Crystal size dependence of the persistent phosphorescence in Sr2ZnSi2O7: Eu2+, Dy3+
Introduction
There has been a revival of interest in materials which evince a long persistent luminescence or phosphorescence [1], [2], [3], [4], [5], [6]. This phenomenon has been reported in a large variety of insulating materials and has most recently focused on doubly activated complex silicates and aluminates. For example, a commercially available material, Luminova, is an aluminate activated by Eu2+ and Dy3+; it is generally thought that the Dy3+ ion in this material creates hole traps which are ultimately responsible for the excitation of the Eu2+ phosphorescence observed. Rare earth doped nano-phosphors have been also prepared and their optical properties have been investigated in recent years, such as Gd2O3:Yb,Er and Gd2O3S:Yb,Er [7], ZnS:Eu [8], and Y2O3:Eu [9]. Spectral shifts have been observed due to the surface/size effects [1], [10]. In this paper, we have studied the behavior of the persistent phosphorescence decay as a function of particle size and find a functionality which is consistent with increase of traps as the surface to volume ratio increases with decreasing size. We report on this as well as the synthesis and optical properties of Sr2ZnSi2O7:Eu2+, Dy3+.
Section snippets
Experimental
The samples were prepared using a sol-gel procedure similar to that described by Jiang and coworkers [4]. In this procedure strontium and zinc acetates and tetraethoxysilane were used as starting materials and water and methanol were selected as solvents for all the salts. Europium and dysprosium chloride were obtained by reacting stoichiometric amounts of lanthanide oxides with hydrochloric acid. The dissolved salt solutions were stirred into the tetraethoxysilane solution and the mixture was
Results and discussion
Fig. 1 shows the X-ray diffraction patterns of the resulting gel powders for different annealing temperatures. This results show that the samples possess good crystallinity and that the material synthesized has the structure of the compounded Sr2ZnSi2O7 rather than the simpler Sr2SiO4. Through increased thermal mobility, the average phosphor particle size is found to increase with increased annealing temperature. The particle size dependence on temperature is illustrated in Fig. 2; as can be
Conclusions
In summary, we believe that this is the first study, though preliminary, of the dependence of persistent phosphorescence or afterglow properties of this class of phosphors to be carried out in nano-materials as a function of particle size. We have shown that it is possible to synthesize phosphor materials in a controllable way through the sol-gel technique and that materials grown in by this technique largely retain the optical properties of the bulk materials. The principal result from our
Acknowledgements
One of the authors (XJW) wishes to thank the supports from ‘One Hundred Talents Program’ of the Chinese Academy of Sciences and from the Cottrell College Science Awards of Research Corporation.
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