Lithium-molybdate nanostructures grown on the Mo(001) surface

Ordered Lisingle bondMo mixed-oxide films of different compositions have been grown on a Mo(001) surface and analysed by means of scanning tunnelling microscopy, low-energy-electron-diffraction and cathodoluminescence spectroscopy. Starting from a disordered LixO ad-layer grown at room temperature, a scheelite-type Li2MoO4 phase develops on the Mo surface after annealing to 700 K. The building blocks of this structure are regular nanorods of approximately 30 nm length, which exhibit strong light emission in the green spectral range upon electron injection. Further annealing induces a restructuring of the film that evolves into various mixed-oxide phases of decreasing Li content. The Li fully desorbs from the surface above 1000 K, leaving behind a nano-crystalline Mo-oxide. Our approach demonstrates that ternary Lisingle bondMo oxides of high structural quality can be grown as thin films, making them accessible to conventional surface science techniques without charging problems

Photon emission spectroscopy of single oxide-supported Ag-Au alloy clusters

The alloying of Ag and Au has been investigated on the level of individual clusters by analyzing the light emission excited by electron injection from an STM tip. Different Ag-Au alloy and shell-core clusters have been prepared at room temperature on a thin Al2O3 film on NiAl(110) by simultaneous and successive deposition of both noble metals. For simultaneous deposition, one Mie-plasmon resonance has been detected with a wavelength position shifting from the pure Au to the Ag value with increasing Ag content. The results are in agreement with calculations based on Mie theory indicating a complete mixing of both materials. For successive deposition, two Mie resonances have been observed, attributed to plasmon excitations in the shell and core of the clusters. Comparing these results to model calculations, a considerable intermixing of the core and shell materials is concluded, which is especially strong in Au shell-Ag core clusters

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

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

Probing the 4f states of ceria by tunneling spectroscopy

Low-temperature scanning tunneling microscopy and spectroscopy have been employed to analyze the local electronic structure of the (111) surface of a ceria thin film grown on Ru(0001). On pristine, defect-free oxide terraces, the empty 4f states of Ce4+ ions appear as the only spectral feature inside the 6 eV oxide band gap. In contrast, occupied states are detected between 1.0 and 1.5 eV below EFermi in conductance spectra of different point and line defects, such as surface oxygen vacancies, grain boundaries and step edges. They are assigned to partially filled 4f states localized at the Ce3+ ions. The presence of excess electrons indicates the oxygen-deficient nature of the direct oxide environment. The f state spectroscopy with the STM allows us to probe the spatial distribution of Ce3+ ions in the ceria surface, providing unique insight into the local reduction state of this chemically important material system. 1