Eu²⁺-activated full color orthophosphate phosphors for warm white light-emitting diodes

Abstract

Eu²⁺-doped RbBaPO₄, NaCaPO₄ and CsMgPO₄ orthophosphate phosphors were studied. X-ray diffraction (XRD) patterns analysis indicated all of the phosphors could be obtained in a pure phase by the solid-state reaction method. The photoluminescence emission (PL) spectra revealed that the phosphors showed broad-band blue, green and red emission with peaks at near 430 nm, 510 nm and 630 nm under a 340 nm UV lamp, corresponding to RbBaPO₄:0.03Eu²⁺, NaCaPO₄:0.01Eu²⁺ and CsMgPO₄:0.07Eu²⁺ samples, respectively. A white light-emitting diode (w-LED) lamp, fabricated by the three samples combining a 370 nm n-UV chip, presented a high color-rendering index (CRI) of 86, as well as a warm correlated color temperature (CCT) of 2751 K. The results indicated that Eu²⁺-activated full color versatile orthophosphate phosphors and mixing strategy of the similar hosts will have potential use in the design of white LEDs.

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I. Introduction

Ever since the first white light-emitting diodes (w-LEDs) became commercially available in 1997, they have drawn a lot of attention for their advantages of high luminous efficiency, energy saving and low power consumption.^1,2 The general way to fabricate white LEDs is based on a blue InGaN LED chip and a yellow phosphor YAG:Ce³⁺, which yields a poor color rendering index.³ In order to improve the color rendering index (CRI), blue, green and red emission phosphors pumped by the ultraviolet/near-ultraviolet (UV/NUV) chips have been widely investigated recently.⁴ The commercialized blue, green and red phosphors for white LED lamps, such as BaMgAl₁₀O₁₇:Eu²⁺ (BAM), Ba₂SiO₄:Eu²⁺ (BSO), CaAlSiN₃:Eu²⁺ (CASN), have generally been used, but there exists the disadvantage that the different host families may lead to a lack of consistency in practical applications. In this work, we have synthesized three kinds of full-color orthophosphate phosphors with a single rare-earth activator, Eu²⁺, to explore the luminescence properties for white light-emitting diodes (w-LEDs) application. Therefore, the same orthophosphate host family, including the blue-emitting RbBaPO₄:0.03Eu²⁺, green-emitting NaCaPO₄:0.01Eu²⁺ and red-emitting CsMgPO₄:0.07Eu²⁺ phosphors,^5–7 are selected as tricolor emitters for NUV LEDs to improve compatibility for their application in white LED lamps.⁸

II. Experimental procedure

RbBaPO₄:0.03Eu²⁺, NaCaPO₄:0.01Eu²⁺ and CsMgPO₄:0.07Eu²⁺ phosphors were prepared via a traditional, high-temperature, solid-state method. The stoichiometric amounts of Rb₂CO₃ (A.R.), Na₂CO₃ (A.R.), Cs₂CO₃ (A.R.), BaCO₃ (A.R.), CaCO₃ (A.R.), (MgCO₃)₄·Mg(OH)₂·5H₂O (A.R.), (NH₄)₂HPO₄ (A.R.) and Eu₂O₃ (99.99%) were employed as raw materials, and they were mixed and ground homogeneously in an agate mortar. The mixture was first preheated at 500 °C for 3 h in air atmosphere in alumina crucibles with covers. After the preliminary products were ground thoroughly in an agate mortar after cooling to room temperature, they were placed in alumina crucibles and annealed at 1150 °C in a CO reducing atmosphere for 3 h with highly pure carbon particles as the reducing agent. The phosphors were then obtained for characterization.

Powder X-ray diffraction (XRD) data was checked for structural phase identification by an X-ray powder diffractometer (Shimadzu, XRD-6000, Cu Kα radiation, λ = 0.15406 nm, 40 kV, 40 mA). The continuous scanning rate (2θ ranging from 5° to 90°) used in phase formation determination was 4°(2θ) min⁻¹ and the step scanning rate (2θ ranging from 10° to 80°) used for Rietveld analysis was 8 s per step with a step size of 0.02°. Powder diffraction data were obtained using the computer software program General Structure Analysis System (GSAS). The morphology of the samples was inspected using a scanning electron microscope (SEM, JEOL, JSM-6490). Room-temperature photoluminescence excitation (PLE) and emission (PL) spectra were characterized on an F-4600 fluorescence spectrophotometer (F-4600, HITACHI, Japan) with a photomultiplier tube operating at 400 V and a 150 W Xe lamp used as the excitation source. The temperature-dependent luminescence properties were measured on the same spectrophotometer combined with a self-made heating attachment and a computer-controlled electric furnace. Optical properties of the white light n-UV LED, including the luminescent spectrum, CRI, CCT, and CIE value of the lamp, were measured by a HAAS-2000 (Ever fine, China) light and radiation measuring instrument. The quantum efficiency (QE) was measured using the integrating sphere on the FLS920 fluorescence spectrophotometer (Edinburgh Instruments Ltd., UK), and a Xe900 lamp was used as an excitation source and white BaSO₄ powder as a reference.

III. Results and discussion

1 Phase structure

Fig. 1 shows the typical powder XRD patterns of RbBaPO₄:0.03Eu²⁺, NaCaPO₄:0.01Eu²⁺ and CsMgPO₄:0.07Eu²⁺ samples, and the standard data reported in the Joint Committee on Powder Diffraction Standards card data (JCPDS-81-647-RbBaPO₄, JCPDS-39-1193-NaCaPO₄ and JCPDS-45-275-CsMgPO₄) are given for comparison.^9–11 We found that no impurity or significant changes were detected in the hosts, which confirmed that these samples were successfully obtained by the solid-state reaction method. Furthermore, the refinements and data processing were performed by the GSAS program¹² for further demonstrating their phase structures. As shown in Fig. 2a–c, there were no impurities in the synthesized samples. We also obtained the Rietveld results presented in Table 1, where we can see that their volumes are 448.86 ų, 1009.16 ų and 474.49 ų, respectively, indicating the incorporation of Eu²⁺ in the host lattice. The reason is that, in the present work, the ionic radii are Ba²⁺_CN=9 = 1.47 Å, Ca²⁺_CN=7 = 1.06 Å, _CN=8 = 1.12 Å and Cs⁺_CN=6 = 1.67 Å, whereas the ionic radii for the six-, seven-, eight- and nine-coordinated Eu²⁺ are 1.17, 1.20, 1.25 and 1.30 Å,¹³ respectively. Note that when doping a small amount of Eu²⁺ to replace the other ions, the unit cell parameters and volume will change somewhat. Moreover, we can see that the values of R_wp and R_p are quite a bit smaller; thus, the results can verify the phase formation.

Fig. 1 XRD patterns of the RbBaPO₄:Eu²⁺, NaCaPO₄:Eu²⁺ and CsMgPO₄:Eu²⁺ samples, the standard patterns of the RbBaPO₄ phase (JCPDS-81-647), NaCaPO₄ phase (JCPDS-39-1193) and CsMgPO₄ phase (JCPDS-45-275).

Fig. 2 Rietveld plots of the (a) RbBaPO₄:Eu²⁺, (b) NaCaPO₄:Eu²⁺ and (c) CsMgPO₄:Eu²⁺ samples.

Table 1 Main parameters for RbBaPO₄:Eu²⁺, NaCaPO₄:Eu²⁺, and CsMgPO₄:Eu²⁺ from results of the GSAS Rietveld refinement

Formula	RbBaPO4:Eu^2+	NaCaPO4:Eu^2+	CsMgPO4:Eu^2+
Space group	Pnma	Pn(21)a	P1n1
α = β = γ, °	90	90	90
2θ-interval, °	10–80	10–80	10–80
a (Å)	7.845	20.403	8.930
b (Å)	5.7285	5.400	5.515
c (Å)	10.039	9.158	9.633
V (Å^3)	448.86	1009.16	474.49
Z	4	4	2
Rwp (%)	2.67	5.13	5.48
Rp (%)	1.69	3.65	3.97

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2 Luminescence properties

Photoluminescence excitation (PLE) and emission (PL) spectra of RbBaPO₄:0.03Eu²⁺, NaCaPO₄:0.01Eu²⁺ and CsMgPO₄:0.07Eu²⁺ phosphors are shown in Fig. 3, where the concentration of Eu²⁺ in the three phosphors all correspond to the maximum of luminescence intensity from previous reports.^5–7 The excitation spectra are all composed of strong broad absorption bands from 200 to 420 nm UV and n-UV range, attributed to 4f⁷(⁸S_7/2)–4f⁶5d transitions of the doped Eu²⁺ ions.¹⁴ Under the excitation at 340 nm, the phosphors exhibited strong blue, green and red emission bands, peaking at 430 nm, 510 nm and 630 nm, which can verify the possibility of their operation as the n-UV chip for w-LEDs application. In order to study the relationship between the coordination environment of Eu²⁺ and emission peaks, the following eqn (1) by van Uitert was employed:¹⁵where E represents the position for the rare-earth ion emission peak (cm⁻¹), Q is the energy position of the lower d-band edge for the free ions (34 000 cm⁻¹ for Eu²⁺), V is the valence of the “active” cation (V = 2 for Eu²⁺), n is the number of anions in the immediate shell around the “active” cation, r is the effective radius of the host cation replaced by the Eu²⁺ ion (Å), and ea is the electron affinity of the atoms that form anions (eV). The value of ea is the uncertainty in different circumstances; here, ea is about 1.60 or 2.19 eV.¹⁵ Therefore, by the semiquantitative calculation, we get the results of E are approximately Ba²⁺_CN=9 = 23 558 cm⁻¹, Ca²⁺_CN=7 = 18 939 cm⁻¹, _CN=8 = 20 333 cm⁻¹ and Cs⁺_CN=6 = 16 325 cm⁻¹, respectively. Fig. 4a) shows the normalized emission spectra for RbBaPO₄:Eu²⁺, NaCaPO₄:Eu²⁺ and CsMgPO₄:Eu²⁺ phosphors under the excitation at 340 nm, and Fig. 4b) shows the coordination environment of the cations Ba²⁺, Ca²⁺ and Cs⁺, which illustrates that Eu²⁺-activated orthophosphate phosphors are definitely able to realize the blue to red emission for white light-emitting diodes (w-LEDs) application. In addition, a qualitative analysis of the relationship between crystal field splitting (ε_cfs) and coordination number or polyhedron size is discussed and is presented in Fig. 5. In this work, ε_c is the centroid shift measured from the excitation spectrum:¹⁶ ε_c (RbBaPO₄:Eu²⁺) < ε_c (NaCaPO₄:Eu²⁺: Ca²⁺_CN=8) < ε_c (NaCaPO₄:Eu²⁺: Ca²⁺_CN=7) < ε_c (CsMgPO₄:Eu²⁺). Dorenbos et al.^17–19 reported that the smaller is the coordination number, the larger is the crystal field splitting in various Ce³⁺-doped phosphors. Similarly, the values of ε_cfs in Eu²⁺-doped phosphors are ε_cfs (RbBaPO₄:Eu²⁺_CN=9) < ε_cfs (NaCaPO₄:Eu²⁺_CN=8) < ε_cfs (NaCaPO₄:Eu²⁺_CN=7) < ε_cfs (CsMgPO₄:Eu²⁺_CN=6). Therefore, we obtain the result that the larger are ε_cfs and ε_c, the smaller is the wavenumber, which indicates that CsMgPO₄:Eu²⁺ shows the longest emission wavelength at 630 nm among the above phosphors under the same excitation wavelength, and contributes much to the 5d crystal field splitting.

Fig. 3 Photoluminescence excitation (PLE) and photoluminescence (PL) spectra of blue-emitting RbBaPO₄, green-emitting NaCaPO₄ and red-emitting CsMgPO₄ phosphors.

Fig. 4 (a) The normalized emission spectra for RbBaPO₄:Eu²⁺, NaCaPO₄:Eu²⁺ and CsMgPO₄:Eu²⁺ phosphors under excitation at 340 nm. (b) Coordination environment of the cations Ba²⁺, Ca²⁺ and Cs⁺, respectively.

Fig. 5 Energy diagram of 5d configuration and coordination numbers of Ba²⁺, Ca²⁺ and Cs⁺ in the RbBaPO₄:Eu²⁺, NaCaPO₄:Eu²⁺ and CsMgPO₄:Eu²⁺ hosts.

3 Surface topography, thermal decay and quantum efficiency (QE) properties

The surface topography of the RbBaPO₄:Eu²⁺, NaCaPO₄:Eu²⁺, CsMgPO₄:Eu²⁺ and the mixture of those tri-color phosphors were inspected using a scanning electron microscope (SEM) to investigate the particle size. Fig. 6 exhibits the SEM images of (a) RbBaPO₄:Eu²⁺, (b) NaCaPO₄:Eu²⁺, (c) CsMgPO₄:Eu²⁺ and (d) the mixture of those tri-color phosphors. It can be seen that all the images are with a wide range of particle size, including agglomerates of the smaller particles, but the particle size of the (a) compound RbBaPO₄:Eu²⁺ is slightly bigger than the other two compounds (b) NaCaPO₄:Eu²⁺ and (c) CsMgPO₄:Eu²⁺. However, when the three phosphors were mixed to acquire the w-LEDs lamp according to the ratio of 1 : 19 : 16, based on the relative intensity of the emission spectrum from the same decay test conditions, the SEM image of the mixed phosphor was given in Fig. 6d, which shows that the particle size of the mixture was similar to those three individual samples. From the above results, it can be expected that the three phosphors, i.e. RbBaPO₄:Eu²⁺, NaCaPO₄:Eu²⁺ and CsMgPO₄:Eu²⁺, should be quite suitable to be fabricated for potential application in white LEDs. The thermal decay and quantum efficiency (QE) properties were also measured. As shown in Fig. 7, the relative intensities of temperature-dependent emission spectra for RbBaPO₄:Eu²⁺ and NaCaPO₄:Eu²⁺ show better thermal stabilities, but CsMgPO₄:Eu²⁺ is inferior to the others under excitation at 340 nm. The PL relative intensity CsMgPO₄:Eu²⁺ at 423 K (150 °C) drops to 46.7% of the initial value at room temperature, and it seems to be a disadvantage to adopt this phosphor for white LEDs application because of the low luminescence thermal stability. However, the other two phosphors all have better thermal stabilities, and when the same orthophosphate phosphors of RbBaPO₄:0.03Eu²⁺, NaCaPO₄:0.01Eu²⁺ and CsMgPO₄:0.07Eu²⁺ are blended, the monolithic thermal stability should be improved for potential application in warm white LEDs. From the thermal stability results, it could be expected that the luminescence intensity of CsMgPO₄:0.07Eu²⁺ would have very low QE. Based on the following eqn (2),²⁰

η_QE = ∫L_S/(∫E_R − ∫E_S) (2)

where L_S is the luminescence emission spectrum of the sample; E_S is the spectrum of the light used for exciting the sample; and E_R is the spectrum of the excitation light without the sample in the sphere, all the spectra were collected using the integrating sphere. The measured internal QE values of RbBaPO₄:0.03Eu²⁺, NaCaPO₄:0.01Eu²⁺ and CsMgPO₄:0.07Eu²⁺ phosphors were determined to be about 54.06%, 36.33% and 13.40% under 340 nm excitation, respectively. The QE values are consistent with the results of the thermal stability property.

Fig. 6 SEM images of the (a) RbBaPO₄:Eu²⁺ and (b) NaCaPO₄:Eu²⁺ (c) CsMgPO₄:Eu²⁺ and (d) the mixture of those tri-color phosphors with a wide range of particle size, including agglomerates of the smaller particles.

Fig. 7 The relative intensities of temperature-dependent emission spectra for RbBaPO₄:0.03Eu²⁺, NaCaPO₄:0.01Eu²⁺ and CsMgPO₄:0.07Eu²⁺ samples under 340 nm excitation.

4 CIE chromaticity coordinates and electroluminescence properties

The CIE chromaticity diagrams for RbBaPO₄:Eu²⁺, NaCaPO₄:Eu²⁺ and CsMgPO₄:Eu²⁺ phosphors are shown in Fig. 8, and the insets show the digital phosphor images under 365 nm UV-lamp excitation. All phosphors show intense emission, which indicates they can be applied to the white light-emitting diodes as the three primary color components. In order to evaluate the actual potential application of the phosphors for pc-WLEDs, a w-LED lamp was successfully fabricated by the combination of a 370 nm n-UV InGaN chip and blue-emitting RbBaPO₄:Eu²⁺, green-emitting NaCaPO₄:Eu²⁺ as well as red-emitting CsMgPO₄:Eu²⁺ phosphors; moreover, the ratio of the content of the three phosphors for the fabrication is 1 : 19 : 16, based on the relative intensity of emission spectrum from the same decay test conditions. Photoluminescence and electroluminescence spectra of the same compounds are shown in Fig. 9. Fig. 9a shows the PL spectra under the excitation at 370 nm before fabrication, which reveals a low color rendering index (CRI) of 72 and a high correlated color temperature (CCT) of 5656 K with coordinates of (0.3295, 0.3052), while the fabricated w-LED lamp generates a warmer CCT of 2751 K and a superior CRI of 86 with coordinates of (0.3744, 0.3153). Note that the inset to Fig. 9b shows a photograph of a white-light emission when driven by a 25 mA current. Moreover, the EL spectra in Fig. 9b shows a distinctive phenomenon that the two emission peaks of RbBaPO₄:Eu²⁺ and NaCaPO₄:Eu²⁺ decreased while the emission peak of CsMgPO₄:Eu²⁺ increased. The PL and PLE spectra of these tri-color phosphors are shown in Fig. 10, and it is found that the excitation peaks for RbBaPO₄:Eu²⁺ (near 420 nm) and NaCaPO₄:Eu²⁺ (near 510 nm) phosphors appear to laterally overlap with emission peaks of the mixed PL spectra from a range of 400 nm to 530 nm, as shown in the dashed area of Fig. 10, where CsMgPO₄:Eu²⁺ phosphor can absorb the blue and green lights emitted by RbBaPO₄:Eu²⁺ and NaCaPO₄:Eu²⁺. This can clarify the mechanism of self-absorption in the mixture between those tri-color phosphors,²¹ which enhances the long wavelength emissions for w-LEDs compared with a blue InGaN LED chip with a yellow phosphor YAG:Ce³⁺. Therefore, the fabrication experiment of the w-LED lamp demonstrates that such phosphors and their mixing strategy are promising for w-LEDs.

Fig. 8 CIE chromaticity diagram and selected phosphor images of blue-emitting RbBaPO₄, green-emitting NaCaPO₄ and red-emitting CsMgPO₄ orthophosphate phosphors under 365 nm UV-lamp excitation.

Fig. 9 Photoluminescence and electroluminescence spectra combined with RbBaPO₄:Eu²⁺, NaCaPO₄:Eu²⁺ and CsMgPO₄:Eu²⁺ orthophosphate phosphors: (a) PL spectra under excitation at 370 nm. (b) EL spectra of w-LED lamp fabricated using a 370 nm n-UV chip.

Fig. 10 Photoluminescence excitation (PLE) spectrum of blue-emitting RbBaPO₄, green-emitting NaCaPO₄ and red-emitting CsMgPO₄ phosphors and photoluminescence (PL) spectra for the mixture of those tri-color phosphors.

IV. Conclusions

In summary, blue-emitting RbBaPO₄:Eu²⁺, green-emitting NaCaPO₄:Eu²⁺ and red-emitting CsMgPO₄:Eu²⁺ phosphors were prepared by a high temperature, solid-state reaction method. The emission peaks near 430 nm, 510 nm and 630 nm for the obtained samples are ascribed to the 4f–5d transition of Eu²⁺. The qualitative analysis between crystal field splitting and coordination number or polyhedron size were employed to explain the observed emission spectra. A white light-emitting diode (LED) lamp was fabricated based on these phosphors combining a 370 nm n-UV chip showing a high color rendering index (CRI) of 86 and a warm correlated color temperature (CCT) of 2751 K, which indicated that the Eu²⁺-activated versatile orthophosphate phosphors have potential application in warm white LEDs.

Acknowledgements

The present work was supported by the National Natural Science Foundation of China (Grant no. 51002146, no. 51272242), the Natural Science Foundation of Beijing (2132050), the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-12-0950), the Beijing Nova Program (Z131103000413047), the Beijing Youth Excellent Talent Program (YETP0635) and the Funds of the State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University (KF201306).

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