Redox-Responsive Luminescence Switching of CePO₄:Tb³⁺ Thin Films Prepared by a Sol-Gel Method

Transparent CePO₄:Tb³⁺ thin films were prepared on silica glass substrates by a sol.gel dip-coating method and their structural and optical properties were investigated in response to redox conditions. CePO ₄:Tb³⁺ was crystallized in a monoclinic monazite structure after heating at 900°C, showing photoluminescence due to doped Tb ³⁺ ions upon excitation with ultraviolet light. The oxidation of the CePO₄:Tb³⁺ film with an aqueous KMnO₄ solution resulted in quenching of luminescence associated with a preferential occurrence of a Ce³⁺-Ce⁴⁺ intervalence charge transfer instead of a Ce³⁺-Tb³⁺ energy transfer. The reduction of the once oxidized CePO₄:Tb³⁺ film with an aqueous ascorbic acid solution recovered luminescence completely, thereby providing on/off switching function to the film. The process of quenching and recovery was investigated by following up changes in a chemical and an electronic state using X-ray photoelectron spectroscopy and optical transmission spectroscopy, respectively. It was also shown that the redox sensitivity of luminescence was dependent on controllable parameters such as reaction time, reaction temperature, and concentration of redox reagents. Repetitive on/off cycles of luminescence were achieved under the advantage of chemically stable inorganic materials.