74 research outputs found
Cobalt on nickel surfaces and the role of carbide on its stability
No full textTransition metal atoms are commonly used in catalysis and photocatalysis, but their potential reactivity reduces with aggregation and alloying. We investigate in particular whether Cobalt adatoms float on Ni surfaces or dissolve
into the metal. Density functional theory calculations have been performed in order to evaluate the stability of different Cobalt adsorption configurations on Nickel
surfaces, mainly at (100) terraces and steps, and the relevant energy barriers for diffusion on terraces and across steps, segregation and dissolution into the substrate. The simulations have been compared with variable temperature scanning tunneling microscopy and low energy electron diffraction. The results show that the Cobalt
adatoms and small aggregates are unstable with respect to the formation of Co-Ni alloys, but the presence of a carbide monolayer on Ni surface improve their stability
Pentacene nanorails on Au(110)
No full textWe studied the molecular orientation of pentacene monolayer phases on the Au(110) surface by means of near-edge X-ray absorption spectroscopy at the carbon K-shell and scanning tunneling microscopy. The highest coverage phase, displaying a (6 x 8) symmetry, is found to be formed by two types of differently oriented molecules mimicking regular arrays of nanorails. Flat-lying molecules, aligned side-by-side with the long molecular axis along the [001] direction, form long crosstie chains extending in the [1 (1) over bar0] direction. In between the adjacent flat chains, additional molecules, tilted by 90 degrees around their molecular axis, line up head-to-tail into rails extending along [1<(1)over bar0]. These molecules are very weakly hybridized with the substrate, as indicated by their lowest unoccupied molecular orbitals, which closely resemble those of the free molecule. The nanorail structure is found to be stable up to 420 K in vacuum and to also remain in place after exposure to air, thus being a template well suited for further self-assembly of organic heterostructures. The tilted quasi-free molecules open the possibility for an optimal lateral pi-coupling to other molecules or molecular assemblies
Dynamics of the O Induced Reconstruction of the Rh(110) Surface: a Scanning Tunneling Microscopy Study
No full textReactivity and Deconstruction of the (1x2)-Rh(110) Surface Studied by Scanning Tunneling Microscopy
No full textHoneycomb on Square Lattices: Geometric Studies and Strain Analysis of Moiré Structures at a Symmetry-Mismatched Interface
No full textMoiré superstructures are common for epitaxial graphene on transition metal surfaces and can strongly influence the mechanical and electronic properties of the overgrown film. To date, most of the reported investigations have focused on the moiré patterns on sixfold close-packed substrates, where the superstructures also exhibit hexagonal modulations. Herein, graphene moiré patterns on a fourfold Ni(100) surface have been investigated on the basis of a simple geometric model assuming the existence of coincidence lattices. The moiré motif changes from stripes to rhombic networks as the misorientation angle between graphene and the substrate increases. The unit cells of the observed moiré superstructures are described within a matrix formalism, allowing the lattice parameters to be accurately determined. In all cases, the graphene lattice was found to be anisotropically strained in order to obtain commensurability in the moiré supercells. Our
work for the first time determines the atomic configurations for a variety of graphene moiré superstructures on a fourfold substrate, on the basis of a coincidence lattice model. The resulting atomic-scale details will serve as a reference for future experimental and theoretical studies of this specific system and will shed light on the geometric and strain analysis of two-dimensional films supported by a symmetry-mismatched surface
Probing the graphene/substrate interaction by electron tunneling decay
No full textThe electronic properties of graphene can be modified by the local
interaction with a selected metal substrate. To probe this effect, Scanning
Tunneling Microscopy is widely employed, particularly by means of local
measurement via lock-in amplifier of the differential conductance and of the
field emission resonance. In this article we propose an alternative, reliable
method of probing the graphene/substrate interaction that is readily available
to any STM apparatus. By testing the tunneling current as function of the
tip/sample distance on nanostructured graphene on Ni(100), we demonstrate that
I(z) spectroscopy can be quantitatively compared with Density Functional Theory
calculations and can be used to assess the nature of the interaction between
graphene and substrate. This method can expand the capabilities of standard STM
systems to study graphene/substrate complexes, complementing standard
topographic probing with spectroscopic information
Effects of the Lattice Expansion on the Reactivity of a 1D Oxide
No full textBy means of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we characterize at the single-atom level the mechanism of the water formation reaction on the (10 × 2)-O/Rh(110) surface, a prototype of a one-dimensional (1D) oxide where the lattice expansion and the segmentation of the surface play a fundamental role. When the reaction is imaged in the 238−263 K temperature range (35 s/image acquisition time), a peculiar comblike propagation mechanism for the reaction front is found. Fast STM measurements (33 ms/image) prove that this mechanism holds also at room temperature, being therefore an intrinsic characteristic of the reaction on the 1D oxide. DFT calculations explain the observed behavior as due to the interplay between the lattice expansion in the initial surface and its relaxation during the reaction that leads to varying configurations for the reactants
Anchoring and bending of pentacene on aluminum (001)
No full textWe study the structural, electronic, and spectroscopic properties of pentacene adsorbed on Al(001) surface, combining density functional theory (DFT) methods including van der Waals interactions with X-ray photoemission (XPS), near-edge X-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). We find a major change of the molecular backbone resulting in a peculiar V- shape bending, due to the direct anchoring of the two central carbons atop two Al atoms underneath. In the most stable adsorption configuration, pentacene is oriented with the long axis parallel to the substrate [110] direction, where such anchoring is favored by optimally matched interatomic distances. Remarkably, due to the generally low degree of order, we measure by STM a significant portion of molecules oriented along the [100] direction, which also display the same V-shape conformation, as driven by the bond of the central carbon atoms of pentacene to a pair of slightly displaced Al atoms.
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