|The Role of Gold Adatoms in Self-Assembled Monolayers of Thiol on Au(111)|
| Edmanuel Torres, P. Ulrich Biedermann, Alexander T. Blumenau|
Max-Planck Institut fuer Eisenforschung GmbH, Max-Planck-Str. 1, 40237 Duesseldorf, Germany
|Self-assembled monolayers (SAM's) of thiol on gold surfaces are of great interest not only because the number potential applications in fields such as molecular electronics, nanotechnology or biosciences, but also because their intrinsic phenomenon of self assembly can be studied. The development of future applications of alkanethiol SAM's in nanosciences requires reproducible methods that strongly depend on the understanding of the binding mechanism of the molecules on the metallic surfaces and the preferred structures. However, in spite of intense studies during the last two decades attempting to understand the self assembling mechanism and the spatial arrangement of the alkanethiol molecules on the surfaces, experiment and theoretical calculations have persistently disagreed: In particular the discussion whether the molecules adsorb on top or at a site close to the bridge site has been one of the most remarkable controversy [1, 2]. The fact that surface reconstructions were not considered could be the origin of the controversial results. A recent STM/DFT study at very low coverage discovered a gold adatom in between two thiol molecules . This was further supported by a PED/GXRD/DFT study at high density coverage . Later theoretical investigations taking gold adatoms into account in thiol-SAM structures have shown better agreement with experiments [5, 6]. In the present work, we have systematically studied adatoms and surface vacancy structures and compared their binding and surface energies. Structural differences between the (√3×√3)R30o also known as α-phase, and the c(4×2) superlattices were investigated. While our results for the alpha phase show that molecules are directly bound to an unreconstructed Au(111) surface on a bridge site slightly shifted to the fcc hollow position, for c(4×2) calculations involving gold adatoms suggest a quasi top position to be the favourable adsorption site. In particular we found that gold atoms added to an α-phase induce a phase transition that ends up in a new c(4×2). Simulated STM images of our most favourable structure including adatoms and vacancies exhibit the zig-zag modulation in intensity, which is characteristic for the δ-phase c(4×2) superstructure. It can be clearly seen that differences not only stem from the inequivalent positions of the S atoms with respect to the surface but also result from variations in tilting and precession angles with respect to the α-phase. |
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