CaF2:Eu films shine novel blue, white or red luminescence though adjustment of the valence state of Eu ions using the electro-deposition method

Abstract

We report the fabrication of Eu-doped CaF₂ films with an adjustable oxidation state of Eu ions such as Eu²⁺, Eu²⁺/Eu³⁺ or Eu³⁺ by the electro-deposition method, which are promising for luminescence converters for LEDs. Usually, the Eu²⁺ ion is unstable and can be very easily oxidized into the Eu³⁺ ion in solution. We reduced Eu³⁺ ions into Eu²⁺ ions by the electrochemical reduction method, and then incorporated Eu²⁺ ions into a CaF₂ film during successive electro-deposition. We controlled the balance between Eu²⁺, Eu²⁺/Eu³⁺ and Eu³⁺ doped in the CaF₂ thin film by regulating the parameters of the deposition solution (pH). More importantly, we controlled the luminescence from the Eu doped CaF₂ film to generate pure blue luminescence, white luminescence, and red luminescence. These results not only provide a method for reducing Eu³⁺ ions into Eu²⁺ ions in solution, but also establish a new preparation technique for Eu²⁺/Eu³⁺ ion doped films and pave a new way for potential applications of Eu²⁺ doped luminescent materials.