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D Stradi,
S Barja,
C Díaz,
M Garnica,
B Borca,
J J Hinarejos,
D Sánchez-Portal,
M Alcamí,
A Arnau, A L Vázquez De Parga,
R Miranda,
F Martín
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ABSTRACT: The interpretation of scanning tunneling spectroscopy (STS) and scanning tunneling microscopy measurements of epitaxial graphene on lattice-mismatched substrates is a challenging problem, because of the spatial modulation in the electronic structure imposed by the formation of a moiré pattern. Here we describe the electronic structure of graphene adsorbed on Ru(0001) by means of density functional theory calculations that include van der Waals interactions and are performed on a large 11 × 11 unit cell to account for the observed moiré patterns. Our results show the existence of localized electronic states in the high and low areas of the moiré at energies close to and well above the Fermi level, respectively. Localization is due to the spatial modulation of the graphene-Ru(0001) interaction and is at the origin of the various peaks observed in STS spectra.
Physical Review B 01/2012; 85. · 3.69 Impact Factor
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D Stradi,
S Barja,
C Díaz,
M Garnica,
B Borca,
J J Hinarejos,
D Sánchez-Portal,
M Alcamí,
A Arnau, A L Vázquez de Parga,
R Miranda,
F Martín
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ABSTRACT: Elaborate density functional theory (DFT) calculations that include the effect of van der Waals (vdW) interactions have been carried out for graphene epitaxially grown on Ru(0001). The calculations predict a reduction of structural corrugation in the observed moiré pattern of about 25% (∼0.4 Å) with respect to DFT calculations without vdW corrections. The simulated STM topographies are close to the experimental ones in a wide range of bias voltage around the Fermi level.
Physical Review Letters 05/2011; 106(18):186102. · 7.37 Impact Factor
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Physical Review B. 01/2011; 84(3):035450.
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B Borca,
S Barja,
M Garnica,
D Sánchez-Portal,
V M Silkin,
E V Chulkov,
C F Hermanns,
J J Hinarejos, A L Vázquez de Parga,
A Arnau,
P M Echenique,
R Miranda
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ABSTRACT: 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.
Physical Review Letters 07/2010; 105(3):036804. · 7.37 Impact Factor
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ABSTRACT: It is shown that the Pb(√3×√3)R30/Si(111) ordered layer is an excellent mirror for neutral He atoms. It focuses more than 15% of the incoming He atoms into the specular peak, and is stable up to 450 K. Moreover, the reflectivity remains almost unchanged in a time scale of several weeks in ultrahigh vacuum. As a consequence, this system is a very good candidate to be used as a mirror in the next generation of the scanning helium atom microscope.
Applied Physics Letters 02/2010; 96(8):081901-081901-3. · 3.84 Impact Factor
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ABSTRACT: 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.
Journal of Physics Condensed Matter 04/2009; 21(13):134002. · 2.55 Impact Factor
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ABSTRACT: a b s t r a c t Many properties of metallic thin films have been shown to oscillate with film thickness due to quantum size effects, i.e. the confinement of electrons inside epitaxial metal overlayers which causes quantization of the electronic states. This is a very general phenomenon and it affects both bulk properties of the films, such as resistivity or superconducting transition temperatures, and surface properties, such as chemical reactivity, diffusivity, thermal stability, i.e surface roughening transitions. In this paper we describe some of these thickness-dependent properties which affect the stability of nanostructures and allow us to tailor their properties. We shall concentrate in the paradigmatic example of thin films of Pb grown on metallic and semiconducting substrates and how one can achieve the growth of highly perfect, atomically flat, epi-taxial films on different substrates, due to the kinetic constrains imposed by the presence of QSE, a topic increasingly important in the production of nanoscale quantum devices.
Surface Science 01/2009; 603:1389. · 1.99 Impact Factor
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ABSTRACT: 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.
Physical Review Letters 03/2008; 100(5):056807. · 7.37 Impact Factor
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ABSTRACT: By means of variable-temperature scanning tunneling microscopy and spectroscopy we studied the thickness-dependent roughening temperature of Pb films grown on Cu(111), whose electronic structure and total energy is controlled by quantum well states created by the spatial confinement of electrons. Large scale STM images are employed to quantify the layer population, i.e., the fraction of the surface area covered by different Pb thicknesses, directly in the real space as a function of temperature. The roughening temperature oscillates repeatedly with bilayer periodicity plus a longer beating period, mirroring the thickness dependence of surface energy calculations. Conditions have been found to stabilize at 300 K Pb films of particular magic thicknesses, atomically flat over microns.
Physical Review Letters 12/2006; 97(18):186104. · 7.37 Impact Factor
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ABSTRACT: The lateral confinement of the surface state electrons of Cu(111) has been studied by Scanning Tunnelling Microscopy and Spectroscopy at low temperature. The confining nanostructures are Cu(111) islands embedded in a semiconducting Cu3N(111) film which completely isolate them from each other. The standing wave pattern observed reflect the shape of the edge of the islands, i.e. the positions of the confining potential as long as the islands are larger than twice the Fermi wavelength of the surface electrons. The interference pattern in smaller islands is more complex, reflecting the collective behavior of the electrons. When the width of the islands is, at least in one dimension, smaller than the Fermi wavelength, there is a clear shift in the energy of the bottom of the surface band towards the Fermi level. The depopulation of the surface state produced by lateral confinement might have important consequences with respect to the reactivity of these nanostructures.
Physics of Condensed Matter 07/2004; 40(4):415-419. · 1.53 Impact Factor
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ABSTRACT: Systematic, quantitative comparisons between scanning tunneling microscopy (STM) experiments and first principles simulations of O(2 x 2)/Ru(0001) have been performed. The shape of the atomic adsorbates in the images depends strongly on the tunneling resistance and changes reversibly from circular (high resistance) to triangular (low resistance). In addition, after adsorption of oxygen on the STM tip we observe a contrast reversal on the surface, confirmed by extensive numerical simulations.
Physical Review Letters 06/2004; 92(20):206101. · 7.37 Impact Factor
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Applied Physics A 04/1998; 66:1117-1120. · 1.63 Impact Factor
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ABSTRACT: The surface morphology and structure of -FeSi2(101) films epitaxially grown on Si(111) has been studied by means of Scanning Tunneling Microscopy (STM). The films are formed by large crystallites which are single domain. Each crystallite has only one of the three possible azimuthal orientations with respect to the substrate. A large density of planar defects, however, is detected on top of each crystallite. They are assigned to intrinsic stacking faults and their existence seems hard to avoid. This high density of intrinsic defects casts serious doubts on the use of -FeSi2 as an optoelectronic material.
Applied Physics A 11/1993; 57(6):477-482. · 1.63 Impact Factor
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ABSTRACT: 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.
Surface Science.
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ABSTRACT: The initial stages of the growth of CaF2 on Cu(1 1 1) have been studied by STM as a function of deposition temperature and coverage. No evidence for a reaction between the CaF2 molecules and the Cu substrate has been found. The islands formed at 700 K and above show uniform heights, triangular shapes with two orientations rotated by 60° and flat top surfaces. They can not be imaged at voltages below +3 V indicating their insulating character. The LEED pattern shows that the film is epitaxial, (1 1 1)- oriented and in registry with the substrate. The observations indicate that epitaxial growth of twinned crystallites of CaF2(1 1 1) takes place on Cu(1 1 1).
Surface Science. 582:14-20.
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ABSTRACT: A comparison of theoretical and experimental STM images for the 2 × 1 superstructure of oxygen adsorbed on Ru(0 0 0 1) is presented. The analysis of STM images unveils the interplay between surface geometry and electronic effects. We use first-principles methods to calculate the electronic structures of both tip and sample which are then used to evaluate the tunnelling current in Bardeen’s approximation. Our simulated images compare well with the measurements and allow us to identify the atomic-scale features seen in the images.
Chemical Physics Letters 405:131-135. · 2.34 Impact Factor
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ABSTRACT: By means of variable temperature scanning tunneling microscope we studied the morphology and electronic structure of Pb films grown on Cu(1 1 1). Due to the spatial confinement of electrons, the islands display quantized energy levels. At 300 K, Pb forms 3D nanostructures with magic heights, that correspond to islands having a quantum well state (QWS) far from the Fermi energy. Below 100 K Pb grows in a quasi-layer-by-layer fashion. The QWS that develop in the films determine their total energy and, accordingly, their thermal stability. Films of particularly magic thickness are stable upon heating to 300 K.
Applied Surface Science 254(1):12-15. · 2.10 Impact Factor
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V. Blum,
Ch. Rath,
S. Müller,
L. Hammer,
K. Heinz,
J. M. García,
J. E. Ortega,
J. E. Prieto,
O. S. Hernán,
J. M. Gallego, A. L. Vázquez de Parga,
R. Miranda
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ABSTRACT: The combination of low-energy electron diffraction intensity analyses and scanning tunneling microscopy was used to investigate the morphology and atomic structure of thin Fe films grown on Au(100) at 400 K. Deposition of only about 0.2 monolayers (ML) Fe is sufficient to lift the reconstruction of the clean substrate. In the initial growth process (<~1 ML) place exchanges between Fe and Au lead to almost two-dimensional subsurface Fe film growth with one layer of Au covering the entire film. This way, gold acts as a “self-surfactant.” Yet, there are deviations from two-dimensional growth, with a second Fe layer beginning to grow before the first one is fully completed and some substitutional disorder developing in the film because of incomplete place exchange. The amount of gold floating on the surface only gradually decreases with further increasing film thickness. At about 2 ML the surface undergoes a complete restructuring during which short “wormlike” chains of atoms form and long-range order is destroyed. Nevertheless, the existence of large terraces of little roughness proves that some surface activity of gold remains. At coverages of several ML, long-range order is reestablished with the Fe film growing in an undistorted bcc arrangement. Although parts of the film are still covered by gold, the surface morphology is not very different from that known for homoepitaxial growth of Fe on Fe(100), i.e., gold has stopped to serve as a “self-surfactant.”
Phys. Rev. B. 59(24).
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ABSTRACT: The relationship between strain and surface electronic structure of Cu(111) films grown on Ru(0001) is studied by a combination of tunneling spectroscopy and ab initio theoretical calculations. Experimentally, the relaxation of the 5.5% in-plane lattice mismatch between Ru(0001) and Cu(111) changes layer-by-layer the lateral lattice parameter of the Cu film, while the surface state, that is above the Fermi level for the pseudomorphic monolayer of Cu, shifts down in energy with increasing thickness until it becomes the occupied surface state of Cu(111). The effects due to strain in the Cu films are distinguished from those due to the proximity to the Ru substrate by detailed comparisons with theoretically expanded Cu(111) and Cu∕Ru(0001) surfaces. Our ab initio calculations indicate that the observed energy shift must be essentially assigned to the decreasing tensile stress of the deposited film.
Phys. Rev. B. 71(12).
