Synthesis and luminescent characteristics of europium dopants in SiO2/Gd2O3 core/shell scintillating nanoparticles

Abstract

Scintillating nanoparticles with a SiO₂ core and a Gd₂O₃ shell, where either the core or the shell was doped with Eu³⁺, were synthesized with a solution precipitation process. Based on transmission electron microscopy (TEM) data, an ∼13 nm Gd₂O₃ shell was successfully coated onto ∼220 nm mono-dispersed SiO₂ nanocores. Concentrations of ∼20 at% Eu³⁺ in the SiO₂/Gd₂O₃ nanoparticles exhibited photoluminescent (PL) emission from the ⁵D₀-⁷F₂ transitions of Eu³⁺ at 609 and 622 nm after being calcined at 800 °C for 2 h. The SiO₂ remained amorphous after calcining, while the Gd₂O₃ crystallized to a cubic structure. Photoluminescence excitation (PLE) data showed that emission from Eu³⁺ could result from direct excitation, but was dominated by the oxygen to europium charge-transfer band (CTB) between 250 and 300 nm. The quantum yield (QY) from SiO₂/Gd₂O₃:Eu³⁺ core/shell nanoparticles excited at wavelengths in the CTB decreased from 25.0% to 16.5% with addition of a SiO₂ shell to form a SiO₂/Gd₂O₃:Eu³⁺/SiO₂ nanostructure. In contrast, the QY increased to 32.2% from a SiO₂/Gd₂O₃:Eu³⁺/Gd₂O₃ nanostructure. This increased luminescence with a Gd₂O₃ surface shell is attributed to reduction of surface quenching sites. Moreover, for a SiO₂:Eu³⁺ core, addition of a Gd₂O₃ capping layer increased the PL intensity by four times, which was attributed to the crystalline Gd₂O₃ layer also acting as an antenna for energy transfer to the SiO₂:Eu³⁺ core.