Materiales nanoestructurados para aplicaciones en dispositivos ópticos

Abstract

In the present work entitled "Nanostructured materials for applications in optical devices" different optically active materials have been obtained. First, rare-earth (RE3+) doped nano-glass-ceramics (nGCs) comprising Sr2GdF7 nanocrystals (NCs) into a silica matrix have been prepared by sol-gel method. Moreover, RE3+ -doped Sr2GdF7 NCs were also synthesized by solvothermal method for comparison purposes. All the obtained nanostructured materials were characterized, structural and spectroscopically for optical applications. Thus, un-doped nGCs, single doped with Ce3+, Eu3+, Sm3+, Dy3+ or Tb3+ and co doped with Ce3+ -Eu3+ , Ce3+ -Sm3+ , Ce3+ -Dy3+ and Ce3+ -Tb3+ nGCs were obtained by adequate thermal treatment of the precursor sol-gel glasses. Eu3+, Sm3+, Dy3+ or Tb3+ ions were selected as RE3+ dopants due to their characteristic emissions in the visible range. Due to their low absorption coefficients, Gd3+ and/or Ce3+ ions are used as co-sensitizers, due to their higher absorption capacities and efficient energy transfers (ET) towards the activator ions to enhance the emissions of RE3+ ions. The structural characterization was carried out by X-Ray diffraction (XRD), transmission electron microscopy images (TEM) and energy dispersive X-Ray spectroscopic (EDS) measurements, allowing us to study the crystalline structure, size and distribution of the NCs in the nGCs and chemical composition. Thus, XRD confirmed the precipitation of tetragonal Sr2GdF7 NCs with an average size around 8.4 nm. TEM images shown the presence of spherical NCs with similar sizes to those obtained from XRD patterns. Finally, EDS measurements confirm the presence of Sr, Gd and F as main constituents of nanocrystalline environments with the expected stochiometric amounts, i.e. (2:1:7) ascribed to Sr2GdF7. On the other hand, the spectroscopic study was performed by emission and excitation spectra along with lifetime measurements. First, in the un-doped nGCs sharp and intense excitation and emission peaks corresponding to Gd3+ ions are observed, at around 273 and 311 nm, respectively. This emission overlapped with excitation peaks of Eu3+, Sm3+, Dy3+ or Tb3+ ions, which suggest a possible energy transfer (ET) from Gd3+ to RE dopant ions. Thus, in the single-doped nGCs, by exciting Gd 3+ ions at 273 nm, besides to 311 nm emission peak of Gd3+ ions, corresponding emissions peaks of Eu3+, Sm3+, Dy3+ or Tb3+ ions are observed, showing much more intense emissions than by direct excitation. Accordingly, luminescence decay of Gd3+ ions in un-doped and single-doped nGCs evidenced a reduction when co-doping with Eu3+, Sm3+, Dy3+ or Tb3+ ions, supporting the ET from Gd3+ to these ions. In the Ce3+ single-doped nGC, excitation and emission bands were observed in the UV/UV-violet region, respectively. The emission band overlapped with excitation peaks of Gd3+ , Eu3+, Sm3+, Dy3+ or Tb3+ ions, suggesting a possible ET to these ions. For the co-doped nGCs, comprising Ce3+ -Eu3+ , Ce3+ -Sm3+ , Ce3+ -Dy3+ or Ce3+ -Tb3+ couples, visible emissions Ce3+ sensitized were observed in all cases. In particular, for Ce3+ -Dy3+ and Ce3+ -Tb3+ co-doped nGCs, direct ET from Ce3+ ions Ce3+→( Dy3+/Tb3+) and mediated through Gd3+ Ce3+→(Gd3+)n→(Dy3+/Tb3+), to these ions were observed. However, for Ce3+ -Eu3+ , Ce3+ -Sm3+ co-doped nGCs, the metal-metal charge transfer mechanism (MMCT) produces quenching of the Ce3+ and Eu3+/Sm3+ emissions, inhibit direct ET from Ce3+ to these ions. In this case, ET from Ce3+ to activator ions only occurs through Gd3+ ions by using the chain scheme Ce3+→(Gd3+)n→( Eu3+/Sm3+). Finally, Sr2GdF7 solvothermal NCs, using Ce3+ and Eu3+ as dopants, where obtained to compare their structure, luminescence and ET mechanisms previously studied. In particular, Ce3+ single-doped and Ce3+ -Eu3+ co-doped NCs, shown similar structural and luminescent properties to those previously observed in the corresponding nGCs. Next, in order to verify the existence of the Gd3+ chain, Ce3+ single-doped Sr2GdF7 NCs were covered by a shell epitaxially grown of Eu3+ single-doped Sr2GdF7, giving rise a Sr2GdF7:Ce3+@Sr2GdF7:Eu3+ core-shell system. By exciting Ce3+ ions from the core NC, red emissions coming from Eu3+ ions located in the shell were observed, pointed out the efficient mechanism of ET from the Ce3+ sensitizer to the Eu3+ activators, through Gd3+ ions chain. Difference observed in the asymmetry ratio of Eu3+ emissions were related with different environments for these ions and confirmed with a Sr2GdF7:Ce3+@Sr2GdF7:Eu3+@Sr2YF7 core-shell-shell system, prepared for comparison purposes.