Structure of aCH3SMonolayer on Au(111) Solved by the Interplay between Molecular Dynamics Calculations and Diffraction Measurements

Abstract

We have investigated the controversy surrounding the (sqrt[3] x sqrt[3]) R30 degrees structure of self-assembled monolayers of methylthiolate on Au(111) by first principles molecular dynamics simulations, energy and angle resolved photoelectron diffraction, and grazing incidence x-ray diffraction. Our simulations find a dynamic equilibrium between bridge site adsorption and a novel structure where 2 CH3S radicals are bound to an Au adatom that has been lifted from the gold substrate. As a result, the interface is characterized by a large atomic roughness with both adatoms and vacancies. This result is confirmed by extensive photoelectron and grazing incidence x-ray diffraction measurements.

Exchange Reactions between Alkanethiolates and Alkaneselenols on Au{111}

“…Are gold adatoms derived from the initial lift of the herringbone reconstruction, 17−19 or are they abstracted from the unreconstructed surface, leaving behind vacancy sites? 5,7 What is the significance of a proposed barbell motif relative to other binding models in the context of molecular-exchange and place-exchange reactions? 20−25 Toward answering such questions, we have explored exchange reactions between alkaneselenols and alkanethiolate monolayers.…”

Of interfaces and spin transport across nanoscale layers

“…3.1(monothiol) [76,85,86]. In addition to Au adatoms, some studies suggest the presence of Au vacancies in the top Au (111) layer [79,81]. Many of these conclusions derive from work on methanethiolate, the smallest possible alkanethiolate molecule.…”

“…The two sulfur head-groups straddle the adatom while residing on an on-top site with respect to the surface, see Fig. 3.1(1ad) [77,79]. Other studies, based upon NIXSW and XPS experiments, favor the Au-adatom-monothiolate motif, where the molecule sits directly on-top of an Au adatom, see Fig.…”

“…DFT studies of the adsorption of methanethiolate SAMs on reconstructed Au(111) surfaces confirm that the gain in adsorption energy of thiolates on the reconstructed surface compensates for the cost of adatom and vacancy creation, which are in the order of 0.5-1 eV [75,79,78,80,82,1]. Several different structural models have been proposed, see Fig.…”

“…This indeed has been the standard structural model until recently [61,62]. Recent experimental data and DFT calculations however suggest that adsorption of alkanethiolates leads to major reconstructions of the Au(111) surface, involving Au adatoms and possible vacancies [75,76,77,78,79,80,81,82,83,84,85,86,1]. Such reconstructions not only change the structure of the underlying Au substrate, but also that of the SAM overlayer and possibly affect the chemical bonding between substrate and SAM.…”