2D‐ and 3D‐Photoluminescence Imaging of Eu(III) Embedded in CeO2 Nanomatrix

2D‐ and 3D‐photoluminescence characteristics of Eu(III) doped in CeO₂ nanoparticles were fully imaged for the first time. Their fundamental natures were also examined by scanning electron microscopy (SEM), X‐ray diffraction (XRD) crystallography, and UV–visible absorption spectroscopy. The magnetic dipole ⁵D₀ → 7F₁ transition was dominated by an indirect transition associated with a O^2?–Ce⁴⁺ charge‐transfer band of CeO₂. The electric dipole ⁵D₀ → ⁷F₂ transition was dominated by a direct transition of Eu(III), indicating that Eu(III) replaces Ce(IV) at octahedral sites (O_h and O) with and without an inversion center. Upon annealing, the photoluminescence intensity caused by direct transition was dramatically decreased, whereas that induced by indirect transition was greatly enhanced. These findings indicate that charge transfer to the Eu(III) at the octahedral (O_h) site with the inversion center is more efficient than that to the Eu(III) site without an inversion center. The absolute quantum yield for the 10 mol% Eu(III)–CeO₂ was found to be ? = 0.007 at an excitation wavelength of 350 nm. The photoluminescence of Tb‐doped CeO₂ was briefly discussed for comparison.