Sol-gel preparation and luminescent properties of CeO₂:Ln (Ln = Eu³⁺ and Sm³⁺) thin films

New transparent CeO₂:Ln (Ln = Eu³⁺ or Sm³⁺) thin-film phosphors have been fabricated by a sol-gel method. The CeO₂:Ln films with the Ln concentrations of 1 at.% showed the strongest photoluminescence (PL) upon ultraviolet (UV) light excitation. At higher Ln concentrations, the PL intensity decreased because of the nonradiative energy transfer. For Eu³⁺ ions doped in the CeO₂ lattice with inversion symmetry, an orangey red emission due to a magnetic-dipole ⁵ D₀ → ⁷F₁ transition (591 nm) was dominant when excited at a wavelength of 340 nm. In contrast, much stronger orange emissions (573 nm) due to ⁴G_5/2 → ⁶H_5/2 were observed for Sm³⁺ ions. Apparently, the Eu³⁺ and the Sm³⁺ ions had different efficiencies of the excitation through the energy transfer from the "Ce⁴⁺-O^2-" charge transfer state induced by the UV irradiation.