Optical properties of MAl12O19:Eu (M = Ca, Ba, Sr) nanophosphors

The MAl12O19:Eu (M = Ca, Ba, Sr) phosphor were synthesized by combustion method and systematically characterized by photoluminescence excitation and emission spectra, concentration quenching, morphology and X-ray mapping with scanning electron microscopy. In SrAl12O19:Eu phosphor two PL emission peaks are observed at about 389 nm and another around 420 nm as well as BaAl12O19:Eu phosphor shows blue emission around 460 nm is observed in the blue region of the spectrum and CaAl12O19:Eu shows only red emission at 592 as well as 615nm. Both phosphors can be efficiently excited in the wavelength range of 250-425 nm, where the near UV (~320 nm) solid state excitation is matched. By combining MAl12O19:Eu (M = Ca, Ba, Sr) phosphor with near UV chops emitting intense blue green (Ba), yellow-red (Ca) and blue purple (Sr) LEDs white LEDs can be produced. Copyright © 2011 VBRI press.


Introduction
Rare-earth and non-rare-earth-doped inorganic phosphors are widely used in a variety of applications, such as lamp industry, radiation dosimetry, X-ray imaging, and colour display. In particular, the luminescent properties of europium-ion doped phosphors have been studied extensively for their applications in these areas [1,2]. Eu 2+ activated phosphors MAl 2 O 4 and MAl 12 O 19 (M = Sr, Ba, Ca, Mg) are well known since the studies by Blasse and Brill [3] in the 1960s. Their researches lead to the conclusion that these compounds were adequate phosphorescent materials because of their high quantum efficiency in the visible region. The emission of Eu 2+ ions varies from blue to red depending on the host lattice due to crystal-field effects [2]. A completely new generation of persistent luminescent phosphors, Eu 2+ doped alkaline earth aluminates, MAl 2 O 4 :Eu 2+ (M = Ca, Sr), has been developed to replace ZnS:Cu [4]. At present, complex aluminates [5] as well as other materials [6] are subject to investigation.
Up to date, very little literatures reported the synthesis of nanostructured MAl 12 O 19 (M = Ca, Ba, Sr) by combustion method. Douy and Capron [7] prepared SrAl 12 O 19 powders by spray-drying aqueous solutions of strontium and aluminium nitrates followed by heating the powders to decompose the nitrates. Chen et al. [8] synthesized Eu 2+ and Dy 3+ co-doped SrAl 12 O 19 by a sol-gel process. In fact, BaAl 12 O 19 :Mn is known as a greenemitting phosphor for PDPs [9], and Pr-or Nd-doped SrAl 12 O 19 crystals has been suggested as one of the potential material with good laser properties [10]. In this article, we reported the synthesis of nanostructured MAl 12 2 ] were injected into the precursor solution or these compositions. The amount of metal nitrates (oxidizers) and urea (fuel) were calculated using the total oxidizing and reducing valencies of the components, which serve as the numerical coefficients so that the equivalence ratio is unity and the heat liberated during combustion is at a maximum. After stirring for about 15 min, precursor solution was transferred to a furnace preheated to 400°C and 500°C, the porous products were obtained.
The phase composition and phase structure were characterized by X-ray diffraction (XRD) pattern using a PAN-analytical diffractometer with Cu Kα radiation (λ = 1.5405 A 0 ) operating at 45kV, 40mA. The morphology and the composition of the products were examined by scanning electron microscopy (SEM, JED-2300) equipped with an energy-dispersive spectrometry (EDS). Energy dispersive spectrometry (EDS) attached to the JEOL 2300 was used to determine the composition of the products. The photoluminescence properties of the phosphor (excitation and emission) were measured using a Shimadzu RF5301PC Spectroflurophotometer at room temperature.

Structural property
The overall structure and phase purity of as synthesized products were characterized by XRD. Fig. 1 shows a typical XRD pattern of the M 1-x Al 12 O 19 :Eu x (M = Ca, Ba, Sr). All the phosphors are hexagonal structure with the space group P63/mmc. The diffraction peaks of BaAl 12 O 19 , SrAl 12 O 19 and CaAl 12 O 19 can be easily indexed with the JCPDS card no 00-026-0135, card no. 00-026-0976 and card no. 00-25-0122 respectively. No other crystalline phases were detected within the detection limit. The XRD results indicated that the as synthesized products are highly pure, single-phase.
The small amount of doped rare earth ions has virtually no effect on the phase structures. The average sizes calculated from XRD reflections for BaAl 12 O 19, CaAl 12 O 19 and SrAl 12 O 19 by application of Scherrer's equation are 40, 51 and 51 nm, respectively. It is known that the full-width at half-maximum (FWHM) can be expressed as a linear combination of the contribution from the lattice strain and crystalline size. The effects of the strain and particle size on the FWHM can be expressed by the following equation: D hkl = 0.9 λ / β cos θ where β is the measured FWHM (in radians), θ is the Bragg angle of the peak, λ is the X-ray diffraction wavelength.  Copyright © 2011 VBRI press.

Photoluminescence properties
Europium ions can be stabilized in host lattice in either divalent or trivalent oxidation state. The incorporation and stabilization of Eu ions in the sample were confirmed by the luminescence investigations. The photoluminescence emission spectra for the M 1-x Al 12 O 19 : Eu x (M = Sr, Ba, Ca) phosphors by 320 nm excitation shown in Fig. 2 a- Fig. 2a, b and c

Morphology and X-ray mapping
The typical morphology and X-ray mapping of powders is shown in Fig. 3, which displays an SEM micrograph of a powder synthesized by combustion method. Fig. 3a shows the SrAl 12 O 19 : Eu powder, which is formed by large porous aggregates, with typical sizes of 1-10 μm (Fig. 3a). In Fig.  3b, micrographs of CaAl 11 O 12 : Eu shows the pores produced by the fast expulsion of gases during the combustion process can be observed. SEM observations with higher magnification showed that these aggregates were constituted by nanoparticles. It is worth remarking that the shape of nanoparticles is typical of powders synthesized by the combustion process, and it has been found for other materials [12,13]. This is an important result, since it confirms that the calcination temperature can be reduced in order to get a higher specific surface area and a smaller crystallite size. Moreover, in some faces the holes are observed which formed during the gas evolution such as N 2 , H 2 and nascent oxygen. Fig. 3a show the SrAl 12 O 19 : Eu phosphor consists of a flower like structure, while more porous and dense nano rod like structure is observed at higher magnification for CaAl 12 O 19 : Eu (Fig. 3b). In contrast, the BaAl 12 O 19 : Eu (Fig. 3c) show phosphor which comprises of nano size spherical particles.

Conclusion
In present work M