Full-color-emitting CaYAl3O7:Pr3+,Ce3+ phosphor for near-UV LED-based white light

doi:10.1016/j.jlumin.2013.11.039

Journal of Luminescence

Volume 152, August 2014, Pages 176-181

https://doi.org/10.1016/j.jlumin.2013.11.039 Get rights and content

Highlights

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A full color emitting phosphor CaYAl₃O₇:Pr³⁺ was developed.
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Structural properties were analyzed using neutron diffraction accompanied with MEM analysis.
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Emission spectral properties were improved by coactivating with Ce³⁺ ions.
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Energy transfer from Pr³⁺ levels to Ce³⁺ levels was observed.

Abstract

A full-color-emitting phosphor, CaYAl₃O₇:Pr³⁺,Ce³⁺ was synthesized through conventional solid-state reaction. The structural properties of the phosphor were analyzed by Rietveld refinement, maximum entropy method, and luminescent properties were analyzed by co-activating with different rare earth ions. Photoluminescence spectra on excitation with 450 nm source were dominated by green band at 490 nm originating from ³P₀→³H₄ transition of Pr³⁺ ions and less intense band at 617 nm due to ¹D₂→³H₄ transition. On codoping with Ce³⁺, phosphor exhibits an additional broad band originating from 5d¹→4f¹ (²F_7/2 and ²F_5/2) of Ce³⁺, centered at around 540 nm due to efficient energy transfer from Pr³⁺ levels to Ce³⁺ levels. With this emission property we recommend CaYAl₃O₇:Pr³⁺,Ce³⁺ as potential single-phase full-color-emitting phosphor for solid-state lighting applications.

Introduction

White LEDs based on the combination of a blue-emitting LED chip with broad band emitting yellow phosphor has been extensively used as a source of illumination over past several years [1], [2]. The search for an ideal phosphor for the purpose has not reached anywhere beyond the Y₃Al₅O₁₂:Ce³⁺ (YAG) phosphor, primarily due to its unparalleled efficiency. However the phosphor which has relatively weak emission in the red spectral region yields poor color rendering index, in combination with blue LEDs [3]. In order to improve the color rendering index of the combination, YAG phosphor in combination with red emitting phosphors was attempted [3], [4]. These types of LEDs offer superior color uniformity, high color rendering index, but luminous efficacy of these white LEDs is low due to reabsorption of emission colors. As a solution of these, a single phased full-color emitting phosphors have been proposed [5], [6], [7], [8].

Single phased full-color-emitting phosphors, by nullifying the chance for reabsorption of the light emitted by the blue, green or red phosphors improve the efficacy of the system. The most common strategy to develop such a phosphor is by codoping with suitable ions and utilizing the energy transfer from activators to coactivators [5], [9]. However host compounds which can emit full-color spectrum when doped with specific activator ions are few in number. Among the full-color emitting phosphors, singly doped full-color-emitting phosphors are the most preferred ones as the energy loss in such phosphors could be the lowest possible, by avoiding the loss associated with inter-ionic energy transfer even though only very few host compounds are suitable for such kind of phosphors.

In this study, we have developed a single phased full-color-emitting phosphor, Ca_1−x−yPr_xM_yYAl₃O₇ and investigated its structure and feasibility of emission with higher color rendering index. By coactivating Ca_1−x−yPr_xM_yYAl₃O₇ with few broad band emitting ions M (M=Ce³⁺, Tb³⁺, Mn²⁺) we have described its structure and optical properties in view of application in solid state lighting devices.

Section snippets

Experimental procedure

The CaY_1−xPr_xAl₃O₇ phosphor was synthesized by conventional solid-state reaction method. The starting materials were CaCO₃ (Aldrich, 99.99%), Y₂O₃ (Aldrich, 99.99%), α-Al₂O₃ (Kojundo, 99.99%), Pr₆O₁₁ (Aldrich, 99.99%), Tb₄O₇ (Aldrich, 99.99%) and MnO₂ (Aldrich, 99.99%). Raw materials in stoichiometric ratios were mixed using an agate mortar and pestle for about 1 h, with acetone as the dispersing medium. The dried mixture was then heated at various temperatures, ranging from 1300 to 1500 °C for 4

Results and discussion

Unit cell representation of CaYAl₃O₇ (CYA) crystal, which structurally belongs to the melilite structure group (tetragonal, ${P \bar{4} 2}_{1} m$ , S.G. # 113)[12] is shown in Fig. 1. The melilite group is composed of minerals which were widely investigated as laser materials and phosphors. These compounds have a general formula ABC₃O₇, where A is Ca, Sr, and Ba; B is La and Gd, Y; and C is Al and Ga. In the unit cell (Z=2) of CYA, Pr and Y atoms occupy only the 4e positions. Half of the 4e sites are expected

Conclusions

Full-color-emitting phosphor CaY_0.7Pr_0.3Al₃O₇ (CYA:Pr³⁺) was synthesized using a solid-state reaction method. MEM analysis assisted by the Rietveld refinement reveals that strong antisite orderings of Ca²⁺/Y³⁺/Pr³⁺ on the 4e sites of tetragonal crystal. Luminescent properties of Ce³⁺ codoped sample, CaY_0.7Pr_0.3Ce_0.5Al₃O₇ indicate that it has typical blue, green-yellow, and red emission bands. The energy transfer mechanism among the activator and coactivator is identified as

Acknowledgments

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology. The authors also appreciate the financial support from the Ministry of Knowledge Economy, the Ministry of Education, Science Technology (MEST), and the National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation.

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Despite having a high color consistency and rendering index, these phosphors had low luminous efficiency. Therefore, it was proposed to develop a single phase full-color emitting phosphor [11,12]. CaYAl3O7 is one such phosphor belonging to the melilite family of materials that is being investigated immensely for its luminescence properties [13–17].
A series of CaYAl₃O₇ phosphors doped with different concentrations of Pb²⁺ were prepared by an auto solution combustion route. The structure, luminescence, and electron paramagnetic resonance (EPR) spectra were analysed. The X-ray diffraction (XRD) patterns identify the tetragonal phase formation of CaYAl₃O₇ with a crystallite size in a range of 37–42 nm. The scanning electron microscope (SEM) images were captured to study the surface morphology of the prepared samples. The emission spectra for the prepared phosphors exhibit a wide emission band centered at 362 nm under 270 nm excitation. Un-doped and Pb²⁺ doped CaYAl₃O₇ phosphor exhibit an EPR spectrum which arises from two noticeable defect centers. Center I with a g-value equal to 1.96 and a relatively broad line width is temporarily defined to an F⁺ - center (an electron trapped at an anion vacancy). Center II with g = 2.0 is also ascribed to an F⁺ - center.
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With the enhancing of people's awareness of environment protection, the studying of phosphor-converted white light emitting diodes (pc-WLEDs) had attracted much attention due to the eminent advantages in high luminescent efficiency, low energy consumption, superior lifetime and excellent reliability. The pc-WLEDs as eco-benign products have become the most potential light source in the next generation [1–10]. Moreover, trivalent rare-earth ions (RE3+)-doped aluminate phosphors are appropriate for light-conversion materials owing to their excellent luminescent efficiency, high thermal and chemical stability [5,7,8,11–14].
The color-tunable GdCaAl₃O₇: RE³⁺ (RE = Eu, Tb, Eu@Tb) phosphors were synthesized via an ultrafast sol-gel auto-combustion synthesis method. The X-ray diffraction patterns confirmed the forming of GdCaAl₃O₇ tetragonal phase, and the scanning electron microscopy images indicated that the obtained products presented honeycomb-like structure due to the gas releasing during the auto-combustion process. The luminescent properties, concentration quenching mechanism, energy transfer mechanism, luminescent dynamics, thermal stability and product practicability of GdCaAl₃O₇: RE³⁺ (RE = Tb, Eu, Eu@Tb) phosphors were investigated systematically. All the GdCaAl₃O₇: RE³⁺ products presented the characteristic emission of doped RE³⁺. Using GdCaAl₃O₇: 50% Tb³⁺@3.0% Eu³⁺ as yellow-emitting phosphors in combination with commercial BAM: Eu²⁺ blue-emitting phosphors and a NUV chip can successfully package a warm white LED device with high color rendering index (CRI, 86.3) and low correlated color temperature (CCT, 3348 K) that was very suitable for near-ultraviolet light-triggered indoor illumination.
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Pr3+ ion is considered to be an attractive activator, which can be produced the blue, green, orange, red and infrared emission due to the rich energy level structure [14]. Ce3+ and Pr3+ doped crystalline materials have been found to be interesting as they find their utility in elementary particle scintillations, surgical gamma probes, white LEDs and X-ray imaging arrays [15–18]. Several studies have already been made on the preparation and optical performance of alkali halides (small size crystals and powders) doped with Ce3+ or Pr3+ ions [14,19–21].
Large size single crystals of NaCl:Ce³⁺,XPr³⁺ (with X = 0.0, 0.002, 0.005, 0.008, 0.02, 0.05 and 0.08) were successfully grown by the melt (Czochralski) method and characterized structurally and optically. X-ray diffraction analysis has revealed the incorporation of Ce³⁺ and Pr³⁺ ions into the NaCl matrix and the creation of vacancy centers (Vʺ_Na) and positive defects (Ce_Na^●● and Pr_Na^●●) to keep the charge balance. Optical absorption spectral analysis has indicated the presence of absorption peaks at 214, 225, 305, 342, 446, 469, 483 and 592 nm. Photoluminescence (PL) spectral analysis (excited with 259 nm) has revealed dominant emission peaks at 386 and 390 nm due to 5d→4f transition of Ce³⁺ ions and sharp lines at 479, 491, 532, 600 and 621 nm due to ³P₁→³H₄, ³P₀→³H₄, ³P₁→³H₅, ³P₀→³H₆ and ³P₀→³F₂ transitions of Pr³⁺ ions. The energy transfer mechanism from Ce³⁺ to Pr³⁺ in the NaCl crystal has been described and the color coordinates of the luminescence of NaCl:Ce³⁺,XPr³⁺ crystals have been calculated by using the emission spectral data. The thermo-luminescence (TL) glow curves of NaCl:Ce³⁺,XPr³⁺ crystals obtained show three peaks at about 83, 150 and 239 °C, suitable trap depths induced by Ce³⁺ and Pr³⁺ and resulted in the characteristic emission. The TL mechanism of NaCl:Ce³⁺,XPr³⁺ crystals has been discussed. The photoluminescence and thermo-luminescence performances have been found to depend strongly on the Pr ion concentration and the maximum intensity has been obtained for the crystal with X = 0.008. Also, the increase in Pr ion concentration has led to gradual decrease of luminescence decay time.

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Journal of Luminescence

Highlights

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussion

Conclusions

Acknowledgments

References (27)

J. Lumin.

J. Lumin.

Chem. Phys. Lett.

Solid State Commun.

J. Alloys Compd.

J. Non-Cryst. Solids

J. Alloys Compd.

Acta Mater.

Bull. Mater. Sci.

Chem. Commun.,

J. Mater. Chem.

Dalton Trans.

Cited by (32)

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Photoluminescence and defect centers in near violet emitting Pb<sup>2+</sup> doped CaYAl<inf>3</inf>O<inf>7</inf> phosphors obtained by urea-nitrate auto combustion synthesis

Sol-gel auto-combustion synthesis and luminescence properties of GdCaAl<inf>3</inf>O<inf>7</inf>: RE<sup>3+</sup> (RE=Eu, Tb, Eu@Tb) phosphors for near-ultraviolet light-triggered indoor illumination

CaGdAl<inf>3</inf>O<inf>7</inf>:Eu<sup>3+</sup> unidimensional nanostructures: Facile electrospinning synthesis, structure and luminescence

Effect of Pr ion concentration on the luminescence properties of NaCl:0.01Ce<sup>3+</sup>,Pr<sup>3+</sup> crystals grown in large size