Effect of europium ion content on physical properties of nanocrystalline zinc oxide and gadolinium oxide

This work report a systematic investigation on the effect of Eu 3+ ions on structural and physical properties of nanocrystalline ZnO (nanopowders-NPs and nanocrystals-NCs) and Gd2O3 (NPs and thin films), which were obtained from starting solutions containing Eu(III) species. NPs were synthesized by Sol-Gel method, using 2-ethylhexanoic acid as solvent; NCs were produced through a modified ethanol-based approach at room temperature, whereas thin films were grown by Sol-Gel method and spin coating process, with acetic acid as solvent. No chelating agent was used. In ZnO NPs, XRD and infrared analysis verified the development of the wurtzite structure without any secondary phase. No significant change in the lattice parameters, as could be expected from Eu3+ incorporation, was observed. Main differences between NPs and NCs ZnO came out from their Photoluminescence (PL) behavior. NPs showed a weak red emission (which could be attributed to a direct excitation of the Eu3+ ions, but no to energy transference from the ZnO), but not visible emission under high energy excitation above the band-gap energy of ZnO (which would indicate that free-defects structures were obtained). In turn, both visible broad band and UV emission were observed for NCs. For the same NCs, in addition to an inhibiting growth effect and a blue shift of band gap, an enhancement of the visible/UV emission ratio with the increase in the content of Eu3+ ions in starting solutions was observed. The generation of defects on the surface of NCs could explain this behavior. Moreover, several lines from Eu3+ were detected in the PL spectra of NCs only when the excitation energy was between that broad band for ZnO and that intra-4f transition energy of Eu3+ ions. This feature would suggest no energy transfer from ZnO host to Eu 3+ ions, which would be adsorbed at the surface of NCs instead. In Gd2O3 NPs, the effects of the annealing temperature have also been systematically investigated. XRD analysis showed that pure and highly crystalline cubic-Gd2O3 host structure was obtained when the precursors, bearing different contents of Eu3+ species (from 0.01 to 0.30) were annealed at different temperatures in air. The average crystallite size ranged between 29nm and 41nm when the annealing temperature varied from 750°C to 950°C, respectively. The levels of Eu3+ concentration in the host did not affect the corresponding crystallite sizes. PL spectra of Eu3+-doped Gd2O 3 NPs showed all transitions of Eu3+, being the 5D0→7F2 transition the most intense. On a common sample-weight basis, it was found that the PL intensity was strongly dependent on both, the annealing temperature and the Eu 3+ content. The highest PL intensity was obtained in NPs annealed at 950°C with 'x'= 0.15 (7.5% w/w of Eu). Unlike to ZnO case, the energy transfer from Gd2O3 host to Eu3+ was clearly verified for all evaluated 'x' values, which confirm the actual incorporation of Eu3+ into the Gd2O 3 lattice. From M-H measurements a paramagnetic behavior between - 5000 and 5000Oe at room temperature was observed for Eu3+-doped Gd2O3 NPs, whose magnetic susceptibility decreases as the Eu3+ content increases. In turn, XRD showed that Gd 2-xEuxO3, thin films with preferential orientation along (400) plane of cubic phase, were obtained. Thin films showed high transparence in the visible region, and their band gap practically was no affected for the Eu3+ content. Also, unlike to NPs, the more efficient excitation to obtain red luminescence was that corresponding to the absorption band of the host Gd2O3 (229nm) for all Eu3+ contents.