreciable amounts of secondary phases, such as Li3PO4 and Ca3(PO4)2.23–25 Recently, More et al.26 reported the pure LiCaPO4:Eu2+ prepared by solid state reaction with CaH(PO4) as a precursor, prepared by the same method used by Lightfoot et al.22 To our knowledge, it is the only reported method that yields a single phase LiCaPO4:Eu2+. However, the synthesis is not straightforward and the process takes a long time (over 3 days) with repeated grinding or extensive ball milling. In this work, we report on the synthesis of Li(Ca1−xEux)PO4 (x = 0.005, 0.01, 0.03) phosphors with no secondary phases using a sol-gel/Pechini method and its luminescence properties. The effect of annealing temperature on the particle size and morphology is also examined.
Experimental
First, LiNO3 (99%, Alfa Aesar), Ca(NO3)2·4H2O (99.99%, Mallinckrodt Chemicals) and Eu2O3 (99.999%, Alfa Aesar) in the desired molar ratios were dissolved in a dilute nitric acid solution. Next, (NH4)2HPO4 (EMD chemicals) as the reagent for PO43− was added to the mixture. Note that before adding (NH4)2HPO4, the pH of the cation mixture should be < 2 to prevent precipitation. After the solution became transparent, 1.1 g of citric acid (CA) (C6H8O7·H2O, EMD chemicals), which acted as a chelating agent for the metal ions, and 1.12 mL ethylene glycol (EG) (C2H6OH, Fisher Scientific), were added (molar ratio of metal: CA: EG = 1:0.5:2). Then, 1.0 g polyethylene glycol (PEG, (C2H4O)nH2O, molecular weight = 20,000, Sigma Aldrich), used as a cross-linking agent, was added to the aqueous solution. The mixture was stirred well to achieve uniformity and were then placed in a water-bath at 80°C so that the solution would hydrolyze into a sol and then into a gel. The gel was heated to 800°C for 1h in air to remove the carbon and organic material. Next, the gel was annealed at 800-1150°C in a slightly reducing atmosphere (a mixture of 5% H2 and 95% N2).
Characterization
The particle morphology and size were measured using a field emission scanning electron microscope (FESEM, XL30, Philips). The crystalline phases of the annealed powders were identified by X-ray diffraction (XRD) and the peak and percentage of the phases were identified by an XRD analysis program (JADE, Materials Data Inc.). Photoluminescence (PL) measurements were taken using a Jobin-Yvon Triax 180 monochromator and SpectrumOne charge-coupled device detection system, using a 450 W Xe lamp as the excitation source. Quantum efficiency (QE) measurements were made using a 400 nm laser diode as the excitation source. Powdered phosphor samples were dispersed in a silicone gel and cured. The samples were then placed in a 30.5 cm sphere and three measurements, of which the average is reported, were taken following the method outlined in DeMello et al.27
Results and Discussion
The structure of LiCaPO4 consists of a three dimensional framework of a vertex-sharing LiO4 and PO4 tetrahedra, which encloses large five-sided channels parallel to the [001] direction occupied by Ca2+ ions as shown in Figure 1. Figure 1 was drawn with the visualization program VESTA.28 The five-sided channels observed in the structures are unusual, and contrast with the more commonly observed six-sided channels in other tetrahedral framework structures, such as the SiO2 polymorphs, quartz, cristobalite and tridymite.20 Figure 2a shows the XRD patterns of Li(Ca1−xEux)PO4 as a function of x. All samples show the main peak of LiCaPO4, at 2θ = 32.82°; the synthesized compound is a single hexagonal phase and matches well with JCPDS 79-1396 for x = 0.005 and 0.01. However, at higher concentrations of Eu2+ (x = 0.03), there is a slight distortion in the hexagonal structure, which is accompanied by the formation of Li3PO4 (2θ = 33.89°) and Ca3(PO4)2 (2θ = 31.01°) impurity phases. The amounts of LiCaPO4, Ca3(PO4)2 and Li3PO4 are estimated to be approximately 80%, 15% and 5%, respectively, as determined by the XRD program (JADE).
Figure 1. Unit cell representation of the crystal structure of LiCaPO4 along the [001] direction. Blue, green, purple, and red spheres represent Li, Ca, P, and O ions, respectively. The polyhedral geometry of LiO4 and PO4 are depicted by blue and purple polyhedral, respectively.
Figure 2. XRD patterns of (a) Li(Ca1−xEux)PO4 (x = 0.005, 0.01, 0.03) phosphors and (b) Li(Ca1−xEux)PO4 (x = 0.01) at various post-synthesis annealing temperatures.
Figure 2b shows the XRD patterns of Li(Ca0.99Eu0.01)PO4 at various annealing temperatures. At 800°C, as mentioned previously, single phase of LiCaPO4 is obtained. However, the peaks for Li3PO4 and Ca3(PO4)2 increase with increasing annealing temperature and the amounts of LiCaPO4, Ca3(PO4)2 and Li3PO4 are estimated to be approximately ∼75%, ∼15% and ∼10% at 1000°C and ∼60%, ∼20% and ∼20% at 1150°C. Thus, 800°C is the optimal annealing temperature to obtain a single phase of LiCaPO4 for x ≤ 0.01 and agrees well with Lightfoot's results.22 Moreover, the sol-gel/Pechini approach is superior to solid-state reaction to obtain a single phase of LiCaPO4 in terms of a reduced synthesis time and simplicity due to the elimination of repeated grinding and ballmilling of the product. Lightfoot22 suggested the significant amount of impurity phase, Ca3(PO4)2, was mainly due to loss of Li at higher annealing temperatures. However, we find another impurity phase, Li3PO4, also forms at higher annealing temperatures, as shown in Figure 2b. A theoretical explanation of the presence of these impurity phases at higher annealing temperature (>800°C) is currently under investigation.
SEM micrographs of Li(Ca0.99Eu0.01)PO4 at various annealing temperatures are shown in Figure 3a–3c. The particles for samples annealed at 800°C are round with diameters 1∼1.5 μm and have a narrow size distribution as shown in Figure 3a. The diameter and the width of size distribution increases with increasing annealing temperature and the morphology changes from spherical to an irregular shape at 900 and 1150°C as shown in Figure 3b–3c. The irregular shape of the particles is most likely due to the presence of impurity phases such as Ca3(PO4)2 and Li3PO4 caused by higher annealing temperature.
Figure 3. SEM micrographs of Li(Ca0.99Eu0.01)PO4 annealed at (a) 800°C, (b) 1000°C, and (c) 1150°C.
Figure 4a shows the PL excitation spectra of Li(Ca0.99Eu0.01)PO4. The broad excitation spectrum covers the wavelength range from UV to the visible region and the phosphors excited efficiently in the region from 280 to 410 nm. Figure 4b presents PL emission spectra of Li(Ca1−xEux)PO4 for various values of x under 380 nm excitation. The PL spectra consist of a strong broad blue band centered around 470 nm and is attributed to the allowed 4f65d → 4f7 transition of Eu2+.4,5 The emission peak is slightly red-shifted from 470 to 473 nm at higher Eu2+ concentrations. The emission intensity increases with increasing x and shows a maximum at x = 0.01 and then decreases at x = 0.03. The decrease in the emission intensity at x = 0.03 is mainly due to the presence of impurity phases in the powders as shown in Figure 1a. Figure 4c shows the PL emission spectra of Li(Ca0.99Eu0.01)PO4 at several annealing temperatures. The emission intensity of the samples annealed at 1000°C is lower than those annealed at 800°C because of material impurity phases, but the intensity of the 1150°C samples is higher than those annealed at 800°C despite having impurity phases. This can be explained by the increased crystallinity of LiCaPO4 as the annealing temperature increased. The quantum efficiencies of the samples annealed at 800°C and 1150°C are 58% and 62%, respectively, under 400 nm excitation. These numbers are lower than our previous samples prepared by solid-state reaction annealed at 1300°C (QE = 88%), most likely because of the larger particle size (>5 μm) and increased crystallinity of the higher annealing temperature.
Figure 4. (a) PL excitation spectra of Li(Ca0.99Eu0.01)PO4, (b) PL emission spectra of Li(Ca1−xEux)PO4 (x = 0.005, 0.01, 0.03) under 380 nm excitation, (c) PL excitation spectra of Li(Ca0.99Eu0.01)PO4 at various annealing temperatures.
In order to increase the emission intensity and crystallinity of the powders, higher annealing temperature is required, but the impurity phases form above 800°C as shown in Figure 2b. Thus, in attempt to obtain high crystallinity and single phase LiCaPO4, the powders annealed at higher temperatures were then re-annealed at 800°C. Figure 5 shows the XRD diffraction of Li(Ca0.99Eu0.01)PO4 annealed at 1150°C for 1h and the powders re-annealed at 800°C for 12h. The powders annealed at 1150°C are mixtures of LiCaPO4, Li3PO4, Ca3(PO4)2 with weight percentages of ∼60%, ∼20% and ∼20%, respectively. However, the powders re-annealed at 800°C are nearly single phase LiCaPO4 with only a small amount of Ca3(PO4)2 (<5%). This suggests that the impurity phases react to form LiCaPO4 during the re-annealing process at 800°C.
Figure 5. XRD patterns of Li(Ca0.99Eu0.01)PO4 annealed at 1150°C and re-annealed at 800°C.
The PL emission spectra of both samples excited at 380 nm is shown in Figure 6. The emission intensity of the re-annealed powder is higher than that of powder annealed at 1150°C. The quantum efficiency of re-annealed powder increased from 62% to 73%. It is obvious that phase purity of Li(Ca1−xEux)PO4 plays a crucial role in improving its luminescence properties. However, the high quantum efficiency values indicate that single phase Li(Ca1−xEux)PO4 is a promising blue-emitter for applications in white-emitting UV-LEDs.
Figure 6. PL emission spectra of Li(Ca0.99Eu0.01)PO4 annealed at 1150°C and re-annealed at 800°C under 380 nm excitation.
Conclusion
Single phase blue-emitting phosphor powders, Li(Ca1−xEux)PO4, (x = 0.005, 0.01, 0.03) were prepared by a sol-gel/Pechini method. This synthesis method is both simple and fast for the production of a single phase powder compared to solid state reaction. The phase purity of Li(Ca1−xEux)PO4 is dependent on x and annealing temperature. The phase pure powders with diameters of 1-1.5 μm and a narrow size distribution are obtained at x = 0.005 and 0.01 annealed at 800°C. Impurity phases are present at higher annealing temperatures (> 800°C) and the spherical morphology transforms to irregular shapes. These phosphors are efficiently excited by both in the deep UV and near UV region (370–410 nm), and show a strong blue emission band centered near 470 nm. By re-annealing the powders originally annealed at high temperatures (having significant amounts of impurity phases) at 800°C, a single phase powder with higher emission intensity was obtained. The quantum efficiency of the single phase Li(Ca1−xEux)PO4 phosphors increases from 58% to 73% by
increasing higher annealing temperature and re-annealing at 800°C. Therefore, Li(Ca1−xEux)PO4 is a potential candidate for application in white-emitting UV-LEDs.
Acknowledgments
This work is supported by the U.S Department of Energy of grant DE-EE0002003.