Ab-Initio Simulation Laboratory

We report calculations of the dissociative adsorption of H-2 at Pd (100) covered with 1/4 monolayer of sulfur using quantum dynamics as well as molecular dynamics and taking all six degrees of freedom of the two H atoms fully into account. The ab-initio potential-energy surface (PES) is found to be very strongly corrugated. In particular, we discuss the influence of tunneling, zero-point vibrations due to the localization of the wave function of the nuclei when narrow valleys of the PES are passed, steering of the approaching H-2 molecules towards low-energy barrier configurations, and the important role of subsurface absorbates for the hydrogen dissociation. It is shown that "established" concepts derived from low-dimensional dynamical studies are not necessarily valid in a high-dimensional treatment. (C) 1998 Elsevier Science B.V. All rights reserved.

Scanning tunneling microscopy (STM) observations of the close-packed C60 fullerene arrays on Si(111)
R3xR3-Ag surface have revealed the presence of dim C60 molecules which constitute 9–12% of all fullerenes. The dim C60 fullerenes reside  1.6 A lower than the bright (“normal”) C60.While the brightC60 are in continuous rotation, the dim C60 are fixed in one of the single orientations, indicating a more tight bonding to the surface. At room temperature (RT), the dynamic switching from bright to dim C60 and vice versa has been detected. Switching slows down with decreasing temperature and becomes completely frozen at 110 K, which implies that the switching is a thermally driven process. RT deposition of  0.1 monolayer of Ag onto C60 array eliminates completely the dim C60 molecules. Experimental results can be understood if one assumes that formation of the dim C60 is associated with disintegration of Ag trimer on Si(111)R3xR3- Ag surface under a given C60 fullerene.

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Adsorption of C-60 onto Si(111)root 3 x root 3-In surface presents a fascinating example of interplay between molecular adsorbate and surface structural defects. It has been found that adsorbing C-60 molecules are trapped by the substitutional Si-defects. In turn, the group of a few adsorbed C-60 can act as a trap for the mobile vacancies of the root 3 x root 3-In reconstruction. Namely, adsorbed C-60 induces a strain in the indium layer, and when a mobile vacancy happens to get into the surface area surrounded by fullerenes, the In atoms between the C-60 and the vacancy shift from the T-4 to the H-3 sites, fixing a vacancy in a given location. (C) 2011 Elsevier B.V. All rights reserved.

Self-assembly of atoms or molecules on a crystal surface is considered one of the most promising methods to create molecular devices. Here we report a stepwise self-assembly of C60 molecules into islands with unusual shapes and preferred sizes on a gold–indium-covered Si(111) surface. Specifically, 19-mer islands prefer a non-compact boomerang shape, whereas hexagonal 37-mer islands exhibit extraordinarily enhanced stability and abundance. The stepwise self-assembly is mediated by the moiré interference between an island with its underlying lattice, which essentially maps out the adsorption-energy landscape of a C60 on different positions of the surface with a lateral magnification factor and dictates the probability for the subsequent attachment of C60 to an island’s periphery. Our discovery suggests a new method for exploiting the moiré interference to dynamically assist the self-assembly of particles and provides an unexplored tactic of engineering atomic scale moiré magnifiers to facilitate the growth of monodispersed mesoscopic structures.

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