A.L. Vázquez de Parga

Periodically Rippled Graphene: Growth and Spatially Resolved Electronic Structure

We grow epitaxial graphene monolayers on Ru(0001) that cover uniformly the substrate over lateral distances larger than several microns. The weakly coupled graphene monolayer is periodically rippled and it shows charge inhomogeneities in the charge distribution. Real space measurements by scanning tunneling spectroscopy reveal the existence of electron pockets at the higher parts of the ripples, as predicted by a simple theoretical model. We also visualize the geometric and electronic structure of edges of graphene nanoislands.

Potential energy landscape for hot electrons in periodically nanostructured graphene.

We explore the spatial variations of the unoccupied electronic states of graphene epitaxially grown on Ru(0001) and observed three unexpected features: the first graphene image state is split in energy; unlike all other image states, the split state does not follow the local work function modulation, and a new interfacial state at +3  eV appears on some areas of the surface. First-principles calculations explain the observations and permit us to conclude that the system behaves as a self-organized periodic array of quantum dots.

Real-Space Imaging of the First Stages of FeSi2 Epitaxially Grown on Si(111): Nucleation and Atomic Structure

The first stages of the growth of iron disilicide on Si(111) have been studied with Scanning Tunnelling Microscopy. Nucleation and growth of two-dimensional, epitaxial islands of disilicide are obtained by means of a Solid Phase Epitaxy method. Atomically resolved images show that the islands present a hexagonal closed-packed symmetry and correspond to the first stage of a new metastable cubic phase of FeSi2 which grows matching its (111) plane to the Si(111) surface. The surface of the islands shows a 2 × 2 reconstruction explained on the basis of the reduction in number of Si dangling bonds. The results allow us to visualize in the real space that metastable phases can be stabilized by heteroepitaxial growth on suitable substrates.

Surface characterization of epitaxial, semiconducting, FeSi2 grown on Si(100)

We have identified the composition and range of thermal stability of FeSi and FeSi2 films grown on Si(100) by solid phase epitaxy and reactive deposition epitaxy. Evidence for the semiconducting character of FeSi2 is obtained from photoemission measurements giving a low density of states at the Fermi level. Si enrichment at the outer surface of the silicides at temperatures much lower than previously thought has been found by depth profiling. Scanning tunneling microscopy reveals a rather inhomogeneous growth with a tendency towards epitaxial growth favored by the presence of surface steps on the Si substrate.

Reactivity of periodically rippled graphene grown on Ru(0001)

We report here the reactivity of epitaxial graphene islands and complete monolayers on Ru(0001) towards molecular oxygen and air. The graphene is prepared by thermal decomposition of ethylene molecules pre-adsorbed on an Ru(0001) surface in an ultra-high vacuum chamber. The graphene layer presents a periodically rippled structure that is dictated by the misfit between graphene and Ru(0001) lattice parameters. The periodic ripples produce spatial charge redistribution in the graphene and modifies its electronic structure around the Fermi level. In order to investigate the reactivity of graphene we expose graphene islands to a partial pressure of oxygen and following the evolution of the surface by STM during the exposure. For the exposure to air we removed the sample from the UHV chamber and we re-introduce it after several hours, taking STM images before and after. The surface areas not covered by the graphene islands present a dramatic change but the graphene structure, even the borders of the islands, remain intact. In the case of a complete graphene monolayer the exposure to oxygen or to air does not affect or destroy the rippled structure of the graphene monolayer.

Self-surfactant effect on Fe/Au(100):: place exchange plus Au self-diffusion

In this paper, we report on the submonolayer growth of Fe on Au(100), a system where a monolayer of the substrate floats at the external surface during growth facilitating the layer-by-layer growth of the Fe film. STM and STS demonstrate that this self-surfactant action results from a combination of several processes: fast place exchange between Fe and the Au atoms of the topmost surface layer, anisotropic diffusion of the Au atoms ejected due to the local lifting of the surface reconstruction, and preferential nucleation on the isolated Fe atoms embedded within the surface. The Au atoms diffuse over the surface in a way almost identical to the homoepitaxial growth of Au on Au(100). As a result, stable, reconstructed islands composed of Au atoms are formed and end up burying the deposited Fe.

Influence of surfactants on atomic diffusion

Abstract We have used Monte Carlo simulations with realistic interatomic potentials, combined with experimental results obtained by He diffraction (thermal energy atom scattering) and STM to investigate the effect of a surfactant agent such as Pb on the mechanisms of atomic diffusion involved in epitaxial metal growth. We find that the main role of the surfactant is to hinder fast diffusion by hopping over the surface, which is the dominant mechanism on a compact face such as Cu(111), and to promote exchange. As a side effect, this facilitates interlayer diffusion and hence layer-by-layer growth, because islands are smaller and have rougher borders; adatoms reaching an edge have more opportunities to cross them by exchange with a step atom.

Geometric and electronic structure of epitaxial iron silicides

The geometric and electronic structure of several iron silicide phases epitaxially grown on Si(111)7×7 have been characterized by means of surface sensitive techniques including scanning tunneling microscopy (STM), x‐ray photoelectron spectroscopy, ion scattering spectroscopy (ISS), and ultraviolet photoelectron spectroscopy. The silicides were grown by solid‐phase and reactive‐deposition epitaxy, and their stability range was determined in situ as a function of iron coverage and annealing temperature. In particular, we have studied the phases appearing in the low‐coverage low‐temperature region. Additionally, the crystallites of the most important FeSi2 phases (γ‐FeSi2 and β‐FeSi2) have been characterized at atomic level with STM, while the surface termination was analyzed with ISS.

A new metastable epitaxial silicide: FeSi2/Si(111)

Abstract A metastable phase of FeSi 2 that is not present in the bulk phase diagram can be grown epitaxially on Si(111). Scanning tunneling microscope images reveal the surface reconstruction of this phase and allows one to model its crystallographic structure. It crystallizes with a cubic structure, probably a fluorite structure like CoSi 2 and NiSi 2 , lattice-matched to Si. The (111) surface has a 2×2 reconstruction according to LEED. The origin of this reconstruction is reported here for the first time. It is due to a superstructure of Si adatoms with local bonding similar to that of the adatoms in the Si(111)7×7 reconstruction.

Observation of Localized Vibrational Modes of Graphene Nanodomes by Inelastic Atom Scattering.

Inelastic helium atom scattering (HAS) is suitable to determine low-energy (few meV) vibrations spatially localized on structures in the nanometer range. This is illustrated for the nanodomes that appear often on graphene (Gr) epitaxially grown on single crystal metal surfaces. The nature of the inelastic losses observed in Gr/Ru(0001) and Gr/Cu/Ru(0001) has been clarified by intercalation of Cu below the Gr monolayer, which decouples the Gr layer from the Ru substrate and changes substantially the out-of-plane, flexural phonon dispersion of epitaxial Gr, while maintaining the nanodomes and their localized vibrations. He diffraction proves that the Cu-intercalated Gr layer is well ordered structurally, while scanning tunneling microscopy reveals the persistence of the (slightly modified) periodic array of Gr nanodomes. A simple model explains the order of magnitude of the energy losses associated with the Gr nanodomes and their size dependence. The dispersionless, low-energy phonon branches may radically alter the transport of heat in intercalated Gr.