Size-Dependent Chromaticity in YBO₃:Eu Nanocrystals: Correlation with Microstructure and Site Symmetry

Abstract

Pure hexagonal-phased Y1 - x BO3: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 (5D0 → 7F2) to the orange emission transition (5D0 → 7F1) (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 1 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.