J. J. Hinarejos

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.

Self-organization of electron acceptor molecules on graphene

Graphene grown on Ir(111) electronically decouples adsorbed molecules from the metallic substrate and allows the study of their self-organization on surfaces. We study two electron acceptor molecules from the same family. The intermolecular interaction, attractive for TCNQ and repulsive for F(4)-TCNQ, dictates the molecular ordering.

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.

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.

Iron silicides grown on Si(100): metastable and stable phases

Abstract The growth of iron silicides on Si(100) by solid phase epitaxy has been investigated by photoelectron spectroscopies. The different iron silicide phases appearing and their stability ranges have been determined. We have found three different ranges depending on the initial Fe coverage deposited. In the case of large initial Fe coverages (above 16 ML), we have found the subsequent formation of ϵ-FeSi and β-FeSi 2 upon annealing. For initial Fe coverages below 3.4 ML, a phase of 1:1 stoichiometry is formed upon annealing, followed by the formation of a well-ordered metastable phase which we identify with FeSi 2 (CsCl). In the intermediate coverage range, while a poorly ordered phase of 1:1 stoichiometry is found for T

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.

Periodically modulated geometric and electronic structure of graphene on Ru(0 0 0 1)

We report here on a method of fabricating and characterizing highly perfect, periodically rippled graphene monolayers and islands, epitaxially grown on single crystal metallic substrates under controlled ultra-high vacuum conditions. The periodicity of the ripples is dictated by the difference in lattice parameters of graphene and substrate, and, thus, it is adjustable. We characterize its perfection at the atomic scale by means of STM and determine its electronic structure in the real space by local tunnelling spectroscopy. There are periodic variations in the geometric and electronic structure of the graphene monolayer. We observe inhomogeneities in the charge distribution, i.e. a larger occupied density of states at the higher parts of the ripples. Periodically rippled graphene might represent the physical realization of an ordered array of coupled graphene quantum dots. The data show, however, that for rippled graphene on Ru(0 0 0 1) both the low and the high parts of the ripples are metallic. The fabrication of periodically rippled graphene layers with controllable characteristic length and different bonding interactions with the substrate will allow a systematic experimental test of this fundamental problem.

Metastable iron silicide phase stabilized by surface segregation on Fe3Si(100)

Abstract The surface properties of single crystalline Fe3Si(100) have been characterized by AES, UPS, LEED, ISS and XPS measurements. The ideal Fe3Si(100) surface may exhibit two possible terminations: pure Fe or mixed Fe/Si (1:1). XPS shows that the thermally equilibrated surface has a larger concentration of Si in the top layers than in the bulk. The thickness of the Si-enriched region is of the order of 2 ML. The segregated overlayer is ordered according to LEED. It corresponds to a metastable phase with the structure of the CsCl and an FeSi composition, stabilized by epitaxy on top of the substrate. The corresponding surface electronic structure, as revealed by UPS, is in good agreement with theoretical calculations. ISS and CO titration experiments support the presence of both Si and Fe at the external surface, the former giving rise to a c(2 × 2) surface reconstruction composed of Si adatoms.

Study of the electronic structure of iron silicides grown on Si(100)2 × 1 by reactive deposition epitaxy

Abstract We have studied the growth of iron silicides on Si(100)2 × 1 by reactive deposition epitaxy. The electronic structure of the surface was monitored with ultraviolet photoelectron spectroscopy during the growth process, while the atomic stoichiometry was determined by X-ray photoelectron spectroscopy. The characteristic features observed in the density of states at the different stages of growth allow us to determine the appearance and the stability range of each silicide. By selecting an adequate temperature range it is possible to directly grow a certain phase on the Si substrate. In addition the sharpness of the features was used to estimate the crystalline quality of the films.