From embedded nanoislands to thin films: Topographic and optical properties of europium oxide on MgO(001) films

Combining scanning tunneling microscopy and cathodoluminescence spectroscopy, we have explored different routes to produce luminescent MgOEu films on a Mo(001) support. Codeposition of Eu and Mg in an O-2 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 D-5(0) excited to the F-7(J) 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.