Dresden 2009 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 42: Poster Session II (Nanostructures at surfaces: arrays; Nanostructures at surfaces: Dots, particles, clusters; Nanostructures at surfaces: Other; Nanostructures at surfaces: Wires, tubes; Metal substrates: Adsorption of O and/or H; Metal substrates: Clean surfaces; Metal substrates: Adsorption of organic/bio moledules; Metal substrates: Solid-liquid interfaces; Metal substrates: Adsorption of inorganic molecules; Metal substrates: Epitaxy and growth; Heterogeneous catalysis; Surface chemical reactions; Ab-initio approaches to excitations in condensed matter; Organic, polymeric, biomolecular films– also with adsorbates; Particles and clusters)

O 42.60: Poster

Mittwoch, 25. März 2009, 17:45–20:30, P2

Diffusion of 1,4-butanedithiol on unreconstructed Au(111) — Andreas Franke and •Eckhard Pehlke — Institut für Theoretische Physik und Astrophysik, Christian-Albrechts-Universität zu Kiel

Extending our previous density-functional studies on the diffusion of alkanethioles on gold surfaces, we now focus on more complex sulfur anchored molecules diffusing on the unreconstructed Au(111) surface. As a benchmark system we choose 1,4-butanedithiol radicals (BDT: S-(CH₂)₄-S) which bind to the gold surface via two sulfur atoms. Molecules that bind to the surface via more than one site are expected to show diffusion properties beyond isotropic diffusion of single adsorbate atoms (e.g. [1]). Unreconstructed Au(111) exists under certain electrochemical conditions [2]. Density functional calculations are carried out using the VASP code by Hafner, Kresse et al. [3]. We compare various adsorption geometries of one BDT per (4x3) unit cell. For the most favourable geometry both S-atoms bind close to fcc-hollow sites with d_S-S=4.9Å. Taking the free, spinpolarized radical as reference, the adsorption energy was calculated to be -3.3 eV. Due to lateral interactions, the adsorption energy changes to -3.8 eV in a (6x6) surface unit cell. We found a diffusion path with a small maximum energy barrier of 0.3 eV which results in a translation perpendicular to the S-S-axis of the molecule. Notably, the energy barrier increases to approx. 0.5 eV in case of the larger (4x6) surface unit cell.

[1] K.-Y. Kwon, et al., Phys. Rev. Lett. 95, 166101 (2005).

[2] M. A. Schneeweiss, et al., Appl. Phys. A 69, 537 (1999).

[3] http://cms.mpi.univie.ac.at/vasp/