Synthesis and characterization of ultra-fine Y₂O₃:Eu³⁺ nanophosphors for luminescent security ink applications

We report a simple method for the synthesis of ultra-fine Eu³⁺-doped yttria (Y₂O₃) nanophosphors with an average diameter of ∼5 nm for development of a transparent colloid that could be used as a luminescent security ink. This has been achieved by suitably substituting Eu³⁺ ions at the favorable C₂ symmetry sites of Y³⁺ ions and quantum mechanically confining the growth of the nanophosphor using a novel acid-catalyzed sol–gel technique. This is one of the few reports that depict the development of a transparent aqueous-stable Y₂O₃:Eu³⁺ colloidal solution for strategic applications related to security codes. High resolution transmission electron microscopy images showed excellent lattice fringes that in turn support the presence of better crystal quality and enhanced photoluminescence (PL) emission from the Y_1.9O₃Eu_0.1³⁺ nanophosphor system. Time resolved emission spectroscopy measurement indicated a PL decay time in the range of a few milliseconds, suitable for making luminescent security ink and other advanced applications in optoelectronic devices and bio-labeling.