Effects of Weak Electric Field on the Photoluminescence Behavior of Bi³⁺-Doped YVO₄:Eu³⁺ Core–Shell Nanoparticles

Tuning the luminescent properties of nanophosphors by modifying the energy level hybridization has been previously achieved by methods such as heat treatments or applying strong magnetic/electric fields. However, these methods can permanently distort the crystal geometry or are limited to the continuous application of strong fields. This work aims to modify the energy level hybridization of doped (Eu³⁺/Bi³⁺) YVO₄ nanoparticles (NPs) via surface functionalization with polarized molecules. Incorporating Bi³⁺ into YVO₄:Eu³⁺ core NPs resulted in a red shift of the excitation edge by ∼30 nm and a decrease in the Eu³⁺ emission lifetime. Moving the Bi³⁺ to the YVO₄ shell layer allowed for the modification of Bi³⁺/VO₄³– energy level hybridization ion pairs without significant quenching of the Eu³⁺ ions in the core. Polarized molecules (NH₂-BZA and NO₂-BZA) were used for selective tuning of the electron density at the interface, impacting the Bi³⁺/VO₄³– energy level hybridization and the luminescent behavior of the NPs. Higher emission lifetimes and systematic photoluminescent response, that is, an increase or decrease in the excitation intensity based on the direction of the dipole moment, were observed for surface-functionalized core–shell NPs compared to the core NPs. Finally, the surface of the core–shell NPs was decorated with d-biotin to elucidate the effect of this biological ligand on the surface electron density and luminescence behavior of the NPs.