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Abstract:
Combining scanning tunnelingmicroscopy and cathodoluminescence spectroscopy, we have explored different
routes to produce luminescentMgOEu films on aMo(001) support. Codeposition of Eu and Mg in an O₂ ambience
turned out to be unsuitable to prepare crystalline mixed oxides with distinct emission properties because of the
large mismatch between the Eu and the Mg ion radius. In contrast, highly luminescent samples were obtained
after annealing MgO-supported Eu particles in oxygen. The optically active species were identified as nanosized
Eu₂O₃ islands embedded in the first MgO layer, while single Eu ions inside the host lattice are of minor
importance. The MgOEu adsorption system exhibits a rich photon spectrum that comprises five emission bands
in the wavelength region between 565 and 725 nm. They are assigned to electron transitions from the ⁵D0 excited
to the ⁷FJ ground states of Eu³⁺, with the J quantum number running from 0 to 4. From the relative intensities
of certain J transitions, we conclude that the respective Eu³⁺ ions occupy sites without inversion symmetry, a
condition that is best fulfilled by Eu species at the perimeter of the Eu₂O₃ nanoislands.With increasing exposure,
a europium-oxide film develops on top of the MgO surface, whose weak spectral signature is compatible with
Eu³⁺ ions in more centrosymmetric surroundings. Our work demonstrates that relevant properties of Eu-based
phosphors, being typically prepared in the form of powder samples, can be generated in thin-film systems as
well, the latter being accessible to a range of surface-science techniques due to their finite conductivity.
