Materials science communication
A novel green-emitting Ca15(PO4)2(SiO4)6:Eu2+ phosphor for applications in n-UV based w-LEDs

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Abstract

A novel green-emitting phosphor Eu2+-doped Ca15(PO4)2(SiO4)6 with a structure derivative of α′-Ca2SiO4 is synthesized by a conventional solid-state reaction method. The photoluminescence properties, the concentration quenching of Eu2+ ion, and the thermal stability are investigated. The Ca15(PO4)2(SiO4)6:Eu2+ phosphor is excited efficiently at a wavelength of 330 nm. At the optimum doping concentration of 0.5 mol% Eu2+ ion, the Ca15(PO4)2(SiO4)6:Eu2+ phosphor has the strongest emission intensity at 491 nm. The main mechanism of concentration quenching is found to be the rapid migration of excitation between the excited Eu2+ ions by exchange coupling. The thermal activation energy barrier is confirmed as 0.244 eV. Furthermore, the performance of this phosphor is tested by the fabrication of a phosphor-converted LED (pc-LED) through the integration of an InGaN-based near-ultraviolet light-emitting diode (LED) and driving under from 50 mA to 300 mA. The pc-LED reveals high color rendering index (CRI) value over 50, which is good for applications in w-LED.

Highlights

► A novel phosphor Ca15(PO4)2(SiO4)6:Eu2+ is synthesized by a solid-state reaction. ► We discussed the mechanism of concentration quenching. ► The Ca15(PO4)2(SiO4)6:Eu2+ phosphor can be excited efficiently with n-UV LEDs.

Introduction

Since the traditional incandescent and fluorescent lamp have many weaknesses (high power consumption, short lifetime, toxic mercury), white light-emitting diodes (w-LEDs) have been considered as a next generation light source [1]. Furthermore, phosphor-converted w-LEDs have been attracted more attention due to their various applications such as display backlighting, LED flashlights, architectural lighting, indicator lamps [2], [3], [4]. One of the conventional methods to fabricate a w-LED is pumping a bright commercial yellow phosphor, YAG:Ce3+, with a blue LED [5]. However, this w-LED generates bright cold white light; it cannot generate warm white light because of poor red emission. Furthermore, there are some problems such as color change with input current and low color rendering index (CRI) due to two-color mixing. Combining tri-color phosphors with n-UV LEDs is a potential alternative method to fabricate w-LEDs [6], since n-UV-phosphor-converted LEDs are expected to have many potential applications due to their excellent color rendering index, high color tolerance, and high conversion efficiency into visible light [7].

Rare-earth-ion-doped alkaline earth silicate phosphors such as CaMgSi2O6:Eu2+ and M2SiO4:Eu2+ (M = Ca, Sr, Ba) are plausible candidates for w-LEDs because they have high luminescence properties and physical/chemical stability [8], [9]. Additionally, Eu2+ ion is one of the most widely used activators, because Eu2+-doped phosphors usually show intense broad band photoluminescence (PL) with a short decay time on the order of tens of nanoseconds. The emission of Eu2+ ion is very strongly dependent on the host lattice and can occur from the ultraviolet to the red region of the electro-magnetic spectrum [10]. In particular, β-Ca2SiO4 host, which has been considered as a green-emitting material for w-LEDs, is stable at room temperature [11], whereas α and α′ phase are not stable without the addition of Ca3(PO4)2 [12]. H. Saalfeld et al. reported that Ca15(PO4)2(SiO4)6 with a structure derivative of α′-Ca2SiO4 was stable at room temperature with the chemical formula, 6Ca2SiO4·1Ca3(PO4)2 [13]. However, to our best knowledge, there has been no report on a Eu2+-doped Ca15(PO4)2(SiO4)6 phosphor with a structure derivative of α′-Ca2SiO4 for potential application as a phosphor for w-LEDs.

In this paper, a Ca15(PO4)2(SiO4)6:Eu2+ phosphor was synthesized by a solid-state reaction, and its luminescence characteristics, e.g., excitation, emission, concentration quenching of Eu2+ ion, and thermal activation energy barriers for luminescence, were investigated in order to search for new phosphor materials with potential applications for n-UV based w-LEDs. We also reported the fabrication and the characteristics of phosphor-converted LEDs (pc-LEDs), which consisted of n-UV LED chips and Ca15(PO4)2(SiO4)6:Eu2+ phosphors.

Section snippets

Preparation of Ca15(PO4)2(SiO4)6:Eu2+ phosphor

Powder samples of Ca15(1−x)(PO4)2(SiO4)6:15xEu2+ (0.0025 ≤ x ≤ 0.015) were prepared by conventional solid-state reaction method. To synthesize Ca15(PO4)2(SiO4)6:Eu2+, the reaction materials CaCO3 (Kojundo 99.99%), (NH4)2HPO4 (Junsei 99%), SiO2 (Kojundo 99.9%) and Eu2O3 (Kojundo 99.9%) were used. Stoichiometric amounts of raw materials were mixed by ball milling using ZrO2 balls and ethanol for 24 h and dried on hot plates in air. Then the mixture was ground well and pre-heat-treated in an

Results and discussion

The differential volume of Ca14.925(PO4)2(SiO4)6:0.075Eu2+ phosphor versus particle diameter is plotted in Fig. 1. As shown in Fig. 1, PSD of the Ca14.925(PO4)2(SiO4)6:0.075Eu2+ phosphor is irregular and the mass-median-diameter is 4.402 μm. The XRD patterns of Ca15(PO4)2(SiO4)6:Eu2+ phosphors are presented in Fig. 2. The patterns (a)–(e) correspond to prepared Ca15(1−x)(PO4)2(SiO4)6:15xEu2+ phosphors (x = 0.0025, 0.005, 0.0075, 0.01 and 0.015, respectively). All of the patterns are in good

Conclusion

A series of novel green emitting phosphors Eu2+ ion-doped Ca15(PO4)2(SiO4)6 with a structure derivative of α′-Ca2SiO4 are synthesized by a conventional solid-state reaction method, and its photoluminescence properties concerning application to pc-LEDs are investigated. The Ca15(PO4)2(SiO4)6:Eu2+ phosphor can be excited efficiently with n-UV LEDs, having broad excitation spectrum in the n-UV region. At the optimum doping concentration of 0.5 mol% Eu2+ ion, the Ca15(PO4)2(SiO4)6:Eu2+ phosphor has

Acknowledgments

This work was partly supported by the IT R&D program of MKE/IITA [2009-F-020-01, Development of Red nitride phosphor and self-assembly phosphorescent layer packaging technology for high rendition LED illumination] and the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MEST) (No. 2009-0094046).

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