Complex molecular surfaces and interfaces: concluding remarks

This paper is derived from our concluding remarks presentation and the ensuing conversations at the Faraday Discussions meeting on Complex Molecular Surfaces and Interfaces, Sheffield, UK, 24th-26th July 2017. This meeting was comprised of sessions on understanding the interaction of molecules with surfaces and their subsequent organisation, reactivity or properties from both experimental and theoretical perspectives. This paper attempts to put these presentations in the wider context and focuses on topics that were debated during the meeting and where we feel that opportunities lie for the future development of this interdisciplinary research area.

Intermixed adatom and surface-bound adsorbates in regular self-assembled monolayers of racemic 2-butanethiol on Au(111)

In situ scanning tunneling microscopy combined with density functional theory molecular dynamics simulations reveal a complex structure for the self-assembled monolayer (SAM) of racemic 2-butanethiol on Au(111) in aqueous solution. Six adsorbate molecules occupy a (10×√3)R30° cell organized as two RSAuSR adatom-bound motifs plus two RS species bound directly to face-centered-cubic and hexagonally close-packed sites. This is the first time that these competing head-group arrangements have been observed in the same ordered SAM. Such unusual packing is favored as it facilitates SAMs with anomalously high coverage (30%), much larger than that for enantiomerically resolved 2-butanethiol or secondary-branched butanethiol (25%) and near that for linear-chain 1-butanethiol (33%).

Controlling the stereochemistry and regularity of butanethiol self-assembled monolayers on au(111)

The rich stereochemistry of the self-assembled monolayers (SAMs) of four butanethiols on Au(111) is described, the SAMs containing up to 12 individual C, S, or Au chiral centers per surface unit cell. This is facilitated by synthesis of enantiomerically pure 2-butanethiol (the smallest unsubstituted chiral alkanethiol), followed by in situ scanning tunneling microscopy (STM) imaging combined with density functional theory molecular dynamics STM image simulations. Even though butanethiol SAMs manifest strong headgroup interactions, steric interactions are shown to dominate SAM structure and chirality. Indeed, steric interactions are shown to dictate the nature of the headgroup itself, whether it takes on the adatom-bound motif RS(•)Au(0)S(•)R or involves direct binding of RS(•) to face-centered-cubic or hexagonal-close-packed sites. Binding as RS(•) produces large, organizationally chiral domains even when R is achiral, while adatom binding leads to rectangular plane groups that suppress long-range expression of chirality. Binding as RS(•) also inhibits the pitting intrinsically associated with adatom binding, desirably producing more regularly structured SAMs.

Effect of coverage and defects on the adsorption of propanethiol on Au(111) surface: a theoretical study

Periodic density functional calculations have been carried out to investigate both the thiol adsorption on Au(111) surface and the reaction mechanism for the formation of the self-assembled monolayers, taking propanethiol as a representative example. The effect of coverage and surface defects (adatoms and vacancies) has been analyzed. It is found that the most stable physisorption (undissociated) site is an adatom site, whereas the chemisorption site for the thiol is a vacancy site or protrusion consisting of a pair of adatoms, followed by one adatom site. The results point out that the thiolate self-assembled monolayer adsorption process occurs preferentially on step edges.

Coverage-dependent formation of chiral ethylthiolate-Au complexes on Au(111)

We studied the coverage-dependent self-assembly of the flat-lying phase of ethylthiolate on Au(111). At low coverage, we observed the formation of short stripes of chiral Au-(SC(2)H(5))(2) complexes that arrange in a disordered phase. The latter grow partly at the expense of the native Au(111) surface reconstruction, which is fully lifted for a coverage of ∼0.60 ML. We found that the lift of the reconstruction and evaporation from step edges are competing adatom sources. Close to saturation coverage (0.70 to 0.75 ML), large, well-ordered domains with a (8 × √3)rectangular superstructure formed. Alternation of chirality was found in adjacent stripes as already reported for other short alkanethiolates. We suggest that, because of a simple geometrical consideration, the chirality should, on the contrary, be preserved in the stripe phase of longer alkanethiolates.