Synthesis, structural investigation and luminescence spectroscopy of nanocrystalline Gd3Ga5O12 doped with lanthanide ions
Introduction
Garnets are among the most important hosts for luminescent active centers, such as lanthanide and transition metal ions. Single crystals of Nd3+- and Yb3+-doped gadolinium gallium garnet Gd3Ga5O12 (GGG) have been studied for their possible application as active elements for diode pumped lasers [1], [2], [3]. On the other hand, lanthanide-doped nanostructured materials have gained importance for possible applications as imaging or display devices in which the resolution is inversely related to the particle size [4], [5]. The luminescence properties have been investigated for Pr3+-, Ho3+- and Er3+-doped GGG nanocrystalline powders prepared by a combustion synthesis [6], [7]; these materials appear to show efficient visible upconversion emission after excitation with IR radiation. To the best of our knowledge, only a few papers have been published on the preparation and optical properties of lanthanide-doped nanocrystalline GGG powders and thin films, obtained using a coprecipitation or Pechini sol–gel process [8], [9].
In the present communication, we report on the synthesis of Ln3+-doped (Ln = Eu, Er) nanocrystalline GGG powders using two preparation methods (coprecipitation and Pechini). After a suitable heat treatment, the obtained powders result to be single phase with a garnet structure. Size distributions and morphological properties of the nanocrystalline samples are obtained from scanning electron microscopy (SEM) images and compared with previous results obtained for GGG nanocrystalline powders prepared by combustion synthesis [10] or other methods. Laser excited Stokes and upconversion emission spectra have been measured and discussed.
Section snippets
Preparation procedure
The coprecipitation and Pechini methods employed for the preparation of the nanocrystalline GGG powders are described in detail in a separate paper [11]. Briefly, for the coprecipitation method, stoichiometric quantities of Gd2O3, Ga(NO3)3 and Ln(NO3)3 (Ln = trivalent lanthanide ion) were dissolved in a HNO3 solution. The obtained solution was added dropwise to a NH3 aqueous solution and the precipitate was filtered and then dried at 60 °C. The samples were heat treated at 500 °C for 30 h, ball
Results and discussion
Fig. 1 shows selected powder X-ray diffraction patterns of Eu3+-doped CP and PE nanocrystalline GGG samples. A heat treatment of the Eu3+-doped CP powders at 900 °C for 2 h produces almost entirely a GGG cubic phase (see Fig. 1). The lattice parameter of this phase is 12.582(±3) Å, considerably larger than the values of 12.377 and 12.383 Å reported for single crystals of precise stoichiometry Gd:Ga = 3:5 [16], [17]. The reasons for this behaviour were already discussed in a previous paper [15] and
Conclusions
Lanthanide-doped GGG nanocrystalline samples have been obtained using coprecipitation and Pechini preparation methods. After a suitable heat treatment, single phase cubic GGG nanopowders have been obtained. The CP and PE samples show different morphologies. The CP samples are composed by irregular agglomerations of small crystallites, forming building blocks quite irregular in shape. Differently, the particles of the PE nanopowders show a spherical shape and have a large size distribution
Acknowledgements
The authors gratefully acknowledge Erica Viviani and Marianna Parisi (DST, Univ. Verona) for expert technical assistance. Thanks are also expressed to Dr. Luca Lutterotti (http://www.ing.unitn.it/∼luttero/) for making available a copy of the programme MAUD running on a personal computer.
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