Luminescence properties of Y3Al5O12:Eu3+-coated submicron SiO2 particles

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Abstract

Silica submicron spherical particles coated with an yttrium aluminum garnet (Y3Al5O12, YAG) layer doped with Eu3+ were prepared by the sol–gel method. The structure and morphology of samples determined by the X-ray powder diffraction measurements and transmission electron microscope images, respectively, indicated that well-crystallized garnet nanocrystallites were formed with successive coating cycles. Similar trends were deduced from the evolution of the luminescence spectra. The ratio of integrated intensities of the 5D0  7F2 and 5D0  7F1 transitions was used to analyze the structural variations in the surroundings of the Eu3+ ion. The effect of coating was analyzed by comparing the luminescence properties of the Y3Al5O12:Eu3+ nanocrystalline powders and composite Y3Al5O12:Eu3+/SiO2 materials.

Introduction

In recent years, the synthesis and study of luminescent core–shell materials has become of growing interest [1], [2], [3], [4], [5], [6], [7]. Both the core and shell can be optically active – depending on the purpose of the layered structure. Accordingly, the core–shell techniques can be used to protect the core material from interaction with the surrounding medium and/or to facilitate the modification of the surface. In the latter case, luminescent core is coated with one or several additional optically inactive layers [1], [2]. The core–shell technology allows the control of the spherical shape, requested for many phosphor materials. The cost of the actual luminescent material can be lowered by coating them on inexpensive cores [3], [4], [5], [6], [7]. One of the non-luminescent host materials frequently used for that purpose is the sol–gel derived silica. Amorphous SiO2 is not only non-toxic, but its surface can be modified, it can be easily covered with practically any type of shell, and it is a common and cheap material.

The rare-earth (R) doped luminophores are widely applied as the luminescent component of the core–shell structures [2], [3], [5], [6], [7]. The desired color of emission from such luminophores can be then achieved by choosing the appropriate rare-earth ion, e.g., Eu3+ for the red [8] or Tb3+ for the green [9]. Besides efficient emission obtained from the Eu3+ ion, its luminescence spectrum, especially the ratio between the electric dipole (5D0  7F2) and magnetic dipole (5D0  7F1) transition intensity, has been found useful in analyzing the changes in local structure around the dopant in different hosts [10], [11].

At present, the rare-earth doped core–shell materials are based mainly on oxide materials as phosphates [12], vanadates [2], [3], [5], tungstates [6] and perovskites [7] as well as binary oxides [13]. As an exception, rare-earth doped lanthanum trifluoride has been described [14], as well. For some applications, other host materials, especially garnets (R3Al5O12) may be preferred [15] for various reasons. In this work, for the first time, the method of preparation and preliminary luminescence studies are presented for the nanocrystalline Eu3+-doped yttrium aluminum garnet (Y3Al5O12:Eu3+, YAG:Eu3+) coated on spherical SiO2 cores.

Section snippets

Experimental

The colloidal suspension of amorphous SiO2 submicron spheres with the size of ca. 550 nm were prepared by using the Stöber method which involves the process of hydrolysis and condensation of tetraethoxysilane (TEOS) in a solution of ethanol, water and ammonia [16]. The obtained SiO2 particles were dried and washed several times with methanol. Simultaneously, aqueous yttrium and europium nitrate solutions were prepared by dissolving high purity Y2O3 (99.999%) and Eu2O3 (99.99%) with ultrapure

Results

The XRD analyses revealed the presence of a nanocrystalline Y3Al5O12:Eu3+ layer coated on an amorphous SiO2 core phase only when the SiO2 spheres were coated more than once. The rather broad but distinct reflection in the XRD patterns at 2θ = 33° belong to the yttrium aluminium garnet phase with cubic structure (space group: Ia3d) while the very broad feature at 2θ = 22° is characteristic for the amorphous SiO2 (Fig. 1). No crystalline phase was observed for the sample coated only once. Several

Discussion

The excitation of the Eu3+ emission occurs most efficiently through the charge transfer states. These states exhibit themselves as a broad band in the UV range – usually below 250 nm. The shoulder of this excitation band the width of which can be more than 4000 cm−1 can be observed in the excitation spectrum of the 5D0  7F1 transition at 590.1 nm (Fig. 4). The sharp and weak 4f–4f excitation lines can be observed, too. These lines are tentatively assigned in Fig. 4 though the high density of the 4f

Conclusions

The novel method of preparation of highly efficient core–shell composite Y3Al5O12:Eu3+/SiO2 material is presented. The effect of coating on the structure, morphology and spectroscopic properties was observed and trends were elucidated. The evolution of the intensity ration between the 5D0  7F2 and 5D0  7F1 transitions can indicate that the nanocrystals grown on the surface of SiO2 cores can have more distorted local environment of the Eu3+ ion than in the dispersed nanocrystals.

The luminescence

Acknowledgements

The authors thank Mr M. Gusowski, Mr P. Głuchowski and Mrs L. Krajczyk for measurement of the emission and excitation spectra, the lifetimes and TEM images, respectively. The exchange program between the Polish Academy of Sciences and the Academy of Finland is acknowledged for the financial support to D.H. and J.H.

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