Patricio Häberle

Surface structure of MgO(001): a medium energy ion scattering study

Abstract We have used medium-energy ion-scattering with channeling and blocking to study the surface structure of MgO(001). the ion scattering data show that UHV-cleaved crystals result in well ordered surfaces, while sputtered/ annealed samples show structural disorder. Surface blocking dips in both the O and Mg spectra are found to occur at angles very close to the corresponding bulk blocking directions, indicating very small surface relaxation and rumpling. A detailed R -factor analysis comparing the experimental spectra with results of Monte Carlo simulations for different structures gives a surface relaxation of −1.0% ± 1.0% and a rumpling of 0.5% ± 1.0%. These results are in good agreement with preliminary results from all-electron total-energy calculations and imply that the surface O 2− ions have small polarizations.

Electronic structure of cubic gallium nitride films grown on GaAs

The composition, surface structure, and electronic structure of zinc blende–GaN films grown on GaAs (100) and (110) by plasma‐assisted molecular beam epitaxy were investigated by means of core and valence level photoemission. Angle‐resolved photoelectron spectra (photon energy 30–110 eV) exhibited emission from the Ga 3d and N 2s levels, as well as a clear peak structure in the valence band region. These peaks were found to shift with photon energy, indicative of direct transitions between occupied and unoccupied GaN bands. By using a free electron final band, we are able to derive the course of the bands along the Γ‐X and Γ‐K‐X directions of the Brillouin zone and to determine the energy of critical points at the X point. The relative energies of the Ga 3d and nitrogen 2s bands were also studied, and a small amount of dispersion was detected in the latter. The resulting band structure is discussed in relation to existing band structure calculations.

Gold nanoparticles grown inside carbon nanotubes: synthesis and electrical transport measurements

The hybrid structures composed of gold nanoparticles and carbon nanotubes were prepared using porous alumina membranes as templates. Carbon nanotubes were synthesized inside the pores of these templates by the non-catalytic decomposition of acetylene. The inner cavity of the supported tubes was used as nanoreactors to grow gold particles by impregnation with a gold salt, followed by a calcination-reduction process. The samples were characterized by transmission electron microscopy and X-ray energy dispersion spectroscopy techniques. The resulting hybrid products are mainly encapsulated gold nanoparticles with different shapes and dimensions depending on the concentration of the gold precursor and the impregnation procedure. In order to understand the electronic transport mechanisms in these nanostructures, their conductance was measured as a function of temperature. The samples exhibit a ‘non-metallic’ temperature dependence where the dominant electron transport mechanism is 1D hopping. Depending on the impregnation procedure, the inclusion of gold nanoparticles inside the CNTs can introduce significant changes in the structure of the tubes and the mechanisms for electronic transport. The electrical resistance of these hybrid structures was monitored under different gas atmospheres at ambient pressure. Using this hybrid nanostructures, small amounts of acetylene and hydrogen were detected with an increased sensibility compared with pristine carbon nanotubes. Although the sensitivity of these hybrid nanostructures is rather low compared to alternative sensing elements, their response is remarkably fast under changing gas atmospheres.

Growth and morphology of ultra-thin Ni films on Pd(100)

A series of Ni films with thickness from 0.2 monolayers (ML) to 12.5 ML were epitaxially grown on a Pd(100) substrate at room temperature. Growth and morphology were investigated by scanning tunneling microscopy (STM), reflection-high-energy-electron diffraction (RHEED) and Auger electron spectroscopy (AES). We found that the strain relief mechanism for the tetragonal distorted films is related with the appearance of 1A high-filaments.

Surface roughness of thin gold films and its effects on the proton energy loss straggling

We present a description of the effect of the surface roughness on the energy straggling associated to the energy loss distributions of protons transmitted through a self supported metallic thin foil. For this purpose we prepared a polycrystalline gold thin films using the standard sputtering method with different deposition rates. The statistics of the surface height distribution induced in these thin films were determined using Atomic Force Microscopy. The measured surface roughness allowed us to quantify the ion energy loss straggling in these samples for different deposition parameters and as a function of the incident ion energy.

Inverse Photoemission Spectroscopy of Multiwall Carbon Nanotubes

Multiwall Carbon Nanotubes (MWCNTs) were synthesized by Chemical Vapor Deposition (CVD). Two different procedures were used to grow MWCNT films roughly, aligned in the direction normal to the SiO2/Si(111) substrate. Inverse Photoemission Spectroscopy measurements, on these samples, show the existence of resonances which could be traced back to a flat graphene sheet. The unoccupied valence band is fairly similar to that shown by graphite except by an additional intensity in the vicinity of the Fermi level. This resonance could be interpreted both as tubes tips end effects or van Hove singularities in the density of states.

Unoccupied electronic states of Au(113): Theory and experiment

We present results from inverse photoemission spectroscopy in the isochromat mode, with angular resolution, from a clean Au(113) surface. To identify the origin of the different resonances, we have performed a first-principles calculation of the bulk band structure in the linear-muffin-tin-orbital formalism. We predict the dispersion of the bulk features, as a function of parallel momentum, considering energy and momentum conservation. We have been able to identify unambiguously two surface resonances and a surface state in the [{bar 1}10] and [33{bar 2}] directions, respectively, as well as various bulk-derived features.

The Many Faces of Graphene as Protection Barrier. Performance under Microbial Corrosion and Ni Allergy Conditions

In this work we present a study on the performance of CVD (chemical vapor deposition) graphene coatings grown and transferred on Ni as protection barriers under two scenarios that lead to unwanted metal ion release, microbial corrosion and allergy test conditions. These phenomena have a strong impact in different fields considering nickel (or its alloys) is one of the most widely used metals in industrial and consumer products. Microbial corrosion costs represent fractions of national gross product in different developed countries, whereas Ni allergy is one of the most prevalent allergic conditions in the western world, affecting around 10% of the population. We found that grown graphene coatings act as a protective membrane in biological environments that decreases microbial corrosion of Ni and reduces release of Ni2+ ions (source of Ni allergic contact hypersensitivity) when in contact with sweat. This performance seems not to be connected to the strong orbital hybridization that Ni and graphene interface present, indicating electron transfer might not be playing a main role in the robust response of this nanostructured system. The observed protection from biological environment can be understood in terms of graphene impermeability to transfer Ni2+ ions, which is enhanced for few layers of graphene grown on Ni. We expect our work will provide a new route for application of graphene as a protection coating for metals in biological environments, where current strategies have shown short-term efficiency and have raised health concerns.

Synthesis, characterization and spectroscopy of carbon based nanoscale materials

We have used chemical vapor deposition (CVD), for the synthesis of carbon nanotubes (CNTs) by the sublimation of iron phthalocyanines (FePcs); by the catalytic decomposition of acetylene over iron nanoparticles, supported on a thermal Si oxide and also by a Pd catalyst supported on alumina. We have explored different means of growing multiple wall CNTs and other carbon materials. Transmission and scanning electron microscopy have been used to characterize the synthesis products. We have also performed inverse photoemission spectroscopy (IPS), to explore the unoccupied electronic states above the Fermi level of CNTs and describe the peculiarities of the nanotubes electronic structure. We have examined by different CNTs samples and determined the origin of the main IPS spectral features.

Photoelectrochemical Activity of Graphene Supported Titanium Dioxide

Thin TiO2 layers grown over few-layers graphene were prepared in order to evaluate the photoinduced chemical response of this composite. Graphene was grown over copper foils by decomposition of acetylene in a standard chemical vapor deposition apparatus. Graphene was subsequently transferred to a silicon substrate, on which the titanium dioxide was grown to form a TiO2/FLG/SiO2/Si composite. The formation of each layered material was verified by Raman spectroscopy and the morphology was characterized by scanning electron microscopy. The photoelectrochemical evaluation of the resulting composite, using it as a photoanode, was accomplished with a potentiostat, a solar simulator, and a three-electrode configuration. The electrochemical response indicates that the new composite preserves the average photoactive properties of the base material and at the same time shows a singular transient response where explicit benefits seem to be derived from the FLG/TiO2 combination.