Unveiling the multifunctional roles of hitherto known capping ligand oleic acid as blue emitter and sensitizer in tuning the emission colour to white in red-emitting phosphors

Abstract

Capping ligands are vital in stabilizing various nanostructures and semiconductor quantum dots in which unusual optical properties, especially white light emission, have been realized. Oleic acid (OA) is a widely used capping ligand. Here, we report blue emission from OA in its free molecular form and further demonstrate this by anchoring OA over the surfaces of Al₂O₃, ZnAl₂O₄(ZA), ZnAl₂O₄:Eu³⁺ (ZA:Eu³⁺), and Y₂O₃:Eu³⁺. White light emission was observed from OA-modified ZA:Eu³⁺ nanophosphor due to mixing of broad blue emission of OA and red emission of Eu³⁺ through energy transfer from OA to Eu³⁺. A detailed study revealed the characteristic binding modes of OA and their dependence on Eu³⁺ concentration, structural inversion in ZA, and the optical properties and surface states in the pristine and OA-modified ZA:Eu³⁺. First principles density functional theory calculations were employed to provide an insight into the HOMO–LUMO levels of OA molecule and, electronic structure of pristine and OA-modified ZA surface. The binding of OA with the ZA:xEu³⁺ surface changes from bridging bidentate to chelating bidentate with increasing Eu³⁺ concentration in the lattice. The surface binding nature of the carboxylate group with the optimized surface of ZA and the creation of mid-gap states were deduced theoretically by using butanoic acid instead of OA. The blue emission from OA and its mixing with Eu³⁺ emission was further confirmed experimentally by anchoring it over Y₂O₃:Eu³⁺ red phosphor. These results show the multifunctional roles of OA as capping ligand, blue emitter and sensitizer in tuning the emission colour of red phosphors into white.