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

Abstract

Combining scanning tunnelingmicroscopy and cathodoluminescence spectroscopy, we have explored different routes to produce luminescentMgO_Eu 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 MgO_Eu 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 ⁷F_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

Optical characterization of europium-doped indium hydroxide nanocubes obtained by Microwave-Assisted Hydrothermal method

“…Whereas the odd-parity components of the crystal field enable electric dipole transitions between 5 D 0 and 7 F 2,4,6 in the first order, a combination of odd and even crystal-field terms gives rise to faint second-order transitions for the remaining J = 0, 3, 5 values. The intensities of the different J transitions, and in particular the hypersensitive J = 2 peak provide a good measure of the local crystal field acting on Eu 3+ ions [45] . Assuming the PL emission peaks can be described with a Gaussian function, the emission envelope in Figure 2 can be fitted to multiple peaks (Figure 3a-e).…”

Chapter model systems in heterogeneous catalysis at the atomic level: a personal view

“…In the first one, the host material was evaporated simultaneously with the dopants in an O 2 ambience of 10 −5 to 10 −7 mbar. The scheme was used for example to load MgO thin films with either Cr, Eu, or Li impurities [82,83]. The main drawback of the technique was the uncontrolled diffusion of the dopants during film annealing, followed by the formation of dopant-rich surface phases.…”

Activating Nonreducible Oxides via Doping

“…We note that the dopant electronic structure has been analyzed also by STM luminescence spectroscopy, using the emission response of the oxide after electron injection from the tip. 5 For example, Cr-doped MgO features a sharp 700 nm emission peak due to e g → t 2g transitions inside the gap, 4 while Li-doped MgO emits 550 nm light arising from the recombination of electrons in Liinduced F 2+ centers and holes in the oxide valence band. 13 The distinct electronic structure of transition metal ions in rocksalt oxides gives rise to an unusual charging mechanism that renders individual dopants visible in the STM, although no atomic resolution is obtained on the insulator and most dopants reside below the surface.…”

“…In the first one, the host material was evaporated simultaneously with the dopants in an O 2 ambience of 10 –5 to 10 –7 mbar. The scheme was used for example to load MgO thin films with either Cr, Eu, or Li impurities. , The main drawback of the technique was the uncontrolled diffusion of the dopants during film annealing, followed by the formation of dopant-rich surface phases (Figure a,b). In contrast, segregation of substrate atoms into the oxide film was exploited in the second approach, used for instance to prepare Mo-doped CaO (Figures c,d.).…”

“…Whereas the e g states are entirely empty, the t 2g levels contain four Mo 4d electrons in the Mo 2+ species, forming the basis for the donor character of the material. We note that the dopant electronic structure has been analyzed also by STM luminescence spectroscopy, using the emission response of the oxide after electron injection from the tip . For example, Cr-doped MgO features a sharp 700 nm emission peak due to e g → t 2g transitions inside the gap, while Li-doped MgO emits 550 nm light arising from the recombination of electrons in Li-induced F 2+ centers and holes in the oxide valence band…”