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Villaseca S.A.,CNRS Jean Lamour Institute | Loli L.N.S.,CNRS Jean Lamour Institute | Ledieu J.,CNRS Jean Lamour Institute | Fournee V.,CNRS Jean Lamour Institute | And 3 more authors.
Journal of Physics Condensed Matter | Year: 2013

Atomic oxygen adsorption on a pure aluminum terminated Al 9Co2(001) surface is studied by first-principle calculations coupled with STM measurements. Relative adsorption energies of oxygen atoms have been calculated on different surface sites along with the associated STM images. The local electronic structure of the most favourable adsorption site is described. The preferential adsorption site is identified as a 'bridge' type site between the cluster entities exposed at the (001) surface termination. The Al-O bonding between the adsorbate and the substrate presents a covalent character, with s-p hybridization occurring between the states of the adsorbed oxygen atom and the aluminum atoms of the surface. The simulated STM image of the preferential adsorption site is in agreement with experimental observations. This work shows that oxygen adsorption generates important atomic relaxations of the topmost surface layer and that sub-surface cobalt atoms strongly influence the values of the adsorption energies. The calculated Al-O distances are in agreement with those reported in Al2O and Al 2O3 oxides and for oxygen adsorption on Al(111). © 2013 IOP Publishing Ltd.

Addou R.,CNRS Jean Lamour Institute | Shukla A.K.,CNRS Jean Lamour Institute | De Weerd M.-C.,CNRS Jean Lamour Institute | Gille P.,Crystallography Section | And 5 more authors.
Journal of Physics Condensed Matter | Year: 2011

We have used the pseudo-tenfold surface of the orthorhombic Al 13Co4 crystal as a template for the adsorption of Cu thin films of various thicknesses deposited at different temperatures. This study has been carried out by means of low energy electron diffraction (LEED), scanning tunnelling microscopy (STM), x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD). From 300 to 573K, Cu adatoms grow pseudomorphically up to one monolayer. At 300K, the -Al(Cu, Co) phase appears for coverages greater than one monolayer. For higher temperature deposition, the -Al(Cu, Co) phase further transforms into the -Al4Cu9 phase. Both and phases grow as two (110) domains rotated by 72°1°from each other. Instead of following the substrate symmetry, it is the orientations of the bipentagonal motifs present on the clean Al13Co 4(100) surface that dictate the growth orientation of these domains. The initial bulk composition and structural complexity of the substrate have a minor role in the formation of the -Al4Cu9 phase as long as the amount of Al and the Cu film thickness reach a critical stoichiometry. © 2011 IOP Publishing Ltd.

Shin H.,Pennsylvania State University | Pussi K.,Lappeenranta University of Technology | Gaudry E.,CNRS Jean Lamour Institute | Ledieu J.,CNRS Jean Lamour Institute | And 7 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

In a combined scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density functional theory (DFT) study of the surface of Al13Co4(100), all techniques have found that after annealing to 1165 K, the surface structure is consistent with a dense Al-rich plane with surface Co atom depletion. Various structure models were considered, and in the LEED study, the best agreement was found with a model that consists of Al-rich terminating planes with no Co atoms, and otherwise a structure similar to the bulk puckered layers. This structure was also found to be stable in the DFT study. The best-fit structural parameters are presented for the two domains of this structure, which contain bipentagons that can be related to the pentagonal bipyramidal structures in the bulk, plus additional glue atoms between them. These domains are not strictly related to each other by symmetry, as they have different surface relaxations. The STM study found significant differences in the surfaces of samples grown by different methods and is able to explain a different interpretation made in an earlier study. © 2011 American Physical Society.

Ledieu J.,CNRS Jean Lamour Institute | Gaudry E.,CNRS Jean Lamour Institute | De Weerd M.-C.,CNRS Jean Lamour Institute | Gille P.,Crystallography Section | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

We report the formation of an ordered C60 monolayer on the Al9Co2(001) surface using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), x-ray and ultraviolet photoelectron spectroscopy (XPS/UPS), and ab initio calculations. Dosing fullerenes at 300 K results in a disordered overlayer. However, the adsorption of C60 with the sample held between 573-673 K leads to a [4, -21,3] phase. The growth of C60 proceeds with the formation of two domains which are mirror symmetric with respect to the [100] direction. Within each domain, the superstructure unit cell contains six molecules and this implies an area per fullerene equal to 91 Å2. The molecules exhibit two types of contrast (bright and dim) which are bias dependent. The adsorption energies and preferred molecular configuration at several possible adsorption sites have been determined theoretically. These calculations lead to a possible scheme describing the configuration of each C60 in the observed superstructure. Several defects (vacancies, protrusions,...) and domain boundaries observed in the film are also discussed. If the sample temperature is higher than 693 K when dosing, impinging C60 molecules dissociate at the surface, hence leading to the formation of a carbide film as observed by STM and LEED measurements. The formation of Al4C3 domains and the molecular dissociation are confirmed by XPS/UPS measurements acquired at different stages of the experiment. The cluster substructure present at the Al9Co2(001) surface dictates the carbide domain orientations. © 2015 American Physical Society.

Hamalainen S.K.,Aalto University | Boneschanscher M.P.,University Utrecht | Jacobse P.H.,University Utrecht | Swart I.,University Utrecht | And 5 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

We have studied the incommensurate moiré structure of epitaxial graphene grown on iridium(111) by dynamic low-energy electron diffraction [LEED I(V)] and noncontact atomic force microscopy (AFM) with a CO-terminated tip. Our LEED I(V) results yield the average positions of all the atoms in the surface unit cell and are in qualitative agreement with the structure obtained from density functional theory. The AFM experiments reveal local variations of the moiré structure: The corrugation varies smoothly over several moiré unit cells between 42 and 56 pm. We attribute these variations to the varying registry between the moiré symmetry sites and the underlying substrate. We also observe isolated outliers, where the moiré top sites can be offset by an additional 10 pm. This study demonstrates that AFM imaging can be used to directly yield the local surface topography with pm accuracy even on incommensurate two-dimensional structures with varying chemical reactivity. © 2013 American Physical Society.

Pussi K.,Lappeenranta University of Technology | Caragiu M.,Ohio Northern University | Hanna K.J.,Pennsylvania State University | Moritz W.,Crystallography Section | Diehl R.D.,Pennsylvania State University
Journal of Physics Condensed Matter | Year: 2015

The multilayer relaxation of the stepped Cu(5 1 1) surface has been studied by quantitative low-energy electron diffraction and analyzed using the CLEED program package. Relaxations with respect to the bulk interlayer spacing of 0.6934% are -9.5%, -10.4%, +8.2% and -1.8% for the first four interlayer spacings, respectively (negative sign corresponds to contraction). The relaxation sequence (- - + -⋯) is thus in agreement with the theoretical prediction. The deeper relaxations are damped in a non-uniform manner and the lateral relaxations are smaller than 2% of the lateral spacing. This result agrees well with theoretical studies of the same surface. The Pendry R-factor for the favored structure is 0.21. © 2015 IOP Publishing Ltd.

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