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Alkylthiolate adsorbates on coinage metal surfaces, and particularly the (111) faces, are perhaps the best-known and most widely exploited of self-assembled monolayers (SAMs). Despite this, the detailed nature of the interface structures has been largely unknown, most quantitative structural studies having been based on total energy calculations, not experiment. In a Warwick/Nottingham collaboration, mainly using the technique of normal incidence X-ray standing waves (NIXSW), but combined with STM and medium energy ion scattering (MEIS), we have now established many of the key features of these interfaces, primarily through model studies of methylthiolate layers, but including some work on longer alkyl chains.
On Cu(111) it is now well established that both methylthiolate and octylthiolate cause a pseudo-(100) reconstruction of the surface in which the thiolate S headgroup occupy 4-fold coordinated hollow sites in a single reconstructed Cu layer of near-square symmetry.
On Ag(111), methylthiolate is well-known to produce a (√7x√7) ordering. We have shown that this structure involves a single 3/ ML of Ag atoms in an hexagonal layer with the S headgroup in 3-fold coordinated hollow sites. For pentylthiolate, a similar reconstruction occurs, but the lateral periodicity within the SAM expands to produce an incommensurate reconstructed layer.
Au(111), the most widely-studied surface, has long been thought to be unreconstructed at the metal/SAM interface, mainly forming (√3x√3) layers alone for the shortest alkyl chains, but with a coexistent (3x2√3)rect. phase for the longer chain SAMs. Interconversion between the two phases has been found to be facile. Our most recent results show that on this surface these SAMs actually involve the self-organisation of Au-S-R moieties, rather than the S-R thiolates. Interaction of the surface with the thiol causes the extraction of surface Au atoms, to which the S headgroup is bonded in atop sites, and the ordered phases involve different occupation of the two inequivalent ‘fcc’ and ‘hcp’ hollow sites by the associated Au adatoms.
Reconstruction therefore plays a key role in all of these thiolate/coinage metal SAM systems. This conclusion represents a paradigm shift in our understanding of these systems. |