Michel A. Van Hove

Surface Crystallography by LEED

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Single-Molecule Resolution of an Organometallic Intermediate in a Surface-Supported Ullmann Coupling Reaction

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Theory of CO adsorption on MgO(100): the influence of intermolecular interactions on the CO orientation

We have studied the organometallic intermediate of a surface-supported Ullmann coupling reaction from 4, 4″-dibromo-p-terphenyl to poly(para-phenylene) by scanning tunneling microscopy/spectroscopy and density functional theory calculations. Our study reveals at a single-molecular level that the intermediate consists of biradical terphenyl (ph)(3) units that are connected by single Cu atoms through C-Cu-C bridges. Upon further increasing the temperature, the neighboring biradical (ph)(3) units are coupled by C-C bonds forming poly(para-phenylene) oligomers while the Cu atoms are released.

Survey of structural and electronic properties of C60 on close-packed metal surfaces

Abstract From periodic Hartree-Fock calculations, the mode of CO chemisorption on MgO(100) is found to vary with the coverage. At low coverage, the best adsoption mode is predicted to be perpendicular to the surface. At higher coverages, the interaction between the adsorbed molecules influences the ordering of the surface. At coverage θ = 1 2 monolayer, lateral effects dominate and CO is adsorbed parallel to the surface, bridging pairs of nearest-neighbor Mg atoms. At θ = 3 4 , for the c(4 × 2) unit cell, the COs are adsorbed differently: whereas one third of the COs remains perpendicular to the surface, the two other thirds are bent on the surface. In this case, we find two geometrical arrangements that are equally favorable from an energetic standpoint. In a first arrangement, the non-perpendicular COs differ: one of them is tilted off-normal while the other one is parallel to the surface and bridges Mg atoms as at θ = 1 2 . This arrangement therefore distinguishes three kinds of COs. It is very close to results obtained by potential energy calculations. In another model, the two non-perpendicular COs are symmetrically positioned relative to the perpendicular one. This model distinguishes only two kinds of COs in a 2:1 ratio and is very close to results derived from spectroscopies at low temperature.

Energetics and dynamics of a new type of extended line defects in graphene.

The adsorption of buckminsterfullerene (C60) on metal surfaces has been investigated extensively for its unique geometric and electronic properties. The two-dimensional systems formed on surfaces allow studying in detail the interplay between bonding and electronic structures. Recent studies reveal that C60 adsorption induces reconstruction of even the less-reactive close-packed metal surfaces. First-principles computations enable access to this important issue by providing not only detailed atomic structure but also electronic properties of the substrate–adsorbate interaction, which can be compared with various experimental techniques to determine and understand the interface structures. This review discusses in detail the ordered phases of C60 monolayers on metal surfaces and the surface reconstruction induced by C60 adsorption, with an emphasis on the different types of reconstruction resulting on close-packed metal surfaces. We show that the symmetry matching between C60 molecules and metal surfaces determines the local adsorption configurations, while the size matching between C60 molecules and the metal surface lattice determines the supercell sizes and shapes; importantly and uniquely for C60, the number of surface metal atoms within one supercell determines the different types of reconstruction that can occur. The atomic structure at the molecule–metal interface is of crucial importance for the monolayer’s electronic and transport properties: these will also be discussed for the well-defined adsorption structures, especially from the perspective of tuning the electronic structure via C60–metal interface reconstruction and via relative inter-C60 orientations.

Electron Stimulation of Internal Torsion of a Surface-Mounted Molecular Rotor

We revealed a novel extended line defect (ELD) containing tetragonal rings embedded in graphene, formed as a growth fault, with its energetic and dynamic behavior studied via first-principles calculations. In our finding based upon the molecular dynamics simulation, transformation between locally stable ELDs in graphene at high temperatures simultaneously with contrastive electronic properties can be applied to predetermine the formation process and reconstruction of ELDs.

BENZENE ADSORPTION ON Pt(111): A THEORETICAL STUDY

A molecular rotor which includes a central rotator group was investigated by scanning tunneling microscopy at 4.9 K as it was grafted on a Cu(111) surface via its two terminal groups. Topographs with submolecular resolution revealed several distinct molecular conformations which we attribute to different angular orientations of the rotator and which are locally stable states according to density functional theory calculations. Time-resolved tunneling current spectra showed that the rotator undergoes a torsional motion around the molecular long axis as stimulated by tunneling electrons in a one-electron process with an excitation energy threshold of 355 meV. Calculations identified an intrinsic axial vibration mode of the rotator group at 370 meV as adsorbed on the surface, which we propose to be the channel for effectively converting the tunneling electron energy into the mechanical energy of the intramolecular torsion.

Manipulating Magnetism at Organic/Ferromagnetic Interfaces by Molecule-Induced Surface Reconstruction

From crystal orbital calculations, benzene is found to chemisorb with nearly equal binding energy on a hollow site and on a bridge site of the Pt(111) face. The chemisorption is stronger and involves larger molecular distortions than on palladium and rhodium surfaces in agreement with experiment. On the hollow site, the benzene molecule undergoes a Kekule distortion. On the bridge site found experimentally (with or without coadsorbed CO), the benzene molecule undergoes a local C2v distortion with long and short C-C bonds also in qualitative agreement with experiment. The favored azimuthal orientation of pure benzene coincides with that found experimentally only in the presence of CO. According to calculations, CO adsorption is found to weaken the benzene adsorption and reduce its metal-induced distortions but preserved the same orientation.

Atomic nitrogen chemisorption on graphene with extended line defects

Fullerenes have several advantages as potential materials for organic spintronics. Through a theoretical first-principles study, we report that fullerene C60 adsorption can induce a magnetic reconstruction in a Ni(111) surface and expose the merits of the reconstructed C60/Ni(111) spinterface for molecular spintronics applications. Surface reconstruction drastically modifies the magnetic properties at both sides of the C60/Ni interface. Three outstanding properties of the reconstructed structure are revealed, which originate from reconstruction enhanced spin-split π-d coupling between C60 and Ni(111): (1) the C60 spin polarization and conductance around the Fermi level are enhanced simultaneously, which can be important for read-head sensor miniaturization; (2) localized spin-polarized states appear in C60 with a spin-filter functionality; and (3) magnetocrystalline anisotropic energy and exchange coupling in the outermost Ni layer are reduced enormously. Surface reconstruction can be realized simply by controlling the annealing temperature in experiments.

Inducing extended line defects in graphene by linear adsorption of C and N atoms

The adsorption of N atoms onto a graphene substrate with extended line defects (ELDs) has been investigated by first-principles calculations. In the presence of recently observed extended line defects, the N adatom can be adsorbed onto both top and bridge sites of the graphene lattice. We demonstrate that chemisorption on ELDs in graphene can substantially affect their structural and electronic properties, depending in particular on specific adsorption sites and density. We also find that magnetism can be induced in ELD-graphene by nitrogenation at suitable N densities; a higher density of N adsorption onto the core carbon atoms of the ELD removes this.