O. Rodríguez de la Fuente

Dislocation emission around nanoindentations on a (001) fcc metal surface studied by scanning tunneling microscopy and atomistic simulations.

We present a combined study by scanning tunneling microscopy and atomistic simulations of the emission of dissociated dislocation loops by nanoindentation on a (001) fcc surface. The latter consist of two stacking-fault ribbons bounded by Shockley partials and a stair-rod dislocation. These dissociated loops, which intersect the surface, are shown to originate from loops of interstitial character emitted along the <110> directions and are usually located at hundreds of angstroms away from the indentation point. Simulations reproduce the nucleation and glide of these dislocation loops.

Extended and Localized Surface Plasmons in Annealed Au Films on Glass Substrates

We present here a study on the surface plasmon resonance (SPR) in Au films deposited onto glass substrates and annealed in air at different temperatures. The initial Au films exhibit the resonant absorption of extended surface plasmons which depends on the film thickness. Thermal treatments promote the modification of the continuous films toward the formation of Au isolated islands. The morphological features of the islands depend on the film initial thickness and annealing temperature. The optical properties of the films are qualitatively modified as a consequence of the morphological changes. For films with initial thickness below 30 nm, the islands exhibit localized SPR while thicker films lead to islands large enough to hold extended SPR.

Molecular dynamics modeling and simulation of void growth in two dimensions

The mechanisms of growth of a circular void by plastic deformation were studied by means of molecular dynamics in two dimensions (2D). While previous molecular dynamics (MD) simulations in three dimensions (3D) have been limited to small voids (up to ≈10 nm in radius), this strategy allows us to study the behavior of voids of up to 100 nm in radius. MD simulations showed that plastic deformation was triggered by the nucleation of dislocations at the atomic steps of the void surface in the whole range of void sizes studied. The yield stress, defined as stress necessary to nucleate stable dislocations, decreased with temperature, but the void growth rate was not very sensitive to this parameter. Simulations under uniaxial tension, uniaxial deformation and biaxial deformation showed that the void growth rate increased very rapidly with multiaxiality but it did not depend on the initial void radius. These results were compared with previous 3D MD and 2D dislocation dynamics simulations to establish a map of mechanisms and size effects for plastic void growth in crystalline solids.

Dislocation emission at the onset of plasticity during nanoindentation in gold

Nanoindentations and the subsequent plastic damage in the form of dislocation configurations have been both generated and imaged with scanning tunnelling microscopy on a reconstructed Au(001) surface, the resulting observations being interpreted in terms of the elastic theory of dislocations in a continuum. The rearranged pileup material around the nanoindentation is described in terms of dislocation emission and glide involving, in particular, multiple cross-slip. ‘Mesas’, shallow protusions stemming from a special dislocation configuration consisting of Schockley partial dislocations encompassing two stacking faults, are shown to glide parallel to the surface under the stress generated by further nanoindentations. The spatial distribution of ‘mesas’ around the nanoindentation traces is shown to be controlled by a balance between the interactions between the different ‘mesas’ and the stresses arising from the nanoindentation itself.

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 Youngs 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.

Dielectric properties of self-assembled layers of octadecylamine on mica in dry and humid environments.

Atomic force microscopy operating in noncontact electrostatic force mode was used to study the interaction of water with films of alkylamines and alkylsilanes on mica. The films efficiently block water adsorption except in exposed mica areas, where it strongly modifies the mobility of surface ions. We also studied the molecular orientation of octadecylamine molecules forming monolayers and multilayer islands. In monolayer films the molecules bind to mica through the amino group, producing a positive contact potential relative to mica (dipole pointing up). In multilayer films the methyl and amino group terminations are exposed in alternating layers that give rise to alternating values of the contact potential. These findings correlate with low and high friction forces measured in the methyl termination and amino terminations.

Growth, structure and magnetism of ε-Fe2O3 in nanoparticle form

We present a novel and easy synthetic path to prepare e-Fe2O3 (∼90%) with a small portion of α-Fe2O3 nanoparticles embedded in an amorphous silica matrix. An exhaustive structural study reveals the higher relative concentration of the e-phase, with an average particle size of 17 nm. Confocal Raman microscopy and X-ray absorption spectroscopy are presented as novel techniques to characterize the e-polymorph. The magnetic properties have been studied in a wide range of temperatures (5–1000 K), detecting blocking effects (∼135 K), collapse effects (50–125 K), Morin (∼268 K) and Neel (∼505 K) transitions.

Dislocation configurations around a nanoindentation in the surface of a fcc metal

We report a scanning tunnelling microscopy investigation of the emission of dislocations around nanoindentations in the form of dislocation arrangements previously called hillocks , consisting of two pairs of Shockley partial dislocations, each encompassing a stacking fault. The spatial arrangement and size distribution of hillocks around the nanoindentation traces are studied. We show that standard dislocation theory for an isotropic continuum can be used to describe the stability of the hillocks, their size and spatial distribution and the broadening of the corresponding extended dislocations near the surface. A model is proposed in which hillocks originate from the split into dislocations partials of primary perfect dislocation loops punched into the crystal by the scanning tunnelling microscope tip. This model implies the operation of a novel dislocation mechanism involving long-range transport of matter across the surface.

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.

MAGNETIC PROPERTIES OF ORGANIC COATED GOLD SURFACES

We review here our recent results of experimental observation of room temperature magnetism in gold nanoparticles (NPs) and thin films. Capping gold surfaces with certain organic molecules leads to the appearance of magnetism at room temperature. The surface bonds between the organic molecules and Au atoms give rise to magnetic moments. These magnetic moments are blocked along the bond direction showing huge anisotropy. In the case of atomically flat surfaces, the magnetic moments are giants. An explanation of this orbital ferromagnetism is given. These results point out the possibility to observe magnetism at nanoscale in materials without typical magnetic atoms (transition metals and rare earths), and are of fundamental value to understand the magnetic properties of surfaces.