Elsevier

Optical Materials

Volume 29, Issue 7, March 2007, Pages 896-900
Optical Materials

Novel red phosphor of Bi3+, Sm3+ co-activated NaEu(MoO4)2

https://doi.org/10.1016/j.optmat.2005.12.010Get rights and content

Abstract

A novel phosphor, NaEu(MoO4)2, co-doped with Bi3+ ions and Sm3+ ions was prepared by solid-state reaction technique, and its photoluminescent properties were investigated. The introducing of Bi3+ ions and Sm3+ ions broadened the excitation band of the phosphor NaEu(MoO4)2 and enhanced the emission intensity of Eu3+ under 395/405 nm light excitation. The emission of the phosphor shows very good CIE (Commission Internationale de l’Eclairage, International Commission on Illumination) chromaticity coordinates (x = 0.66, y = 0.34). The phosphor may be applied in fabrication of phosphor-converted light-emitting diodes (LEDs).

Introduction

Phosphor-converted light-emitting diodes (LEDs) technique is an important kind of solid-state illumination [1], [2], [3], [4]. In these devices, the tricolor phosphors are pumped by UV-InGaN chips or blue GaN chips and generate white light [5], [6]. Presently, the main phosphors for near UV InGaN-based LEDs are BaMgAl10O17:Eu2+ for blue, ZnS:(Cu+, Al3+) for green, and Y2O2S:Eu3+ for red [7]. However, the red phosphor Y2O2S:Eu3+ shows much lower efficiency than that of green, blue phosphors [7]. Hence, it is urgent to search for new red phosphors that can be efficiently excited around 400 nm.

To obtain a red-emitting phosphor with proper CIE chromaticity coordinates, it is without doubt that the Eu3+ activated phosphors are considered firstly, because the lowest excited level (5D0) of the 4f6 configuration is situated below the 4f55d configuration for Eu3+, and it mainly shows very sharp 5D07F2 red emission lines around 612 nm when Eu3+ ions occupy the lattice sites without centro-symmetry. Double molybdates ALn(MoO4)2 (A = Li+, Na+, K+, Rb+, Cs+; Ln = trivalent rare earth ions), which share scheelite-like (CaWO4) iso-structure, show excellent thermal and hydrolytic stability, and are considered to be efficient luminescent hosts [8], [9], [10], [11]. As argued in our previous work [12], Ln3+ ions occupy the lattice sites without centro-symmetry in the host lattice. Therefore, the phosphors with CIE chromaticity coordinates close to the NTSC (National Television Standard Committee) standard values (x = 0.67, y = 0.33) were achieved in NaEu(MoO4)2.

The phosphors used in the present UV LEDs are excited by about 400 nm photons, though Eu3+ ions show 7F05L6 transition at about 395 nm in most hosts that are near the excitation energy, but this transition is parity-forbidden, so it is a narrow line and cannot absorb the excitation energy efficiently. In order to strengthen and broaden the absorption around 400 nm, one of the important approaches is introducing co-activator in the phosphor. We consider that Sm3+ and Bi3+ are probably two eligible co-activators. On one hand, Sm3+ ions exhibit strong line absorption that belongs to 6H5/2  4K11/2 transition at about 405 nm in many host lattices, which is close to the emission wavelength (400 nm) of InGaN-based LED; therefore, the absorption can be strengthened by means of Sm3+ and Eu3+ co-doping. On the other hand, Bi3+ that shows broad band-like absorption can also be selected. It is well known that the Bi3+ ions are very good sensitizers of luminescence in many hosts [13], [14], [15], [16]. It can efficiently absorb the UV-light and transfer the energy to the luminescent center, then the emission intensity of the luminescent center would be strengthened.

Our objective is to search for novel red phosphors that show strong and broad absorption around 400 nm for LEDs. Hence, the Bi3+ and Sm3+ ions were co-doped into phosphor NaEu(MoO4)2 and the photoluminescent properties of the co-doped phosphors were investigated.

Section snippets

Experimental

Samples NaEu1−xBix(MoO4)2 (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90 and 1.00) and NaEu0.80−xBi0.20Smx(MoO4)2 (x = 0.02, 0.04, 0.06, and 0.08) were prepared by solid-state reaction technique at high temperature. The starting stoichiometric mixtures were (NH4)6Mo7O24 · 4H2O (AR grade), NaHCO3 (AR grade), Bi2O3, Eu2O3 and Sm2O3 (99.99% purity). They were first ground and pre-fired at 500 °C for 4 h, then heated at 800 °C for 4 h.

The structure of these phosphors was

NaEu1−xBix(MoO4)2

The XRD patterns of NaEu1−xBix(MoO4)2 (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90 and 1.00) were measured. As examples, the patterns of the phosphors for x = 0, 0.10, 0.50 and 1.00 are shown in Fig. 1. Curve (a) is very consistent with the JCPDS 51-1508 [NaBi(MoO4)2], showing that the sample has a single phase with the scheelite structure. The other three curves are very similar to curve (a); it means they are of the iso-structure with the single phase, and Eu3+

Conclusions

Bi3+ and Sm3+ co-doped NaEu(MoO4)2 phosphors were prepared by solid-state reaction technique. The obtained NaEu0.76Bi0.20Sm0.04(MoO4)2 phosphor shows broadened excitation band around 400 nm, and enhanced red emissions due to Eu3+ f–f transitions under ∼400 nm light excitation. The CIE chromaticity coordinates (x = 0.66, y = 0.34) of the phosphor are close to NTSC standard values. All the results indicated that this novel red phosphor is a suitable candidate for the fabrication of near UV InGaN based

Acknowledgement

This work was financially supported by a research grant from the Guangdong province government (ZB2003A07).

References (16)

  • S. Neeraj et al.

    Chem. Phys. Lett.

    (2004)
  • F. Shi et al.

    J. Solid State Chem.

    (1996)
  • R. Mueller-Mach et al.

    IEEE J. Select. Top. Quant. Electron.

    (2002)
  • J.-H. Yum et al.

    J. Electron. Soc.

    (2003)
  • J.S. Kim et al.

    Appl. Phys. Lett.

    (2004)
  • J.K. Park et al.

    Appl. Phys. Lett.

    (2004)
  • Y. Narukawa et al.

    Jpn. J. Appl. Phys.

    (2002)
  • J.K. Sheu et al.

    IEEE Photon. Technol. Lett.

    (2003)
There are more references available in the full text version of this article.

Cited by (180)

View all citing articles on Scopus
View full text