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

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

Autocatalytic growth of ZnO nanorods from flat Au(111)-supported ZnO films

Physical vapour deposition of ZnO on an Au(111) support has been investigated as a function of the oxygen chemical potential by means of scanning tunnelling microscopy and luminescence spectroscopy. Whereas a layer-by-layer growth of ZnO is revealed in oxygen excess, formation of oxide nanorods with large height-to-diameter ratio prevails at lower oxygen chemical potentials. We ascribe the formation of 3D nanostructures in the latter case to traces of Au atoms on the surface that promote trapping and dissociation of the incoming oxygen molecules. The Au residuals, acting as catalyst for the oxide formation, are indeed found on top of the ZnO nanorods

Training-induced inversion of spontaneous exchange bias field on La1.5Ca0.5CoMnO6

In this work we report the synthesis and structural, electronic and magnetic properties of La1.5Ca0.5CoMnO6 double-perovskite. This is a re-entrant spin cluster material which exhibits a non-negligible negative exchange bias effect when it is cooled in zero magnetic field from an unmagnetized state down to low temperature. X-ray powder diffraction, X-ray photoelectron spectroscopy and magnetometry results indicate mixed valence state at Co site, leading to competing magnetic phases and uncompensated spins at the magnetic interfaces. We compare the results for this Ca-doped material with those reported for the resemblant compound La1.5Sr0.5CoMnO6, and discuss the much smaller spontaneous exchange bias effect observed for the former in terms of its structural and magnetic particularities. For La1.5Ca0.5CoMnO6, when successive magnetization loops are carried, the spontaneous exchange bias field inverts its sign from negative to positive from the first to the second measurement. We discuss this behavior based on the disorder at the magnetic interfaces, related to the presence of a glassy phase. This compound also exhibits a large conventional exchange bias, for which there is no sign inversion of the exchange bias field for consecutive cycles

Compensation temperatures and exchange bias in La1.5Ca0.5CoIrO6

We report on the study of magnetic properties of the La1.5Ca0.5CoIrO6 double perovskite. Via ac magnetic susceptibility we have observed evidence of weak ferromagnetism and reentrant spin glass behavior on an antiferromagnetic matrix. Regarding the magnetic behavior as a function of temperature, we have found that the material displays up to three inversions of its magnetization, depending on the appropriate choice of the applied magnetic field. At low temperature the material exhibit exchange bias effect when it is cooled in the presence of a magnetic field. Also, our results indicate that this effect may be observed even when the system is cooled at zero field. Supported by other measurements and also by electronic structure calculations, we discuss the magnetic reversals and spontaneous exchange bias effect in terms of magnetic phase separation and magnetic frustration of Ir4+ ions located between the antiferromagnetically coupled Co ions.Comment: 10 pages, 8 figures and supplemental materia

Probing the luminescence of single Eu₂O₃ nano-islands on MgO(001) with scanning tunneling microscopy

STM imaging and cathodoluminescence spectroscopy are used to explore the morphology and optical properties of Eu2O3 nano-islands grown on MgO(001) thin films. The global emission properties are governed by intense photon peaks at around 600 nm, arising from forced-dipole transitions in the 4f-state manifold of Eu3+ ions placed in a non-centrosymmetric crystal field. Local optical spectra obtained in the tunneling mode of the STM reveal that the Eu2O3 islands are responsible for the distinct optical response and not isolated Eu3+ ions inside the MgO matrix. Our work demonstrates that thin-film model systems can be used to elucidate fundamental properties of Eu-containing phosphors on the nanometer scale

Assessing the film-substrate interaction in germania films on reconstructed Au(111)

Purely amorphous germania bilayer films are grown on a reconstructed Au(111) surface. The presence of the film affects the native configuration of the Au soliton walls, as observed with scanning tunneling microscopy. They partly avoid the film islands, and partly penetrate under film patches. This behavior indicates a weaker film-substrate interaction than the one reported for other oxide films on reconstructed Au(111). Moreover, this new system highlights the impact of the metal support on the structure of ultrathin films of germania: With decreasing film-substrate interaction the amorphous phase is promoted. Density functional theory calculations confirm and rationalize the experimental observations. This work provides a useful generalization of the relationship between film structure and adhesion energy

Structure and registry of the silica bilayer film on Ru(0001) as viewed by LEED and DFT

Silica bilayers are stable on various metal substrates, including Ru(0001) that is used for the present study. In a systematic attempt to elucidate the detailed structure of the silica bilayer film and its registry to the metal substrate, we performed a low energy electron diffraction (I/V-LEED) study. The experimental work is accompanied by detailed calculations on the stability, orientation and dynamic properties of the bilayer at room temperature. It was determined, that the film shows a certain structural diversity within the unit cell of the metal substrate, which depends on the oxygen content at the metal-bilayer interface. In connection with the experimental I/V-LEED study, it became apparent, that a high-quality structure determination is only possible if several structural motifs are taken into account by superimposing bilayer structures with varying registry to the oxygen covered substrate. This result is conceptually in line with the recently observed statistical registry in layered 2D-compound materials

Defect complexes in Li-doped MgO

Magnesium oxide (MgO) is used in a variety of industrial applications due to its low cost and structural stability. In heterogeneous catalysis, MgO and Li-doped MgO have been studied as catalysts for the oxidative coupling of methane. In this work, we analyze the structure and stability of defect complexes comprising Li dopants and oxygen vacancies in MgO, combining scanning tunnelling microscopy, photon-emission experiments, and density-functional theory computations. The experimental results strongly indicate that after annealing Li-doped MgO to temperatures of 600 K and higher, Li evaporates from the surface, but Li defects, such as substitutional defects, interstitials, or defect complexes comprising Li remain in the bulk. Our calculations show that bulk defect complexes containing F2+ color centers, that have donated their two electrons to two adjacent Li defects, are the most stable configurations at realistic pressure and temperature condition

IDH1 mutations in a Brazilian series of Glioblastoma

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Albert Einstein Jewish HospitalUniversidade de São Paulo Faculdade de Medicina Department of NeurologyUniversidade de São Paulo Faculdade de Medicina Department of PathologyCancer Institute of São PauloFundação Pio XII Barretos Cancer HospitalFederal University of São Paulo School of Medicine Department of NeurologyFederal University of São Paulo School of Medicine Department of PathologyNove de Julho HospitalAlbert Einstein Jewish HospitalUNIFESP, EPM, Department of NeurologyUNIFESP, EPM, Department of PathologyFAPESP: 04/12133-6SciEL