Alkanethiol self-assembled monolayer (SAM) phases on Au(111) have been assumed to involve direct S head group bonding to the substrate. Using x-ray standing wave experiments, we show the thiolate actually bonds to gold adatoms; self-organization in these archetypal SAM systems must therefore be governed by the movement of these Au-S-R moieties on the surface between two distinct local hollow sites on the surface. The results of recent ab initio total energy calculations provide strong support for this description, and a rationale for the implied significant molecular mobility in these systems. Self-assembled monolayers (SAMs) of molecular systems have attracted huge interest over the last decade or more (e.g., [1][2][3]). The archetypal SAM system of straightchain alkanethiols [CH 3 CH 2 nÿ1 SH] chemisorbed on Au(111) is the subject of many investigations, yet the structure of the molecule-substrate interface, which strongly influences the molecular ordering, remains in doubt [4]. Here we show that a quantitative investigation of this interface structure leads us to conclude that the true nature of the self-assembly in these SAMs is not organization of adsorbed molecules on the Au(111) surface, but rather organization on the surface of Au-thiolate complexes formed by the bonding of the deprotonated thiol molecule to single Au adatoms. Our results resolve two major puzzles in these systems: (i) why do all previous total energy calculations fail to agree with experiment over the adsorption site of the simplest methylthiolate species (n 1, CH 3 S) [5,6]; (ii) why is interchange between the several ordered structures of the molecules in their standing-up orientation so facile? This modified view of the interface has major implications for our understanding of the local ordering in these SAMs, and the influence of the corrugated adsorbate-substrate potential relative to the interadsorbate forces.

No abstract