Size-dependent chromaticity in YBO3:Eu nanocrystals:: Correlation with microstructure and site symmetry

Pure hexagonal-phased Y1-xBO3:Eu-x (x = 0-0.30) nanocrystals with different particle sizes were prepared by a facile sol-gel pyrolysis process, and their photoluminescence spectra evidently showed a size-dependent characteristic because the ratio of the red emission transition (D-5(0) --> F-7(2)) to the orange emission transition (D-5(0) - F-7(1)) (R/O) was much higher in the smaller particles. Both XRD patterns and IR spectra demonstrated that the lattices of YBO3:Eu nanocrystals were distorted and that as the particle size became smaller, the lattices became more distorted. Studies on the charge transfer (CT) bands indicated that Eu3+ ions can be excited preferentially by different excitation sources in both the bulk and the nanocrystals, suggesting that at least two different types of intrinsic luminescent sites, site 1 and site 2, coexist in YBO3:Eu. Site-selective excitation spectra also revealed that a particular site, site 3, existed concurrently in the nanocrystals. Site 2, the site with relatively inferior symmetry of the intrinsic sites, was identified to be of C, symmetry. Meanwhile, for the nanosized samples, Eu3+ ions exhibited enhanced R/O values in both sites 2 and 3, which might be ascribed to the distorted lattices, and thus displayed the observed superior color chromaticity. A pronounced energy transfer between site 2 and site 3 in the nanocrystals was also observed while Eu3+ concentration was increased to the quenching concentration, which indicated that site 3 may be a disordered surface site surrounding the interior sites.