Surface-enhanced luminescence of Cr3+-doped ZnAl2O4 and MgAl2O4 using Ag@SiO2 and Au@SiO2 core–shell nanoparticles

Abstract

In this study, we investigate chromium-doped spinels (CDSs) and their characteristic red photoluminescence (PL) due to the strong crystal field and octahedral coordination geometries of the Cr³⁺ ions. An increase in the luminescence efficiency of the CDSs caused by the interaction with plasmon excitation is studied to achieve more efficient luminescence. We successfully synthesise CDSs using the citrate sol–gel method and metallic nanoparticles (MNPs) using the classical citrate reduction method. In addition, we cover the MNPs with silica shells, which are used to modify the surface. The surface-enhanced effect exerted by the surfaces modified with Ag@SiO₂ and Au@SiO₂ core–shell nanoparticles on the PL of the two CDSs is studied. The silica shell of the MNPs is used as a separator between the MNPs@SiO₂-modified surface and the CDS. An enhancement of the PL is found for the CDS coupled to MNPs compared with the uncoupled CDSs. The PL enhancement factors and lifetimes are also investigated. The interaction between the CDSs and MNPs is investigated, and several factors are found to influence the PL of the CDSs. The main mechanism explaining the PL enhancement is proposed. A better understanding of the surface-enhanced luminescence of the CDSs may lead to further improvements in these systems.