Electrophoretically Deposited Y₂O₃:Bi³⁺,Eu³⁺ Nanosheet Films with High Transparency for Near-Ultraviolet to Red Light Conversion

Fluorescent films were fabricated by depositing Y₂O₃:Bi³⁺,Eu³⁺ nanosheets, which emit red light under near-UV irradiation. The Y₂O₃:Bi³⁺,Eu³⁺ nanosheets were obtained by calcining hydroxide precursor nanosheets synthesized through a hydrothermal method. An aqueous dispersion of positively charged Y₂O₃:Bi³⁺,Eu³⁺ nanosheets with polyethyleneimine adsorbed to the surface was prepared for their deposition. Fluorescent nanosheets were electrophoretically deposited on a transparent conductive substrate under a constant voltage. The obtained nanosheet films were dense and uniform and showed excellent photostability against the excitation light. Growth of the nanosheet film caused a decrease in transmittance and an increase in the photoluminescence intensity. The former effect was attributed to light scattering from inner voids and the rough surface of the film. A polyvinylpyrrolidone (PVP) coating on the film improved the transmittance to be greater than 70% over the visible region. These effects were attributed to antireflection effects at the film surface owing to the low refractive index of PVP. Furthermore, suppression of light scattering by coating the rough surface with a smooth PVP film and filling of voids in the nanosheet film with PVP also improved the transmittance.