RESUMEN
Chemical wave patterns and V-oxide redistribution in catalytic methanol oxidation on a VOx/Rh(110) surface have been investigated in the 10-4â mbar range with low-energy electron microscopy (LEEM) and micro spot low-energy electron diffraction (micro-LEED) as inâ situ methods. V coverages of θV=0.2 and 0.4 MLE (monolayer equivalents) were studied. Pulses display a c(2×2) pattern in the reduced part and (1×2) and c(2×8) structures in the oxidized part of the surface. At θV=0.4 MLE (1×2)/(1×4) patterns with streaks along the [001]-direction at the 1/8 positions are present on the oxidized part of the surface. This phase can be assigned to V-oxide. On a tentative basis, an excitation mechanism for pulses is presented, Annealing the surface to 990â K under reaction conditions results in a macroscopic hole pattern in which holes of low VOx coverage are surrounded by a V-oxide layer. Chemical waves propagate inside the holes as well as on the VOx covered parts of the surface. The results demonstrate for the first time that also in supported oxidic overlayers selforganization processes can take place leading to chemical waves and a large scale redistribution of the oxide.
RESUMEN
Low-energy electron microscopy (LEEM) taken as intensity-voltage (I-V) curves provides hyperspectral images of surfaces, which can be used to identify the surface type, but are difficult to analyse. Here, we demonstrate the use of an algorithm for factorizing the data into spectra and concentrations of characteristic components (FSC3 ) for identifying distinct physical surface phases. Importantly, FSC3 is an unsupervised and fast algorithm. As example data we use experiments on the growth of praseodymium oxide or ruthenium oxide on ruthenium single crystal substrates, both featuring a complex distribution of coexisting surface components, varying in both chemical composition and crystallographic structure. With the factorization result a sparse sampling method is demonstrated, reducing the measurement time by 1-2 orders of magnitude, relevant for dynamic surface studies. The FSC3 concentrations are providing the features for a support vector machine-based supervised classification of the surface types. Here, specific surface regions which have been identified structurally, via their diffraction pattern, as well as chemically by complementary spectro-microscopic techniques, are used as training sets. A reliable classification is demonstrated on both example LEEM I-V data sets.
RESUMEN
Materials exhibiting reversible resistive switching in electrical fields are highly demanded for functional elements in oxide electronics. In particular, multilevel switching effects allow for advanced applications like neuromorphic circuits. Here, we report a structurally driven switching mechanism involving the so-called "dead" layers of perovskite manganite surfaces. Forming a tunnel barrier whose thickness can be changed in monolayer steps by electrical fields, the switching effect exhibits well-defined and robust resistive states.
RESUMEN
The growth, morphology, structure, and stoichiometry of ultrathin praseodymium oxide layers on Ru(0001) were studied using low-energy electron microscopy and diffraction, photoemission electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. At a growth temperature of 760 °C, the oxide is shown to form hexagonally close-packed (A-type) Pr2O3(0001) islands that are up to 3 nm high. Depending on the local substrate step density, the islands either adopt a triangular shape on sufficiently large terraces or acquire a trapezoidal shape with the long base aligned along the substrate steps.
RESUMEN
Vanadium dioxide (VO2) features a pronounced, thermally-driven metal-to-insulator transition at 340 K. Employing epitaxial stress on rutile [Formula: see text] substrates, the transition can be tuned to occur close to room temperature. Striving for applications in oxide-electronic devices, the lateral homogeneity of such samples must be considered as an important prerequisite for efforts towards miniaturization. Moreover, the preparation of smooth surfaces is crucial for vertically stacked devices and, hence, the design of functional interfaces. Here, the surface morphology of [Formula: see text] films was analyzed by low-energy electron microscopy and diffraction as well as scanning probe microscopy. The formation of large terraces could be achieved under temperature-induced annealing, but also the occurrence of facets was observed and characterized. Further, we report on quasi-periodic arrangements of crack defects which evolve due to thermal stress under cooling. While these might impair some applicational endeavours, they may also present crystallographically well-oriented nano-templates of bulk-like properties for advanced approaches.
RESUMEN
Cerium oxide is often applied in today's catalysts due to its remarkable oxygen storage capacity. The changes in stoichiometry during reaction are linked to structural modifications, which in turn affect its catalytic activity. We present a real-time in situ study of the structural transformations of cerium oxide particles on ruthenium(0001) at high temperatures of 700 °C in ultra-high vacuum. Our results demonstrate that the reduction from CeO2 to cubic Ce2O3 proceeds via ordered intermediary phases. The final reduction step from cubic to hexagonal Ce2O3 is accompanied by a lattice expansion, the formation of two new surface terminations, a partial dissolution of the cerium oxide particles, and a massive mass transport of cerium from the particles to the substrate. The conclusions allow for new insights into the structure, stability, and dynamics of cerium oxide nanoparticles in strongly reducing environments.
RESUMEN
When applying mechanical stress to a bulk metallic glass it responds with elastic and/or plastic deformations. A comprehensive microscopic theory for the plasticity of amorphous solids remains an open task. Shear transformation zones consisting of dozens of atoms have been identified as smallest units of deformation. The connexion between local formation of shear transformations zones and the creation of macroscopic shear bands can be made using statistical analysis of stress/energy drops or strain slips during mechanical loading. Numerical work has proposed a power law dependence of those energy drops. Here we present an approach to circumvent the experimental resolution problem using a waiting time analysis. We report on the power law-distributed deformation behaviour and the observation of a crossover in the waiting times statistics. This crossover indicates a transition in the plastic deformation behaviour from three-dimensional random activity to a two-dimensional nano shear band sliding.