Formation of titanium monoxide (001) single-crystalline thin film induced by ion bombardment of titanium dioxide (110).

A plethora of technological applications justify why titanium dioxide is probably the most studied oxide, and an optimal exploitation of its properties quite frequently requires a controlled modification of the surface. Low-energy ion bombardment is one of the most extended techniques for this purpose and has been recently used in titanium oxides, among other applications, to favour resistive switching mechanisms or to form transparent conductive layers. Surfaces modified in this way are frequently described as reduced and defective, with a high density of oxygen vacancies. Here we show, at variance with this view, that high ion doses on rutile titanium dioxide (110) induce its transformation into a nanometric and single-crystalline titanium monoxide (001) thin film with rocksalt structure. The discovery of this ability may pave the way to new technical applications of ion bombardment not previously reported, which can be used to fabricate heterostructures and interfaces.

Surface defects and their influence on surface properties.

Surface defects have a profound influence on many attributes of materials, therefore experimental techniques and specific studies focused on their controlled generation and properties are mandatory. We have carried out a thorough study of the role of surface defects on a variety of physico-chemical properties of metals and oxides, using different experimental techniques and molecular dynamics simulations. In particular, we have studied the defects formed upon bombardment with Ar+ ions in a reconstructed Au(100) surface at very low ion doses. At room temperature, the pristine defects are mainly single vacancies, which diffuse by collective atomic motions, then cluster and collapse, resulting in 2D dislocation dipoles. These dislocations exhibit an enhanced chemical reactivity due to the elastic stress of their cores. We have also performed indentation tests of flat and stepped Au(111) samples with an atomic force microscope, revealing noticeable differences in their mechanical behavior when probed at the nanoscale. Thus, the stepped sample has a 20% smaller Young's modulus, 40% smaller yield point and 50% smaller shear stress. These differences, as well as reversible, quasiplastic behavior of the stepped sample up to a critical load, are due to the active role of steps as dislocation nucleation centers. In contrast, a TiO2(110) surface, modified with ion bombardment, does not show noticeable changes in its nanomechanical properties, which is an indication of the very different mechanical responses of oxides compared to simple metals at the nanoscale. Finally, we show how surface defects affect the chemical activity of a Pt(111) surface when exposed to methanol. The nature of the adsorbed species and the dynamics of the surface reactions are modified in the presence of surface defects, rendering the defective surface into a more robust state against catalytic poisoning.

Plastic properties of gold surfaces nanopatterned by ion beam sputtering.

We review the mechanical properties of defective roughened surfaces with the major emphasis on nanoindentation work. We also report novel results in which force versus penetration curves and AFM images of the nanoindented surface are compared for a flat surface of Au(001) and an Ar(+) bombarded one, both with a high and a low flux of ions. We have found that bombarded surfaces yield at a lower stress than untreated flat ones. Surfaces bombarded at high flux show a large roughness and their yield point, marking the onset of surface plasticity, decreases with respect to that of the flat surface or of the surface bombarded with a low flux. The present results are compared with earlier work on nanoindented vicinal surfaces in which the sole surface modifications with respect to the flat surface were the presence of a high density of steps. It is concluded that a softening effect due to the bombardment-induced nanostructure of the surface dominates over the hardening one due to defect creation and interaction in the surface neighbourhood.

Structure and magnetism in ultrathin iron oxides characterized by low energy electron microscopy.

We have grown epitaxial films a few atomic layers thick of iron oxides on ruthenium. We characterize the growth by low energy electron microscopy. Using selected-area diffraction and intensity-versus-voltage spectroscopy, we detect two distinct phases which are assigned as wüstite and magnetite. Spin-polarized low energy electron microscopy reveals magnetic domain patterns in the magnetite phase at room temperature.

Uncommon dislocation processes at the incipient plasticity of stepped gold surfaces.

Gold vicinal surfaces (788), with a high density of steps, along with (111) flat surfaces taken as a reference, have been nanoindented and their resulting penetration curves and related defect structure comparatively analyzed by AFM and atomistic simulations. Stepped surfaces are shown to yield at smaller loads than (111) ones in agreement with calculations of the critical resolved shear stress needed to nucleate a dislocation. In the stepped surfaces, a novel intermediate state is identified in which the penetration curves depart from a Hertzian behavior prior to the appearance of pop-ins. This state is shown to result from heterogeneous nucleation at preexisting surface steps of dislocation loops, most of which retract and vanish when the indenter load is removed.

Structural origin of the Sn 4d core level line shape in Sn/Ge(111)-(3x3).

High-resolution photoemission of the Sn 4d core level of Sn/Ge(111)-(3x3) resolves three main components in the line shape, which are assigned to each of the three Sn atoms that form the unit cell. The line shape found is in agreement with an initial state picture and supports that the two down atoms are inequivalent. In full agreement with these results, scanning tunnel microscopy images directly show that the two down atoms are at slightly different heights in most of the surface, giving rise to an inequivalent-down-atoms (3x3) structure. These results solve a long-standing controversy on the interpretation of the Sn 4d core-level line shape and the structure of Sn/Ge(111)-(3x3).

Observation of a Mott insulating ground state for Sn/Ge(111) at low temperature.

We report an investigation on the properties of 0.33 ML of Sn on Ge(111) at temperatures down to 5 K. Low-energy electron diffraction and scanning tunneling microscopy show that the (3x3) phase formed at approximately 200 K, reverts to a new ((square root 3)x(square root 3))R30 degrees phase below 30 K. The vertical distortion characteristic of the (3x3) phase is lost across the phase transition, which is fully reversible. Angle-resolved photoemission experiments show that, concomitantly with the structural phase transition, a metal-insulator phase transition takes place. The ((square root 3)x(square root 3))R30 degrees ground state is interpreted as the formation of a Mott insulator for a narrow half-filled band in a two-dimensional triangular lattice.

Electron confinement in surface states on a stepped gold surface revealed by angle-resolved photoemission.

STM images show that vicinal Au(788) surfaces are made up of a uniform array of (111)-oriented terraces of similar width ( approximately 3.8 nm). This uniformity makes it possible to study the electronic structure of the resulting step superlattice by angle-resolved photoemission. We show that for this terrace array the surface state appears to be broken up into one-dimensional quantum-well levels, indicating total electron confinement within the terraces. The angular resolution allows the probability density of the terrace quantum well state to be mapped in reciprocal space, complementing nicely the wave function measured in real space by STM.

NEXAFS multiple scattering calculations of KO2.

Since many years the oxidation of alkali metals has being attracted much interest due to the catalytic properties of metal promoters and the simple electronic structure of alkali atoms. The alkali-oxides phase diagram indicates that the interaction of oxygen with alkali metals can lead to the formation of different atomic O2- ions and molecular O2(-) and O(2)2- ions. Potassium superoxide has been prepared in situ and high resolution O k-edge absorption NEXAFS spectra have been measured at the VUV beam-line at ELETTRA facility. The experimental data have been analyzed by multiple scattering approach deriving many geometrical and electronic details. In particular, we have found that the growth material structure is of the KO2 type with an O-O distance of about 1.35A and that the transition involving single pi molecular empty state of the superoxide O2(-) anion has a fine structure. Multiple Scattering self consistent calculation indicates that the bond between oxygen anion and K atom is totally ionic and that the fine structure is essentially due to solid state effects.

Electron wave function at a vicinal surface: switch from terrace to step modulation.

The Cu(111) surface state has been mapped for vicinal surfaces with variable step densities by angle-resolved photoemission. Using tunable synchrotron radiation to vary the k dependence perpendicular to the surface, as well as the (k) dependence, we find a switch between two qualitatively different regimes at a miscut of 7 degrees (17 A terrace width). For larger miscut angles the step modulation of the wave function dominates, and for smaller miscut angles the terrace modulation dominates. These observations resolve an apparent inconsistency between prior photoemission and STM results.